A METHOD FOR ISOLATING AND SEQUENCING TRYPANOSOME CELLS TO INVESTIGATE SPECIES ASSOCIATIONS IN MULTIPLE MORPHOTYPE INFECTIONS

Trypanosome infections containing multiple morphologies have been described from all classes of vertebrates, including mammals, birds, non-avian reptiles, amphibians, and fish. These mixed infections make it challenging to evaluate trypanosome diversity, as it is not immediately clear whether the forms present in the bloodstream represent different species or a single pleomorphic species. Amphibians are common hosts for trypanosomes and are often infected by multiple trypanosome morphologies in the bloodstream. Based on morphological observations and life cycle studies, many authors have considered multiple trypanosome morphotypes found infecting the same frogs to be a single pleomorphic species. However, molecular evidence supporting pleomorphic trypanosome species in amphibians is lacking, primarily because linking sequence data to bloodstream trypanosome morphology in mixed infections is extremely challenging. Here we present methods to isolate individual trypanosome cells of 6 morphotypes from frog blood for nested PCR of the 18S rRNA and gGAPDH genes. Single trypanosome cells were isolated by dilution and 3 DNA extraction methods, and 5 nested PCR primer regimes were utilized to optimize amplification from very low starting concentrations. The success rates of extraction methods ranged from 29 to 50% with the use of a Direct PCR kit having the highest success rate. Although the success rate varied in the different combinations of extraction methods and primer regimes, multiple individuals of all 6 trypanosome morphotypes were sequenced for both genes in a novel way that links sequence data to cell morphology by observing isolated cells with a microscope before PCR amplification. All 6 morphologically distinguishable morphotypes coinfecting a frog were genetically distinct. The only other recent molecular study on amphibian trypanosomes also found genetic differences between morphotypes in multiple infections. Together these studies suggest that the occurrence of pleomorphism may be overestimated in amphibian trypanosomes. The methods presented here offer a promising solution to characterize trypanosome diversity within multiple morphotype infections.

MOLECULAR IDENTIFICATION OF JUVENILE NEOECHINORHYNCHUS SPP. (PHYLUM: ACANTHOCEPHALA) INFECTING OSTRACOD AND SNAIL HOSTS PROVIDES INSIGHT INTO ACANTHOCEPHALAN HOST USE

The role of invertebrates in some acanthocephalan life cycles is unclear because juvenile acanthocephalans are difficult to identify to species using morphology. Most reports suggest acanthocephalans from turtle definitive hosts use ostracods as intermediate hosts and snails as paratenic hosts. However, laboratory studies of the life cycle suggest that ostracods and snails are both required hosts in the life cycle. To elucidate the role of ostracods and snails in acanthocephalan life cycles better, we collected 558 freshwater snails of 2 species, including Planorbella cf. Planorbella trivolvis and Physa acuta, from 23 wetlands in Oklahoma, U.S.A., and examined them for acanthocephalan infections. Additionally, we examined 37,208 ostracods of 4 species, Physocypria sp. (morphotype 1), Cypridopsis sp., Stenocypris sp., and Physocypria sp. (morphotype 2) for juvenile acanthocephalans from 2 wetlands in Oklahoma. Juvenile acanthocephalans were morphologically characterized, and the complete internal transcribed spacer (ITS) region of nuclear rDNA was sequenced from acanthocephalans infecting 11 ostracod and 13 snail hosts. We also sampled 10 red-eared slider turtles, Trachemys scripta elegans, and 1 common map turtle, Graptemys geographica, collected from Oklahoma, Arkansas, and Texas and recovered 1,854 adult acanthocephalans of 4 species. The ITS of 17 adult acanthocephalans of 4 species from turtle hosts were sequenced and compared to juvenile acanthocephalan sequences from ostracod and snail hosts from this study and GenBank to determine conspecificity. Of the 23 locations sampled for snails, 7 (30%) were positive for juvenile acanthocephalans in the genus Neoechinorhynchus. The overall prevalence and mean intensity of acanthocephalans in Planorbella cf. P. trivolvis and P. acuta were 20% and 2 (1-6) and 2% and 1 (1), respectively. In contrast, only 1 of 4 species of ostracods, Physocypria sp. (morphotype 1), was infected with larval/juvenile Neoechinorhynchus spp. with an overall prevalence of 0.1% and a mean intensity of 1 (1-2). Although 4 species of acanthocephalans infected turtle definitive hosts, including Neoechinorhynchus chrysemydis, Neoechinorhynchus emydis, Neoechinorhynchus emyditoides, and Neoechinorhynchus pseudemydis, all the ITS sequences from cystacanths infecting snail hosts were conspecific with N. emydis. In contrast, the ITS sequences from larval/juvenile acanthocephalans from ostracods were conspecific with 2 species of acanthocephalans from turtles (N. emydis and N. pseudemydis) and 1 species of acanthocephalan from fish (Neoechinorhynchus cylindratus). These results indicate that N. emydis infects freshwater snails, whereas other species of Neoechinorhynchus appear not to infect snail hosts. We document new ostracod and snail hosts for Neoechinorhynchus species, including the first report of an ostracod host for N. pseudemydis, and we provide novel molecular barcodes that can be used to determine larva, juvenile, and adult conspecificity of Neoechinorhynchus species.

Survival of larval and cyst stages of gordiids (Nematomorpha) after exposure to freezing

Hairworms infect terrestrial arthropods and are 1 of the most understudied groups of parasites. Recently, life cycles of 2 gordiids (Paragordius varius and Paragordius obamai) have been domesticated in the laboratory. We tested the viability of laboratory reared and post-frozen larval and cyst stages of the North American gordiid, P. varius , frozen at -80 C for 7 mo, and the viability of field collected and post-frozen cysts of the African (P. obamai) and North American ( P. varius ) gordiid frozen at -20 C for 2 mo. All snails exposed to post-frozen or control P. varius larvae became infected with cysts, and there was no significant difference in prevalence or mean intensity of cysts among control or experimental snail groups. As with larvae, no significant differences were observed in prevalence or mean intensity of emerging worms from crickets infected with post-frozen or control P. obamai or P. varius cysts. All female P. obamai and P. varius worms from control and post-frozen cyst infections laid eggs and larvae hatched from some of these eggs. Survival and cyst formation of P. varius larvae exposed to different combinations of drying and/or freezing temperatures indicated that gordiid larvae have the ability to survive drying and freezing, but survival significantly increases during freezing at lower temperatures. The major contribution of our study is the demonstration that gordiid larval and cyst stages can survive freezing temperatures to infect and develop in the next host.

Diabetes and cardiovascular outcomes: finding the silver lining

Type 2 diabetes mellitus and cardiovascular disease create a pernicious synergism that now threatens the public health in both developed and developing countries. As such, there has been a concerted therapeutic effort to mitigate the effects of hyperglycemia on adverse cardiovascular outcomes. Despite compelling epidemiological evidence linking diabetes to cardiovascular disease and mechanistic evidence linking hyperglycemia to cardiomyocyte and endothelial cell toxicity, clinical trials designed to examine the effects of tight glycemic control on CV outcomes have been disappointing. The apparent paradox requires a re-examination of the premise as well as consideration of new therapeutic approaches beyond tight glycemic control alone. In this review, we will review the evidence that links diabetes to adverse cardiovascular outcomes and will examine the mechanistic evidence whereby hyperglycemia causes cellular damage in experimental models. We will extrapolate from information gleaned from recent clinical trials and discuss a new therapeutic approach that embraces glycemic control, but with less collateral side effects and perhaps by mechanisms that are also cardio-protective.

Mechanisms whereby rapid RV pacing causes LV dysfunction: perfusion-contraction matching and NO

Incessant tachycardia induces dilated cardiomyopathy in humans and experimental models; mechanisms are incompletely understood. We hypothesized that excessive chronotropic demands require compensatory contractility reductions to balance metabolic requirements. We studied 24 conscious dogs during rapid right ventricular (RV) pacing over 4 wk. We measured hemodynamic, coronary blood flow (CBF), myocardial O(2) consumption (MVO(2)) responses, myocardial nitric oxide (NO) production, and substrate utilization. Early pacing (6 h) resulted in decreased heart rate (HR)-adjusted coronary blood flow (CBF), MVO(2) (CBF/beat: 0.33 +/- 0.02 to 0.19 +/- 0.01 ml, P < 0.001, MVO(2)/beat: 0.031 +/- 0.002 to 0.016 +/- 0.001 ml O(2), P < 0.001), and contractility [left ventricular (LV) first derivative pressure (dP/dt)/LV end-diastolic diameter (EDD): 65 +/- 4 to 44 +/- 3 mmHg x s(-1) x mm(-1), P < 0.01], consistent with flow-metabolism-function coupling, which persisted over the first 72 h of pacing (CBF/beat: 0.15 +/- 0.01 ml, MVO(2)/beat: 0.013 +/- 0.001 ml O(2), P < 0.001). Thereafter, CBF per beat and MVO(2) per beat increased (CBF/beat: 0.25 +/- 0.01 ml, MVO(2)/beat: 0.021 +/- 0.001 ml O(2) at 28 days, P < 0.01 vs. 72 h). Contractility declined [(LV dP/dt)/LVEDD: 19 +/- 2 mmHg x s(-1) x mm(-1), P < 0.0001], signifying flow-function mismatch. Cardiac NO production, endothelial NO synthase expression, and fatty acid utilization decreased in late phase, whereas glycogen content and lactate uptake increased. Incessant tachycardia induces contractile, metabolic, and flow abnormalities reflecting flow-function matching early, but progresses to LV dysfunction late, despite restoration of flow and metabolism. The shift to flow-function mismatch is associated with impaired myocardial NO production.

SIV cardiomyopathy in non-human primates

While cardiac myocytes are not generally considered conventional cellular targets of retroviral infection with HIV-1, the increasing recognition of AIDS related cardiomyopathy has raised important questions as to the viral pathogenesis. Our laboratory has explored the role of simian immunodeficiency viral (SIV) infection in non-human primates as a suitable large-animal model to examine cardiac involvement. Our data suggest that in the presence of inflammatory myocarditis, SIV is localized to CD4 bearing inflammatory cells and not cardiac myocytes, suggesting that the heart may be an innocent bystander in AIDS cardiomyopathy.

Role of cardiac nerves in the cardiovascular response to cocaine in conscious dogs

BACKGROUND

Although the cardiovascular toxicity of cocaine is well recognized, considerable controversy remains as to the relative contribution of local norepinephrine reuptake inhibition versus central stimulatory effects of cocaine in eliciting its cardiovascular actions. The purpose of the present study was to determine the role of cardiac nerves in mediating the left ventricular (LV) and coronary hemodynamic responses to cocaine.

METHODS AND RESULTS

We studied the cardiovascular response to acute cocaine administration (1 mg/kg) in 10 intact, conscious dogs and 6 dogs with ventricular denervation (VD). There were no significant differences in baseline hemodynamic parameters or plasma catecholamines between the 2 groups. In response to acute cocaine, LV and coronary hemodynamic responses were enhanced in the VD dogs. The enhanced systemic pressor and heart rate responses in VD dogs suggest that cardiac nerves mitigate the response to cocaine through ventricular mechanoreceptors rather than mediating the responses.

CONCLUSIONS

These data suggest that peripheral blockade of norepinephrine reuptake is not the principal mechanism of the acute cardiac effects of cocaine. Rather, cardiac nerves modulate the effects of cocaine through baroreflex mechanisms. Thus, individual differences in baroreflex sensitivity may explain the hemodynamic variability observed in response to cocaine.

Increased susceptibility to development of triggered activity in myocytes from mice with targeted disruption of endothelial nitric oxide synthase

Nitric oxide generated by cardiac myocytes or delivered by drugs has been shown to regulate cardiac contractile function and has been implicated in suppressing some cardiac arrhythmias, although this remains controversial. We examined the ability of the soluble cardiac glycoside, ouabain, to trigger arrhythmic contractions in ventricular myocytes isolated from mice lacking a functional endothelial nitric oxide synthase gene (eNOS(null)). Arrhythmic activity, defined as aftercontractions, was induced with ouabain (50 micromol/L) and recorded using a video-motion detector in isolated, electrically driven single ventricular myocytes from adult eNOS(null)or from their wild-type (WT) littermates. The rate of ouabain-induced arrhythmic contractions was significantly higher in eNOS(null)myocytes than in WT myocytes. Application of the NO donor S-nitroso-acetylcysteine (SNAC) significantly diminished the frequency of arrhythmic contractions in eNOS(null)myocytes. The antiarrhythmic effect of NO, whether generated by eNOS in WT cells or by SNAC, could be partially reversed by 1H-[1,2,4]oxadiazolo-[4, 3-a]- quinoxalin-1-one (ODQ), a specific soluble guanylyl cyclase inhibitor. Ouabain significantly increased intracellular cGMP in WT but not eNOS(null)hearts, and this cGMP response was blocked by ODQ. Since cardiac glycoside- induced aftercontractions are activated by the transient inward current (I(ti)), the role of NO in ouabain (100 micromol/L)- induced I(ti)was examined using the nystatin-perforated patch-clamp technique. The frequency of ouabain-induced I(ti)was significantly higher in eNOS(null)myocytes than in WT myocytes, and this could be suppressed by SNAC. These data demonstrate that NO derived from myocyte eNOS activation suppresses ouabain-induced arrhythmic contractions by a mechanism that might involve activation of guanylyl cyclase and elevation of cGMP.

Coronary vascular responses to short-term cocaine administration in conscious baboons compared with dogs

OBJECTIVES

Cardiovascular complications of cocaine use represent an important clinical problem, yet the mechanisms by which cocaine predisposes to myocardial ischemia are poorly understood.

BACKGROUND

The effects of cocaine on the coronary circulation have been studied extensively in experimental animal models, but have failed to recapitulate the clinical findings reported in humans who use cocaine.

METHODS

We studied 12 conscious, chronically instrumented dogs and 5 conscious, chronically instrumented baboons to determine whether there were important species differences in the response to cocaine.

RESULTS

Comparable doses of intravenous cocaine caused similar increases in left ventricular systolic, diastolic and mean arterial pressure in the two species. However, the peak coronary blood flow response in baboons (+8 +/- 3 from 47 +/- 6 ml/min) was less compared with dogs (+15 +/- 4 from 41 +/- 4 ml/min), while the coronary vascular resistance response was greater in baboons (+0.60 +/- 0.09 from 1.94 +/- 0.09 mm Hg/ml/mm) compared with dogs (+0.35 +/- 0.09 from 2.24 +/- 0.10 mm Hg/ml/min). Although myocardial oxygen consumption responses were similar between species, there was a significant difference (p < 0.05) in oxygen delivery between baboons (+164 +/- 47 from 705 +/- 59 ml of oxygen per minute) and dogs (+397 +/-51 from 656 +/- 33 ml of oxygen per minute) that was attributable to a significant (p < 0.05) increase in hemoglobin concentration in dogs (+2.1 +/- 0.5 g/dl) that was not observed in baboons. Consequently, cocaine caused a significant increase in myocardial oxygen extraction and decreased coronary sinus pH in baboons, but not dogs.

CONCLUSIONS

Cocaine caused greater coronary vasoconstriction and greater requirements for oxygen extraction in baboons compared with dogs.

Dilated cardiomyopathy associated with simian AIDS in nonhuman primates

BACKGROUND

Cardiomyopathy is being recognized with increasing frequency in patients with AIDS, yet the relationship between HIV infection and cardiac contractile dysfunction remains obscure. The purpose of the present study was to determine if infection with simian immunodeficiency virus (SIV) in nonhuman primates is associated with cardiac dysfunction and myocardial injury.

METHODS AND RESULTS

Left ventricular size and function were determined by 2D echocardiography in 16 rhesus macaques before and at weekly intervals following infection with cloned pathogenic SIV(mac) 239 or the highly attenuated SIV(mac) 239 nef deletion mutant. A second group of 15 rhesus macaques chronically infected with pathogenic (n=6) or nonpathogenic (n=9) virus were studied at >2 years following infection. Cardiac tissues from 24 rhesus macaques chronically infected (>2 years) with pathogenic SIV were reviewed for evidence of cardiac pathology. Acute infection (<6 weeks) with either pathogenic or nonpathogenic SIV caused neither contractile dysfunction nor cardiac pathology. However, LV ejection fraction was significantly (P<0.05) depressed (43+/-7%) in rhesus macaques chronically infected with pathogenic SIV compared with rhesus macaques chronically infected with nonpathogenic SIV (61+/-3%). Furthermore, two thirds of rhesus macaques that succumbed to simian AIDS had myocardial pathology including lymphocytic myocarditis (n=9) and coronary arteriopathy (n=6), with complete vessel occlusion (n=4) and associated myocardial infarction and necrosis.

CONCLUSIONS

This unique model is valuable in understanding the pathogenesis of cardiac injury associated with retroviral infection in a relevant nonhuman primate model of AIDS.

Progressive loss of myocardial ATP due to a loss of total purines during the development of heart failure in dogs: a compensatory role for the parallel loss of creatine

BACKGROUND

Whether myocardial ATP content falls in heart failure is a long-standing and controversial issue. The mechanism(s) to explain any decrease in ATP content during heart failure have not been identified.

METHODS AND RESULTS

Cardiac dysfunction, heart failure, and a prolonged steady state of heart failure were induced by chronic right ventricular pacing for 1 to 2 weeks, 3 to 4 weeks, and 7 to 9 weeks in dogs. Cardiac function and myocardial O(2) consumption (Mf1.gif" BORDER="0">O(2)) were measured with the dogs in the conscious state. ATP, total purine, and creatine were measured in biopsy specimens obtained at each stage. ATP and the total purine pool progressively fell at rates of 0.12 and 0.15 nmol. mg protein(-1). d(-1), despite an increase in Mf1.gif" BORDER="0">O(2). The rate of loss of creatine was 1.06 nmol. mg protein(-1). d(-1), 7 times faster than the depletion of total purine.

CONCLUSIONS

(1) ATP contents progressively decreased during heart failure as a result of a loss of the total purine pool. The loss of purines may be due to inhibition of de novo purine synthesis. (2) Loss of creatine is an early marker of heart failure and may serve as a compensatory mechanism minimizing the reduction of the total purine pool in the failing heart.

Beta-adrenergic receptor blockade arrests myocyte damage and preserves cardiac function in the transgenic G(salpha) mouse

Transgenic (TG) mice with cardiac G(salpha) overexpression exhibit enhanced inotropic and chronotropic responses to sympathetic stimulation, but develop cardiomyopathy with age. We tested the hypothesis that cardiomyopathy in TG mice with G(salpha) overexpression could be averted with chronic beta-adrenergic receptor (beta-AR) blockade. TG mice and age-matched wild-type littermates were treated with the beta-AR blocker propranolol for 6-7 months, starting at a time when the cardiomyopathy was developing but was not yet severe enough to induce significant cardiac depression (9.5 months of age), and ending at a time when cardiac depression and cardiomyopathy would have been clearly manifest (16 months of age). Propranolol treatment, which can induce cardiac depression in the normal heart, actually prevented cardiac dilation and the depressed left ventricular function characteristic of older TG mice, and abolished premature mortality. Propranolol also prevented the increase in myocyte cross-sectional area and myocardial fibrosis. Myocyte apoptosis, already apparent in 9-month-old TG mice, was actually eliminated by chronic propranolol. This study indicates that chronic sympathetic stimulation over an extended period is deleterious and results in cardiomyopathy. Conversely, beta-AR blockade is salutary in this situation and can prevent the development of cardiomyopathy.

A novel heart failure model induced by sequential coronary artery occlusions and tachycardiac stress in awake pigs

A heart failure model was developed using conscious pigs subjected to serial myocardial infarctions followed by intermittent rapid ventricular pacing. Aortic and atrial catheters, left ventricular (LV) pressure gauge, LV dimension crystals, ascending aortic flow probe, pacing leads, and two coronary artery occluders were implanted in 15 pigs. The initial distal left circumflex coronary artery (LCX) occlusion produced a modest infarct, i.e., 18 +/- 3% of LV, and the second proximal LCX occlusion, performed 48 h later, enlarged the infarct to 33 +/- 2% of the LV with only modest changes in LV function. Thereafter, the pigs were subjected to ventricular pacing at 220 beats/min, which was maintained for 7 days and terminated for 3 days. This pacing cycle was repeated two more times and resulted in significantly impaired LV function and systemic hemodynamics. For example, after the second cycle of pacing, LV rate of pressure change (dP/dt, -41 +/- 4% from 2,778 +/- 112 mmHg/s), velocity of circumferential fiber shortening (V(cf): -53 +/- 6% from 1.1 +/- 0.1 s(-1)), and cardiac index (CI: -42 +/- 5% from 122 +/- 4 ml. min(-1). kg(-1)) were reduced significantly, whereas LV end-diastolic diameter (EDD: +34 +/- 6% from 39 +/- 2 mm), total peripheral resistance (TPR: +75 +/- 16% from 0.79 +/- 0.05 U), and mean left atrial pressure (LAP) (+21 +/- 1 mmHg from 5 +/- 1 mmHg) were increased significantly. Importantly, 3 wk after cessation of the final pacing cycle, LV dP/dt (-40 +/- 5%), V(cf) (-48 +/- 9%), and CI (-30 +/- 4%) remained depressed, whereas LV EDD (+39 +/- 5%), TPR (+43 +/- 9%), and LAP (+13 +/- 4 mmHg) were still increased. In contrast, hemodynamic impairment in six conscious pigs subjected to pacing only did not persist when pacing was terminated. Thus this model could provide a unique opportunity to study both the effects of preclinical therapeutic interventions and the mechanisms involved in the development of heart failure.

Beta-adrenergic receptor-G protein-adenylyl cyclase signal transduction in the failing heart

The beta-adrenergic receptor signal transduction pathway is critical for rapid adjustments to increased cardiovascular demand (e.g., during exercise). In the face of chronic stimulation of this pathway, as occurs in the pathogenesis of heart failure, beta-adrenergic receptor stimulation may become maladaptive. Under these conditions, elevation of circulating catecholamines and depletion of cardiac tissue stores of norepinephrine occur in the failing heart, resulting in desensitization. Whether or not stimulation or inhibition of the beta-adrenergic receptor signaling pathway is beneficial in heart failure is controversial. One approach to address this question is to specifically overexpress a component of the beta-adrenergic receptor signaling pathway in a transgenic mouse heart. We have characterized young and old adult mice with overexpressed cardiac G(s alpha) which couples the beta-adrenergic receptor to adenylyl cyclase. In younger animals, beta-adrenergic receptor stimulation results in an augmented heart rate and cardiac contractility. Over the life of the animal, however, a picture of cardiomyopathy develops. The result is a dilated heart with a large amount of fibrosis and myocyte hypertrophy, degeneration atrophy, and apoptosis. Conversely, chronic beta-adrenergic receptor blockade prevents the development of cardiomyopathy. These experiments support the point of view that chronic beta-adrenergic stimulation during the development of heart failure is deleterious and that protecting the heart with chronic beta-adrenergic receptor blockade is salutary, conceptually consistent with results of recent clinical trials examining the effects of beta-adrenergic receptor blockers in patients with heart failure.

Mechanisms of desensitization to a PDE inhibitor (milrinone) in conscious dogs with heart failure

The goal of this study was to determine the extent to which the effects of milrinone were desensitized in heart failure (HF) and to determine the mechanisms, i.e., whether these effects could be ascribed to changes in cAMP or phosphodiesterase (PDE) activity in HF. Accordingly, we examined the effects of milrinone in seven conscious dogs before and after HF was induced by rapid ventricular pacing at 240 beats/min. The dogs were chronically instrumented for measurements of left ventricular (LV) pressure and first derivative of LV pressure (dP/dt), arterial pressure, LV internal diameter, and wall thickness. Milrinone (10 micrograms . kg-1. min-1 iv) increased LV dP/dt by 1,854 +/- 157 from 2,701 +/- 105 mmHg/s (P < 0.05) before HF. After HF the increase in LV dP/dt in response to milrinone was attenuated significantly (P < 0.05); it increased by 615 +/- 67 from 1,550 +/- 107 mmHg/s, indicating marked desensitization. In the presence of ganglionic blockade the increases in LV dP/dt (+445 +/- 65 mmHg/s) in response to milrinone were markedly less (P < 0.01), and milrinone increased LV dP/dt even less in HF (+240 +/- 65 mmHg/s). cAMP and PDE activity were measured in endocardial and epicardial layers in normal and failing myocardium. cAMP was decreased significantly (P < 0.05) in LV endocardium (-26%) but not significantly in LV epicardium (-14%). PDE activity was also decreased significantly (P < 0.05) in LV endocardium (-18%) but not in LV epicardium (-4%). Thus significant desensitization to milrinone was observed in conscious dogs with HF. The major effect was autonomically mediated. The biochemical mechanism appears to be due in part to the modest reductions in PDE activity in failing myocardium, which, in turn, may be a compensatory mechanism to maintain cAMP levels in HF. Reductions in cAMP and PDE levels were restricted to the subendocardium, suggesting that the increased wall stress and reduced coronary reserve play a role in mediating these changes.

Differential regulation of inotropy and lusitropy in overexpressed Gsalpha myocytes through cAMP and Ca2+ channel pathways

We investigated the mechanisms responsible for altered contractile and relaxation function in overexpressed Gsalpha myocytes. Although baseline contractile function (percent contraction) in Gsalpha mice was similar to that of wild-type (WT) mice, left ventricular myocyte contraction, fura-2 Ca2+transients, and Ca2+ channel currents (ICa) were greater in Gsalpha mice in response to 10(-8) M isoproterenol (ISO) compared with WT mice. The late phase of relaxation of the isolated myocytes and fura-2 Ca2+ transients was accelerated at baseline in Gsalpha but did not increase further with ISO. In vivo measurements using echocardiography also demonstrated enhanced relaxation at baseline in Gsalpha mice. Forskolin and CaCl2 increased contraction similarly in WT and Gsalpha mice. Rp-cAMP, an inhibitor of protein kinase, blocked the increases in contractile response and Ca2+ currents to ISO in WT and to forskolin in both WT and Gsalpha. It also blocked the accelerated relaxation in Gsalpha at baseline but not the contractile response to ISO in Gsalpha myocytes. Baseline measurements of cAMP and phospholambation phosphorylation were enhanced in Gsalpha compared with WT. These data indicate that overexpression of Gsalpha accelerates relaxation at end diastolic but does not affect baseline systolic function in isolated myocytes. However, the enhanced responses to sympathetic stimulation partly reflect increased Ca2+ channel activity; i.e the cellular mechanisms mediating these effects appear to involve a cAMP-independent as well as a cAMP-dependent pathway.

Lack of desensitization and enhanced efficiency of calcium channel promoter in conscious dogs with heart failure

The goal of this study was to compare responses to a calcium promoter, BAY y 5959, and dobutamine (Dob) in heart failure (HF). Dogs (n = 9) were chronically instrumented and studied in the conscious state before and after pacing-induced HF. In the control state, BAY y 5959 (20 microgram. kg-1. min-1) increased the first derivative of left ventricular (LV) pressure (dP/dt) by 83 +/- 8% and mean arterial pressure (MAP) by 8 +/- 2% and decreased heart rate (HR) by 30 +/- 3%. With Dob (10 microgram. kg-1. min-1) LV dP/dt rose similarly (+80 +/- 6%), but HR also rose (+25 +/- 4%) (P < 0.05 vs. BAY y 5959). After HF developed, BAY y 5959 still increased LV dP/dt by 108 +/- 8% and MAP by 21 +/- 2% and decreased HR by 28 +/- 4%, whereas Dob increased LV dP/dt by only 50 +/- 7% (P < 0.05 vs. BAY y 5959) and MAP by 7 +/- 3%, and HR did not change (+3 +/- 3%) (P < 0.05 vs. BAY y 5959). In HF, cardiac work increased more (P < 0. 05) with BAY y 5959 (+105 +/- 13%) compared with Dob (+47 +/- 11%), yet myocardial oxygen consumption increased similarly with the two drugs. Accordingly, mechanical efficiency increased more (P < 0.05) with BAY y 5959 (+73 +/- 14%) than with Dob (+17 +/- 12%). These data indicate that 1) increases in contractility mediated directly by Ca2+ are relatively resistant to desensitization in HF; and 2) the calcium-channel promoter can produce increases in myocardial contractility and cardiac work similar to those of Dob at a significantly lower oxygen cost, thereby enhancing mechanical efficiency in HF.

Overexpression of myocardial Gsalpha prevents full expression of catecholamine desensitization despite increased beta-adrenergic receptor kinase

Inotropic and chronotropic responses to catecholamines in young adult transgenic mice overexpressing myocardial Gsalpha are enhanced. One might predict that over the life of the animal, this chronically enhanced beta-adrenergic receptor stimulation would result in homologous catecholamine desensitization. To test this hypothesis, old transgenic Gsalpha mice and age-matched controls were studied physiologically in terms of responsiveness of left ventricular function (ejection fraction) to isoproterenol in vivo and in vitro in terms of beta-adrenergic receptor signaling. Old transgenic mice still responded to isoproterenol with augmented (P < 0.05) left ventricular ejection fraction (+44+/-3%) compared with age-matched controls (+24+/-1%). Although total beta-adrenergic receptor density was reduced in the old transgenic mice, and G protein receptor kinase 2 (beta-adrenergic receptor kinase) levels were increased, the fraction of receptors binding agonist with high affinity as well as isoproterenol- and G protein-stimulated adenylyl cyclase activities were enhanced. Thus, classical catecholamine desensitization is not effective in attenuation of persistently enhanced responses to sympathetic stimulation in mice overexpressing myocardial Gsalpha. To support this conclusion further, experiments were performed with chronic isoproterenol, which elicited effective desensitization in wild-type controls, but failed to elicit desensitization in overexpressed Gsalpha mice. The results of this study suggest that the lack of protective desensitization mechanisms may be responsible in part for the dilated cardiomyopathy which develops with chronic sympathetic stress over the life of these animals.

Depressed heart rate variability and arterial baroreflex in conscious transgenic mice with overexpression of cardiac Gsalpha

Recently, we developed a transgenic mouse with cardiac-specific Gsalpha overexpression (TG mouse), which exhibits enhanced postsynaptic beta-adrenergic receptor signaling, ultimately developing a cardiomyopathy. The goal of the present study was to determine whether cardiac Gsalpha overexpression alters autonomic cardiovascular control, which could shed light on the mechanism responsible for the later development of cardiomyopathy. Mean arterial pressure was increased (P<.05) in conscious, chronically instrumented TG mice (123+/-1 mm Hg) compared with age-matched wild-type (WT) control mice (103+/-1 mm Hg). Respiratory frequency was increased (P<.05) in TG mice (269+/-26/min) compared with WT mice (210+/-20/min). By use of telemetric techniques, baseline heart rate (HR) was elevated (P<.05) in conscious, untethered TG mice (696+/-13 bpm) compared with WT mice (568+/-28 bpm). Intrinsic HR, after propranolol and atropine or after ganglionic blockade with hexamethonium, was not different between TG and WT mice. Both the normal minute-to-minute and circadian variations of HR observed in WT mice were markedly blunted in TG mice. HR variability was assessed by the time-domain and frequency-domain methods. At baseline, time-domain analysis indices were reduced (P<.05) in TG mice compared with WT mice. Although the low frequency (LF) component was higher (P<.05) than the high frequency (HF) component in WT mice, the LF component was less (P<.05) than the HF component in TG mice. In addition, arterial baroreflex regulation of HR was markedly blunted in TG mice in response to both nitroglycerin-induced hypotension and phenylephrine-induced hypertension. The reduced LF/HF ratio in TG mice was surprising in view of enhanced beta-adrenergic signaling and may be due to reduced neural tone secondary to the elevated arterial pressure or alterations in arterial baroreflex control. Dobutamine infusion in WT mice also resulted in depressed HR variability. The combination of elevated baseline HR, arterial pressure, and respiratory frequency suggests that enhanced beta-adrenergic signaling in TG mice results in reduced HR variability, in terms of both minute-to-minute variability and the lack of circadian variations in HR. The lack of normal HR variability in general and the failure of HR to decline, even during sleep, may actually be critical mechanisms contributing to the ultimate development of cardiomyopathy in these animals.

Effects of chronic beta-adrenergic receptor stimulation in mice

The goal of the present study was to determine the effects of chronic beta-adrenergic receptor stimulation with isoproterenol (ISO) on cardiac tissue, systemic trophic changes and on beta-adrenergic receptor desensitization in mice. Mice (n=36) received continuous ISO (30 microg/g/day) via osmotic minipump for 13 days. Left ventricle (LV)/body weight (BW) ratio was increased by 27% in ISO v control (CON) mice. The extent of cardiac hypertrophy induced by chronic ISO was offset in part by concomitant increases in body weight, which were greater in ISO than CON mice (22 v 8%), and occurred with increases in both muscle mass and brown fat to BW ratios. Histological analysis of mice revealed a three-fold increase in subendocardial interstitial connective tissue with no evidence of acute cellular necrosis or chronic inflammation. Acute i.v. ISO challenges induced dose-dependent increases in LV fractional shortening (FS) and ejection fraction (EF) using echocardiography (9 MHz), which were attenuated after chronic ISO, i.e. physiological desensitization was observed. Cellular mechanisms of beta-adrenergic receptor desensitization included decreases in beta-adrenergic receptor density (-49%) and decreased basal (-45%) and ISO-stimulated (-61%) adenylyl cyclase activities. Lesser decreases in forskolin-stimulated adenylyl cyclase activity (-16%) and adenylyl cyclase mRNA levels for both type V (-17%) and type VI (-23%) isoforms were observed following chronic ISO. Thus, chronic ISO (30 microg/g/day) induced cardiac hypertrophy without cellular necrosis, increased weight gain and clear physiological desensitization in mice, with more extensive biochemical mechanisms than expected from simple catecholamine-specific (homologous) desensitization.

Effects of renin inhibition compared to angiotensin converting enzyme inhibition in conscious dogs with pacing-induced heart failure

OBJECTIVE

To compare the effects of angiotensin converting enzyme inhibition (ACEI) (captopril 1 mg/kg i.v.) to direct renin inhibition (CP80794 3 mg/kg i.v.) on left ventricular and systemic hemodynamics and peripheral blood flows in advanced congestive heart failure (CHF).

METHODS

Conscious chronically instrumented dogs (n = 14) were treated with captopril, 1 mg/kg, i.v., or CP80794, 3 mg/kg, i.v., before and after development of advanced CHF induced by 4-7 weeks of rapid ventricular pacing. After advanced CHF, comparisons between the inhibitors were made at equihypotensive doses.

RESULTS

In advanced CHF, both agents caused comparable reductions in mean arterial pressure (MAP) (-22% from 79 +/- 4 mmHg) and comparable increases (P < 0.01) in cardiac output (CP80794, 1.4 +/- 0.3 to 1.8 +/- 0.1 l/min; captopril, 1.4 +/- 0.1 to 1.9 +/- 0.1 l/min). Neither agent had a significant effect on LV contractility. In contrast, CP80794 caused a greater (P < 0.05) increase in renal blood flow (66 +/- 6% from 64 +/- 5 ml/min) compared to captopril (33 +/- 4% from 66 +/- 7 ml/min).

CONCLUSIONS

Renin inhibition with CP80794 and ACEI with captopril caused comparable hemodynamic effects in advanced CHF. However, CP80794 caused significantly greater increases in renal blood flow and suppressed renin activity to a greater degree than captopril.

Effects of cardiac denervation on development of heart failure and catecholamine desensitization

BACKGROUND

Two signatures of heart failure are activation of the sympathetic nervous system and catecholamine desensitization. However, whether or not the elimination of cardiac nerves affects either the progression of heart failure or catecholamine desensitization is not clear.

METHODS AND RESULTS

We studied 8 dogs with selective ventricular denervation (VD) (surgical technique) and 10 intact dogs, chronically instrumented for measurement of left ventricular (LV) and arterial pressures, LV dP/dt, LV internal diameter, and wall thickness before and after heart failure was induced by rapid pacing (240 bpm) for 3 to 4 weeks. VD was confirmed by the absence of reflex effects induced by intracardiac veratrine and depletion of tissue norepinephrine and by supersensitive responses to norepinephrine. During the development of heart failure, LV end-systolic and end-diastolic stresses and heart rate increased, while myocardial contractility, as reflected by LV dP/dt and mean velocity of circumferential fiber shortening corrected for heart rate (Vcf(c)), decreased in both intact and VD dogs. However, the increases in LV end-diastolic stress and decreases in LV dP/dt as well as the relationship between LV systolic stress and Vcf(c) in heart failure were less (P<.05) in VD dogs. The responses of LV dP/dt and heart rate to both isoproterenol and norepinephrine in intact dogs were reduced in heart failure. The physiological desensitization to the inotropic effects of isoproterenol and norepinephrine was less in dogs with VD (P<.05), but chronotropic responses were similar because atrial innervation remained intact. Plasma norepinephrine levels were not different in VD dogs (592+/-79 pg/mL) compared with intact dogs (576+/-81 pg/mL) in heart failure.

CONCLUSIONS

Dogs with selective VD tolerated the development of heart failure better than intact dogs and demonstrated significantly less catecholamine desensitization. The latter indicates that intact ventricular innervation is required for physiological expression of catecholamine desensitization despite comparable elevation of plasma catecholamines during the development of heart failure.

Cardiomyopathy induced by cardiac Gs alpha overexpression

The goal of this study was to determine whether chronic endogenous sympathetic stimulation resulting from the overexpression of cardiac stimulatory G protein alpha subunit (Gs alpha) in transgenic mice (15.3 +/- 0.1 mo old) resulted in a clinical picture of cardiomyopathy. The left ventricular ejection fraction, measured by echocardiography, was reduced in older mice with Gs alpha overexpression (50.4 +/- 5.4%) compared with age-matched control mice (70.9 +/- 1.6%; P < 0.05). When ejection fractions were compared at similar heart rates, the Gs alpha mice exhibited a greater left ventricular end-diastolic dimension than control mice (4.3 +/- 0.2 vs. 3.7 +/- 0.1 mm; P < 0.05). Baseline heart rates were elevated in conscious Gs alpha mice (722 +/- 27 beats/min; n = 5) compared with control mice (656 +/- 28 beats/min; n = 5). Moreover, electrocardiographic monitoring demonstrated a high incidence of arrhythmias. Increased mortality compared with control mice (31.6 vs. 3.0%; P < 0.01) was also observed. Thus older mice with Gs alpha overexpression exhibit many of the features of dilated cardiomyopathy. This study supports the concept that chronic sympathetic stimulation over an extended period of time, i.e., over the life of an animal, is deleterious and actually may result in cardiomyopathy.

Neurally mediated cardiac effects of forskolin in conscious dogs

Because major cardiovascular disease states are characterized by defects in adenylyl cyclase regulation, it becomes important to understand the mechanisms by which adenylyl cyclase activators affect inotropy and chronotropy in intact conscious animals. Accordingly, we examined the inotropic and chronotropic responses to forskolin in 11 normal conscious, chronically instrumented dogs and 3 dogs with ventricular denervation (VD). Left ventricular first derivative of pressure (LV dP/dt) increased by 96 +/- 7%, P < 0.05, in response to forskolin (50 nmol.kg-1.min-1) in normal dogs and by significantly less, 52 +/- 14%, in VD dogs. Circulating norepinephrine (NE) levels increased similarly in both groups (from 226 +/- 18 to 389 +/- 33 pg/ml in normal dogs, from 177 +/- 23 to 329 +/- 71 pg/ml in VD dogs). In the presence of ganglionic blockade, the increase in LV dP/dt in response to forskolin was reduced (+62 +/- 4%) in normal dogs but was unchanged in VD dogs (+52 +/- 12%). Ganglionic blockade abolished the increase in circulating NE levels in both groups. Increases in heart rate in the presence of ganglionic blockade (+54 +/- 6 beats/min) were less than in the presence of atropine alone (+92 +/- 10 beats/min). Notably, the LV dP/dt and heart rate responses to forskolin were further attenuated by beta-adrenergic receptor blockade in the presence and absence of ganglionic blockade. Morphine also attenuated the increases in both LV dP/dt and plasma NE in response to forskolin. Increases in LV dP/dt in response to NKH-477 (30 micrograms/kg), a water-soluble forskolin derivative, were similar before and after ganglionic blockade (+63 +/- 8 and +51 +/- 10%, respectively). However, in vitro experiments in LV sarcolemmal membrane preparations demonstrated that stimulation of adenylyl cyclase by forskolin and NKH-477 was not affected by beta-adrenergic receptor blockade. These results indicate that in conscious dogs, inotropic and chronotropic effects of forskolin are not only due to direct activation of adenylyl cyclase, but the effects also are mediated by neural mechanisms and potentiated by the prevailing level of sympathetic tone.

Beta-adrenoceptor desensitization during the development of canine pacing-induced heart failure

1. The goal of this review is to emphasize four major points regarding the development of catecholamine desensitization in heart failure (HF). 2. Catecholamine desensitization occurs prior to the development of HF (i.e. after 1 day of rapid pacing, physiological responses to beta-adrenoceptor stimulation are depressed by over 50%, yet no evidence of HF is observed for 3-4 weeks of rapid pacing). 3. Multiple mechanisms in the beta-adrenoceptor cascade are involved. In HF there are decreases in beta 1-adrenoceptors, high affinity beta-adrenoceptors, adenylyl cyclase activity and messenger RNA and increases in Gi. 4. Not all mechanisms appear simultaneously (i.e. early decreases occur in high affinity beta-adrenoceptors and adenylyl cyclase; late increases in Gi and decreases in beta-adrenoceptor density evolves). 5. Mechanisms distal to cAMP generation also play a role (i.e. alterations in ryanodine receptor binding and excitation-contraction coupling also occur).

Mechanism of the systemic, left ventricular, and coronary vascular tolerance to a binge of cocaine in conscious dogs

BACKGROUND

Prior experimental studies have emphasized the cardiovascular effects of acute, single doses of cocaine. However, cardiovascular complications are most often reported in chronic users, who have been exposed to repetitive doses of cocaine. It remains unclear whether there is tolerance or sensitization to the systemic, left ventricular, and coronary hemodynamic effects of a binge of cocaine.

METHODS AND RESULTS

We studied 11 conscious, chronically instrumented dogs to determine the systemic pressor, inotropic, chronotropic, and coronary vascular resistance responses to cocaine (1 mg/kg IV) administered every 25 minutes for five doses. There was progressive tolerance to the systemic pressor (mean arterial pressure: first dose, +42 +/- 4% from 97 +/- 2 mm Hg; fifth dose, +8 +/- 3% from 116 +/- 7 mm Hg; P < .01) and heart rate (first dose, +45 +/- 8% from 93 +/- 5 bpm; fifth dose, +8 +/- 2% from 109 +/- 9 bpm; P < .01) responses and abolition of the positive inotropic (left ventricular dP/dt: first dose, +19 +/- 4% from 2824 +/- 75 mm Hg/s; fifth dose, -3 +/- 5% from 2531 +/- 436 mm Hg/s; P < .01) and coronary vasoconstrictor (coronary vascular resistance: first dose, +38 +/- 9% from 1.9 mm Hg.mL-1.min-1; fifth dose, -7 +/- 2% from 2.6 +/- 0.2 mm Hg. mL-1.min-1; P < .01) responses to a binge of cocaine despite progressive increases in peak plasma cocaine concentrations. In contrast, both the plasma norepinephrine and epinephrine responses were attenuated with repetitive exposure to cocaine, whereas myocardial alpha and beta-adrenergic responsiveness was maintained.

CONCLUSIONS

Repetitive cocaine administration is associated with the development of early and progressive tolerance to systemic, left ventricular, and coronary vascular effects of cocaine. The mechanism of the tolerance involves neither impaired myocardial nor coronary vascular responsiveness to adrenergic stimulation but, rather, attenuated catecholamine responses to repetitive cocaine administration.

Adverse effects of chronic endogenous sympathetic drive induced by cardiac GS alpha overexpression

To study the physiological effect of the overexpression of myocardial Gsalpha (protein levels increased by approximately threefold in transgenic mice), we examined the responsiveness to sympathomimetic amines by echocardiography (9 MHz) in five transgenic mice and five control mice (both 10.3 +/- 0.2 months old). Myocardial contractility in transgenic mice, as assessed by left ventricular (LV) fractional shortening (LVFS) and LV ejection fraction (LVEF) was not different from that of control mice at baseline (LVFS, 40 +/- 3% versus 36 +/- 2%; LVEF, 78 +/- 3% versus 74 +/- 3%). LVFS and LVEF values in transgenic mice during isoproterenol (ISO, 0.02 micrograms/kg per minute) infusion were higher than the values in control mice (LVFS, 68 +/- 4% versus 48 +/- 3%; LVEF, 96 +/- 1% versus 86 +/- 3%; P < .05). Norepinephrine (NE, 0.2 micrograms/kg per minute) infusion also increased LVFS and LVEF in transgenic mice more than in control mice (LVFS, 59 +/- 4% versus 47 +/- 3%; LVEF, 93 +/- 2% versus 85 +/- 3%; P < .05). Heart rates of transgenic mice were higher than those of control mice during ISO and NE infusion. In three transgenic mice with heart rates held constant, LV dP/dt rose by 33 +/- 2% with ISO (0.02 micrograms/kg per minute) and by only 13 +/- 2% in three wild-type control mice (P < .01). NE (0.1 micrograms/kg per minute) also induced a greater effect on LV dP/dt in the three transgenic mice with heart rates held constant compared with three wild-type control mice (65 +/ 8% versus 28 +/- 4%, P < .05). Pathological and histological analyses of older transgenic mouse hearts (16.0 +/- 0.8 months old) revealed hypertrophy, degeneration, atrophy of cells, and replacement fibrosis reflected by significant increases in collagen volume in the subendocardium (5.2 +/- 1.4% versus 1.2 +/- 0.3%, P < .05) and in the cross-sectional area of myocytes (298 +/- 29 versus 187 +/- 12 micron2, P < .05) compared with control mouse hearts. These results suggest that Gsalpha overexpression enhances the efficacy of the beta-adrenergic receptor-Gs-adenylyl cyclase signaling pathway. This in turn leads to augmented inotropic and chronotropic responses to endogenous sympathetic stimulation. This action over the life of the animal results in myocardial damage characterized by cellular degeneration, necrosis, and replacement fibrosis, with the remaining cells undergoing compensatory hypertrophy. As a model, this transgenic mouse offers new insights into the mechanisms of cardiomyopathy and heart failure and provides a new tool for their study.

Splenic contraction-induced increases in arterial O2 reduce requirement for CBF in conscious dogs

We investigated the extent to which sympathomimetic amines induced splenic contraction and associated increases in arterial O2 content (CaO2) and how these mechanisms affected control of the coronary circulation by sympathomimetic amines in conscious dogs. Blood hemoglobin (Hb) and CaO2 increased by 16 +/- 2 and 18 +/- 2%, respectively, during norepinephrine (NE, 0.8 micrograms.kg-1.min-1 iv) in the intact, conscious state after splenic contraction. Phenylephrine (PE) induced similar effects. After either alpha 1-adrenergic-receptor blockade or splenectomy, these effects were abolished. Isoproterenol (Iso) also decreased splenic thickness, which was abolished after ganglionic, alpha-, or beta 1/beta 2-adrenergic-receptor blockade. Direct infusions of NE and PE into the splenic artery decreased splenic thickness and increased Hb and CaO2, whereas Iso had no effect. After splenectomy, NE did not increase CaO2, but coronary blood flow (CBF) increased more (73 +/- 6%) vs. before splenectomy (49 +/- 7%) without any differences before and after splenectomy in the responses of pressures, contractility, and myocardial O2 consumption (MVO2). In contrast, renal, mesenteric, and iliac artery blood flows were not significantly different in response to sympathomimetic amines before and after splenectomy. These data indicate that sympathomimetic amines induced splenic contraction either directly or reflexly via alpha-adrenergic-receptor stimulation. The consequent increase in Hb and CaO2 allows for equivalent increases in MVO2, but at a smaller increase in CBF.

Role of blood doping in the coronary vasoconstrictor response to cocaine

BACKGROUND

The mechanism by which cocaine induces myocardial ischemia remains controversial. Most prior studies have postulated that cocaine-induced coronary vasoconstriction limits myocardial oxygen delivery during times of increased myocardial oxygen demand.

METHODS AND RESULTS

To determine the contribution of altered myocardial metabolic demands to the coronary vasoconstrictor effects of intravenous cocaine (COC 1 mg/kg), we studied 13 conscious, chronically instrumented dogs in the intact state and with heart rate held constant with atrial pacing in the presence and absence of beta-adrenergic blockade with propranolol (2 mg/kg) to limit the inotropic and chronotropic effects of cocaine on associated increases in myocardial oxygen consumption. In the intact state, COC caused a prompt increase in coronary blood flow (+30 +/- 3%, P < .01) that returned rapidly to baseline within 10 minutes, whereas coronary vascular resistance did not increase significantly (+17 +/- 6%, P < .05) until 15 minutes after COC. Notably, myocardial oxygen consumption increased (+57 +/- 4%, P < .01) to a greater extent than oxygen delivery (+42 +/- 3%, P < .01) during the first 2.5 minutes, requiring increased oxygen extraction (from 75 +/- 1% to 80 +/- 1%, P < .01), although only transiently. Thereafter, enhanced oxygen delivery matched the required oxygen consumption without further need to extract additional oxygen. Surprisingly, the enhanced oxygen delivery associated with COC in conscious dogs did not depend on persistent increases in coronary blood flow but rather was due to enhanced arterial oxygen content (+22 +/- 4%, P < .01) as a result of a significant "blood doping" effect with associated increases in circulating hemoglobin from 12.1 +/- 0.4 to 14.2 +/- 0.6 g/dL (P < .01), which persisted for 60 minutes.

CONCLUSIONS

The myocardial oxygen requirements associated with COC administration have a significant impact on both the magnitude and the mechanism of the coronary vasoconstrictor effects of COC in conscious dogs. Furthermore, the enhanced myocardial oxygen delivery associated with COC administration is not dependent solely on coronary blood flow responses but is due to a significant "blood doping" effect associated with COC.

Hypotensive response to atrial natriuretic peptide administration is enhanced with age

BACKGROUND

Plasma levels of atrial natriuretic peptide (ANP) increase with age. To test the hypothesis that the cardiovascular response to ANP is age dependent and to examine the role of the increased levels of ANP in the hypotensive response to orthostatic challenge, we compared the hemodynamic response of young and elderly subjects to ANP infusion and orthostatic challenge.

METHODS

Blood pressure, heart rate, forearm cutaneous resistance, plasma ANP, and plasma norepinephrine were measured in the supine position and following upright tilt before and after a 60-minute infusion of atrial natriuretic peptide (0.05 microgram/kg/min) in 7 young (27 +/- 4 years) and 5 elderly (74 +/- 4 years) normotensive, healthy subjects.

RESULTS

Prior to ANP infusion, the response to upright tilt was similar in both groups. Infusion of ANP produced similar steady state plasma levels of ANP in both groups (young: 435 +/- 49 pg/ml; elderly: 429 +/- 32 pg/ml). Supine systolic blood pressure decreased by 4 +/- 2 mmHg in the young subjects and by 18 +/- 8 mmHg in the elderly subjects after infusion (p < .08). In contrast, changes in supine heart rate, forearm cutaneous resistance, and plasma norepinephrine were similar in both groups (delta heart rate: young +5 +/- 3 beats/min, elderly +4 +/- 2 beats/min; delta forearm cutaneous resistance: young -38 +/- 9%, elderly -40 +/- 6%; delta norepinephrine: young +55 +/- 11%, elderly: +43 +/- 13%). ANP infusion abolished the vasoconstrictor response normally associated with orthostatic challenge in both groups, despite a significant release of catecholemines and an enhanced heart rate response. This resulted in significant systolic blood pressure reduction in both young (-7 +/- 2 mmHg, p < .05) and elderly subjects (-16 +/- 4 mmHg, p < .05). The drop in systolic blood pressure in response to upright tilt and ANP infusion was four times larger in the elderly subjects (change from pre-ANP level: young -8 +/- 3 mmHg, elderly -32 +/- 5 mmHg, p < .005).

Intravenous cocaine induces platelet activation in the conscious dog

BACKGROUND

Cocaine consumption has been associated with thrombosis of coronary and peripheral arteries. Since cocaine has been found to induce platelet activation in vitro, we sought to establish whether cocaine induced platelet activation in vivo.

METHODS AND RESULTS

Chronically instrumented, conscious dogs were infused with cocaine (1 mg/kg), norepinephrine (0.2 to 0.4 mg/kg), or saline intravenously over 1 minute. Activated canine platelets were identified in whole blood collected from an indwelling aortic catheter by flow cytometric detection of the binding of a monoclonal antibody directed against the activation-dependent antigen P-selectin. Infusion of cocaine resulted in an elevation of mean arterial pressure (91 +/- 3 to 128 +/- 7 mm Hg [P < .01]) and heart rate (87 +/- 9 to 125 +/- 11 beats per minute [P < .01]). A similar change (P = NS) in mean arterial pressure followed norepinephrine infusion (100 +/- 5 to 137 +/- 13 mm Hg [P < .04]), whereas saline infusion had no effect. Cocaine resulted in a substantial but delayed increase in platelet P-selectin expression (14 +/- 7% [P < .08], 31 +/- 12% [P < .04], and 55 +/- 22% [P < .04] at 17, 22, and 27 minutes after drug infusion, respectively). The magnitude of this increase was similar to that found in blood treated ex vivo with the agonists ADP or PAF (23 +/- 7% and 53 +/- 15%, respectively). No significant increase in P-selectin expression was detected in the blood of animals that received norepinephrine or saline. Serum cocaine concentrations were highest immediately after infusion (538 +/- 55 ng/mL at 2 minutes) but declined rapidly (185 +/- 22 and 110 +/- 25 ng/mL at 17 and 32 minutes after infusion); in contrast, the increase in benzoylecgonine concentrations was delayed (from < 25 ng/mL in all but one animal [34 ng/mL] at 2 minutes to 46 +/- 4 and 71 +/- 11 ng/mL at 17 and 32 minutes, respectively, after infusion).

CONCLUSIONS

Intravenous cocaine induces activation of individual circulating platelets; this effect is not reproduced by infusion of norepinephrine at doses sufficient to exert similar hemodynamic effects. The delay in detection of activated platelets after treatment with cocaine may result from the adhesion and subsequent detachment of activated platelets; alternatively, cocaine metabolites, rather than the drug itself, may induce platelet activation.

Increased susceptibility to thiazide-induced hyponatremia in the elderly

Hyponatremia is a common cause of morbidity in the elderly, and thiazide diuretics are often implicated. Eleven healthy young volunteers, eight healthy old volunteers, and five elderly patients with a history of thiazide-induced hyponatremia were studied to determine susceptibility to thiazide-induced hypoosmolality in age. Each of the healthy subjects ingested a water load (20 mL/kg) after 3 days of hydrochlorothiazide (HCTZ) (100 mg/day) or placebo. Although there were no differences in minimum Uosm between young and old, the healthy old had lower hourly free water clearances (CH2O) as compared with the young and a greater decline in serum osmolality in response to water loading (P < 0.05). HCTZ impaired minimum urine osmolality and CH2O and delayed recovery of serum osmolality after the water load in both healthy young and old (P < 0.005, placebo versus HCTZ), but the impairment in the latter two parameters was greater in the healthy elderly (P < 0.05, young versus old). Vasopressin levels were not different between healthy young and old (1.9 +/- 0.3 versus 2.0 +/- 1.0 pm with placebo; 3.0 +/- 0.7 versus 4.4 +/- 1.0 with HCTZ). Five of the young subjects were restudied after the addition of ibuprofen (400 mg thrice daily) to the thiazide and placebo regimens. Creatinine clearance was not changed, but free water clearance and serum osmolality after water loading were significantly reduced to a degree similar to that seen in the elderly subjects on the thiazide regimen (P < 0.05), suggesting an important role for renal prostaglandins in the defense against hyponatremia.

Decrease in myocardial ryanodine receptors and altered excitation-contraction coupling early in the development of heart failure

BACKGROUND

Rapid ventricular pacing for 1 day reduced myocardial contractile function without inducing heart failure in conscious, chronically instrumented dogs. After 4 to 7 weeks of pacing, myocardial contractility was depressed further and overt signs of congestive heart failure, eg, ascites, dyspnea, and edema, were evident.

METHODS AND RESULTS

The mechanical restitution response, a physiological index of calcium release, was depressed at 1 day of rapid ventricular pacing. Postextrasystolic potentiation was also depressed by a similar amount, 14 +/- 3%, at 1 day after pacing. The response to isoproterenol 0.2 microgram/kg per minute was depressed by a significantly greater amount (P < .05), 52 +/- 7%, at 1 day after pacing. 3H-ryanodine receptor binding fell from 1013 +/- 25 to 808 +/- 42 fmol/mg after 1 day of pacing and remained depressed at similar levels (782 +/- 61 fmol/mg) at 4 to 7 weeks when heart failure was manifest. Ryanodine receptor affinity was unchanged from control values. Neither dihydropyridine binding nor affinity for 3H-PN200-110 was changed from control levels. Within 5 days after recovery from 1 day of pacing, physiological responses to isoproterenol, postextrasystolic potentiation, and mechanical restitution recovered, as did 3H-ryanodine binding density.

CONCLUSIONS

These findings suggest that the changes in excitation-contraction coupling and potentially the sarcoplasmic reticulum calcium release channel occur early in the development of heart failure and therefore may be important in the pathogenesis of the contractile abnormalities in this disease state.

Left ventricular systolic dysfunction precedes diastolic dysfunction during myocardial ischemia in conscious dogs

We studied the initial effects of regional and global left ventricular (LV) ischemia induced by left circumflex and left main coronary artery occlusion (CAO), respectively, on indexes of systolic and diastolic LV function in conscious dogs to determine whether diastolic abnormalities precede systolic dysfunction or vice versa during the onset of either regional or global myocardial ischemia. With regional myocardial ischemia, within four beats after left circumflex CAO, there was a significant decrease in end-systolic wall thickness in the ischemic zone followed by significantly enhanced postsystolic wall thickening in the nonischemic zone at beat 6. Both peak negative first derivative of left ventricular pressure (LV dP/dt) and the isovolumic relaxation half-time (T 1/2) were prolonged, but later (i.e., by the 9th beat). During sustained CAO T1/2 was normalized shortly after postsystolic thickening in the nonischemic zone had disappeared despite persistent regional systolic asynchrony and shortened ejection time. Thus postsystolic thickening in the nonischemic zone played a major role in the early, transient changes in isovolumic relaxation after acute induction of regional ischemia. With global myocardial ischemia, induced by left main coronary occlusion, indexes of systolic function (e.g., LV dP/dt, ejection fraction, and velocity of circumferential endocardial fiber shortening) were also depressed significantly before (by 5-15 beats) indexes of LV diastolic function [e.g., time constant of isovolumic relaxation and LV myocardial and chamber stiffness (by 35-45 beats)]. Similar results were observed in the presence of autonomic blockade, when heart rate did not change with CAO. Thus, during the induction of either acute regional or acute global LV ischemia in conscious dogs, LV systolic dysfunction occurs before diastolic dysfunction.

Factors involved in delaying the rise in peripheral resistance in developing heart failure

The development of heart failure (HF) on peripheral vascular control was studied in 10 conscious dogs with measurements of cardiac output (CO) and left ventricular (LV), arterial, and right atrial pressures. At 3 wk after pacing-induced HF, CO was not decreased from 2.5 +/- 0.2 l/min, whereas LV dP/dt fell (from 2,858 +/- 71 to 1,409 +/- 69 mmHg/s) and LV end-diastolic pressure increased (from 4.8 +/- 0.4 to 27.3 +/- 1.1 mmHg) (P < 0.05). At 4-7 wk after pacing, CO was significantly decreased (to 1.6 +/- 0.1 l/min; P < 0.05), but total peripheral resistance (TPR) did not rise, despite increases in plasma norepinephrine and renin activity (P < 0.05). In the presence of ganglionic blockade, TPR was still not increased in HF. In vitro studies in isolated femoral artery segments demonstrated reduced intrinsic tone (0.028 +/- 0.007 g/mg; P < 0.05) as compared with vessels from sham-operated controls (0.124 +/- 0.023 g/mg), whereas the intracellular calcium level was not altered in HF. Thus, during the development of HF, severe contractile dysfunction precedes the fall in CO, which, in turn, precedes the rise in TPR. The delayed rise in TPR appears to involve a reduction in intrinsic peripheral vascular tone, despite neurohumoral activation.

Excitation-contraction coupling in isolated myocardium from dogs with compensated left ventricular hypertrophy

To examine the relationship between left ventricular (LV) function and intracellular calcium modulation in the presence of myocyte hypertrophy, we compared LV muscles from nine dogs with compensated LV hypertrophy (LVH) induced by chronic aortic banding with 11 controls. Mechanical properties were studied in LV muscles (control, n = 25; LVH, n = 23) stretched to the length at which maximal isometric tension developed at 30 degrees C and stimulated at 0.33 Hz; a subset of LVH muscles was loaded with the intracellular calcium indicator aequorin. In LV myocardium from dogs with compensated LVH, both the contraction duration and calcium transients were prolonged at baseline, and the response to phosphodiesterase inhibitors was impaired in keeping with findings in both human and experimental models of pressure-overload hypertrophy and failure. However, in contrast to findings in the failing myocardium, the positive inotropic response to increasing intracellular calcium was preserved in myocardium from dogs with compensated LVH.

Downregulation of adenylylcyclase types V and VI mRNA levels in pacing-induced heart failure in dogs

We have shown that the heart expresses two distinct forms of adenylylcyclase mRNA, types V and VI. In this study we have characterized the expression of these two mRNA species in heart failure generated by overdrive pacing at a rate of 240 beats/min. After 4 wk, left ventricular end-diastolic pressure and heart rate increased significantly with the appearance of signs of heart failure, i.e., edema, ascites, and exercise intolerance. Basal as well as forskolin-stimulated adenylylcyclase activities decreased significantly, which was accompanied by a reduction in the steady state mRNA levels of adenylylcyclase types V and VI. These data suggest that in this model of cardiomyopathy, the downregulation of adenylylcyclase catalytic activity results, at least in part, from a reduction in the steady state levels of types V and VI adenylylcyclase mRNA levels.

Exercise induces cardiac dysfunction in both moderate, compensated and severe hypertrophy

BACKGROUND

Ventricular hypertrophy begins as a physiological adaptation to cardiac overload but progresses to a pathological state. We examined whether the extent of hypertrophy influenced the response to exercise in terms of its effects on regional and global ventricular function and transmural myocardial blood flow distribution.

METHODS AND RESULTS

Left ventricular (LV) hypertrophy was induced by aortic banding in puppies. The effects of treadmill exercise were compared in sham-operated control dogs (n = 7) and in dogs with moderate LV hypertrophy (47% increase in LV wt/body wt, n = 7) with normal baseline levels of LV systolic and diastolic wall stress and dogs with severe LV hypertrophy (85% increase in LV wt/body wt, n = 18), which exhibited elevated levels of LV systolic wall stress at baseline. The dogs with severe LV hypertrophy were further subdivided into those with either elevated or normal baseline levels of LV end-diastolic pressure and wall stress. The response to exercise in dogs with moderate LV hypertrophy was directionally similar to that of sham-operated control dogs for systemic hemodynamics and global and regional LV function, ie, full and subendocardial wall thickening rose, as did mean and diastolic arterial pressures, shortening fraction, and Vcf. The endocardial/epicardial blood flow ratio did not fall during exercise in these two groups. However, relations comparing either LV shortening, Vcf, or wall thickening with LV systolic wall stress during exercise demonstrated depressed myocardial function in the dogs with moderate LV hypertrophy. In contrast, in dogs with severe LV hypertrophy, exercise reduced LV shortening fraction, Vcf, mean and diastolic arterial pressures, and full and subendocardial wall thickening, and the endocardial/epicardial blood flow ratio fell to 0.73 +/- 0.07. There were no differences observed between the two subgroups with severe LV hypertrophy, but the global and regional wall function responses to exercise were more severely impaired than those in dogs with moderate LV hypertrophy.

CONCLUSIONS

Responses of global and regional LV function and transmural myocardial blood flow distribution to exercise were clearly abnormal in dogs with severe LV hypertrophy with elevated baseline levels of LV systolic wall stress whether or not baseline levels of LV end-diastolic wall stress were elevated. Thus, it required more severe LV hypertrophy as well as elevated levels of LV wall stress to elicit qualitatively abnormal regional and global hemodynamic responses to exercise. However, even with moderate LV hypertrophy, which was well compensated under baseline conditions, qualitatively impaired contraction-afterload relations were observed during the stress of exercise.

Effects of anaesthesia and recent surgery on diastolic function

OBJECTIVE

The aims were to determine the effects and the extent to which halothane anaesthesia affects diastolic function both immediately after and remote from surgery and to investigate whether the effect is due to alterations in loading conditions.

METHODS

Eight mongrel dogs were studied under halothane anaesthesia (0.5-1.5 end tidal vol%) with the chest closed, after acute instrumentation with left ventricular pressure transducers, left atrial and aortic catheters, and left ventricular diameter and wall thickness crystals. The same dogs were then studied in the fully conscious state, 2-3 weeks later. An additional four dogs were studied in the conscious state and then again under halothane anaesthesia remote from acute instrumentation. The left ventricular isovolumetric relaxation time constant, tau, as well as myocardial and chamber stiffness constants were used as indices of diastolic function.

RESULTS

Following halothane anaesthesia and recent surgery, tau was prolonged significantly compared to the conscious state, at 30(SEM 1) v 22(1) ms (p < 0.01), but there were no changes in either myocardial or chamber stiffness. While tau remained sensitive to increased heart rate and enhanced contractility and was prolonged by increasing afterload in both the anaesthetised and conscious states, it was consistently prolonged following halothane anaesthesia and recent surgery even at matched levels of contractile states, heart rates and loading conditions, compared to the conscious state, at 26(1) v 19(1) ms (p < 0.01). When the effects of halothane anaesthesia were examined after full recovery from surgery, tau was still prolonged under halothane anaesthesia, at 29(2) v 20(1) ms (p < 0.01), compared to the conscious state, but in contrast to the findings following halothane anaesthesia and recent surgery, it was fully normalised [19(1) v 19(1) ms] when contractile state and loading conditions were matched.

CONCLUSIONS

Left ventricular diastolic function is influenced markedly by halothane anaesthesia and recent surgery, and to a degree comparable to many pathological states. The effects of halothane anaesthesia and recent surgery appear to prolong the isovolumetric relaxation time constant independently of heart rate, contractility, and loading conditions and are most likely to be due to the combined direct effects of anaesthetics and acute instrumentation.

Effects of recombinant human IGF-I on glucose and leucine kinetics in men

To examine the effects of recombinant human (rh) insulin-like growth factor I (IGF-I), insulin, and saline on metabolic parameters, we studied 20 young nonobese healthy men. Euglycemic clamps with 240-min IGF-I infusions at two doses (49 and 33 pmol.kg-1 x min-1, n = 8 and 12 subjects) were performed and compared with hyperinsulinemic-euglycemic clamps (2.25 pmol.kg-1 x min-1, n = 9). Leucine and glucose kinetics were examined with L-[1-13C]leucine and [3-3H]glucose. Glucose rate of appearance (Ra) declined equivalently in the 49 pmol.kg-1.min-1 IGF-I and insulin clamps but remained at basal levels during the 33 pmol.kg-1 x min-1 IGF-I infusions. In contrast, Rd of glucose was increased by 176% in the 49 pmol.kg-1 x min-1 IGF-I and 78% in the 33 pmol.kg-1 x min-1 IGF-I infusions. Furthermore, to prevent hypoglycemia after the termination of both rhIGF-I infusions, it was necessary to infuse glucose for an additional 2-20 h. Ra of leucine was suppressed significantly by both IGF-I and insulin, whereas leucine oxidation was not affected by either hormone. Therefore, the rate of disappearance of leucine expressed as the difference between Ra and oxidation rates was significantly reduced in all clamps. In addition, IGF-I significantly suppressed beta-cell secretion without affecting the other glucoregulatory hormones. In contrast to insulin, IGF-I had no apparent effect on lipolysis, as measured by changes in nonesterified fatty acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Depressed beta-adrenergic receptor- and endothelium-mediated vasodilation in conscious dogs with heart failure

Peripheral vasodilator responsiveness was examined in pacing-induced heart failure (HF) in 11 conscious dogs chronically instrumented for measurement of systemic (total peripheral resistance [TPR]) and local (iliac blood flow) vascular resistance. Dose responses to isoproterenol (ISO), acetylcholine (ACh), and nitroglycerin (NTG) were examined in the same dogs before pacing (control) and after 4 to 7 weeks of rapid ventricular pacing, which induced congestive HF, characterized by increased left ventricular end-diastolic pressure (6.7 +/- 0.4 [control] versus 28 +/- 1.5 [HF] mm Hg) and decreased cardiac output (-30 +/- 5%) and left ventricular dP/dt (-53 +/- 3%), as well as ascites and peripheral edema. In the control state, TPR fell by 57 +/- 2% in response to ISO (100 ng/kg), by 61 +/- 3% in response to ACh (3 micrograms/kg), and by 55 +/- 2% in response to NTG (10 micrograms/kg). In HF, smaller decreases (P < .05) in TPR were observed with the same doses of ISO (-50 +/- 2%) and ACh (-49 +/- 2%) but not with NTG (-58 +/- 3%). Depressed responses to systemic ISO and ACh, but not NTG, were observed in HF in the presence of ganglionic blockade and also after local administration of smaller doses of the drugs in the absence of ganglionic blockade, but where systemic effects were not elicited. Inhibition of nitric oxide synthase increased TPR to a greater degree before HF (+154 +/- 28% [control]) than after (+80 +/- 22% [HF]) and eliminated the depressed responses to ACh but not to ISO. beta-Adrenergic receptor density, as determined by 125I-cyanopindolol binding in membrane preparations from mesenteric vessels was significantly decreased after HF (130 +/- 3 [control] versus 100 +/- 8 [HF] fmol/mg, P < .05) without any change in affinity. Thus, peripheral vascular beta-adrenergic receptor downregulation occurs in HF, independent of altered endothelium-mediated peripheral vasodilation.

Reduced 3H-ouabain binding site (Na,K-ATPase) concentration in ventricular myocardium of dogs with tachycardia induced heart failure

The present study evaluates 3H-ouabain binding site (Na,K-ATPase) concentration in left ventricular myocardium of dogs with heart failure induced by tachycardia as a result of ventricular pacing. Samples of left ventricle were obtained from 10 dogs exposed to pacing of 240 beats/min for 3 to 4 weeks and eight sham-operated controls. Na,K-ATPase was quantified using vanadate facilitated 3H-ouabain binding to intact samples. At time of sacrifice paced dogs showed clinical signs of heart failure, a significant 257% increase in left ventricular end diastolic pressure and a significant 46% decrease in left ventricular dP/dt compared with control. There was no significant change in left ventricular mass. 3H-ouabain binding concentration was significantly reduced by 16%. Evaluation of 3H-ouabain binding kinetics revealed no significant difference between myocardium from paced and control dogs: Equilibrium binding conditions were at the various concentrations used obtained after similar incubation time; nonspecific uptake and retention of 3H-ouabain was 0.9-0.8% of total uptake and retention obtained in the standard assay; apparent dissociation constant (KD) was 6.5 x 10(-8)-6.6 x 10(-8) mol/l; loss of specifically bound 3H-ouabain during washout at 0 degrees C occurred with a half-life time (T1/2) of 120 and 121 h. Hence, total 3H-ouabain binding site concentration in left ventricular myocardium was (mean +/- SEM) 1110 +/- 56 and 1317 +/- 68 pmol/g wet weight, 8.54 +/- 0.43 and 10.05 +/- 0.52 pmol/mg protein, and the total amount of 3H-ouabain binding sites in the entire left ventricle 121 +/- 6 and 162 +/- 8 nmol in paced (n = 10) and control (n = 8) dogs (p < 0.05), respectively. In conclusion, the present study reports a significant reduction in left ventricular myocardium 3H-ouabain binding site concentration in tachycardia induced heart failure. This observation supports the concept of a relationship between Na,K-ATPase concentration and contractile capacity and may be of pathophysiological importance in tachycardia and heart failure.

Exhaustion of Frank-Starling mechanism in conscious dogs with heart failure

The goal of this study was to elucidate the ability of the left ventricle to accommodate an increase in preload (Frank-Starling mechanism) in the presence of congestive heart failure (CHF) but in the absence of the complicating effects of hypertrophy and fibrosis. To accomplish this, the effects of volume loading were examined in eight conscious dogs during the control state and after 3 wk of right ventricular pacing (240 beats/min). CHF increased heart rate (by 16 +/- 5 from 92 +/- 5 beats/min), left ventricular (LV) end-diastolic pressure (by 17 +/- 2 from 10 +/- 1 mmHg), and LV end-diastolic volume (EDV; by 23 +/- 4 from 57 +/- 3 ml). Despite reduced LV ejection fraction (from 54 +/- 3 to 31 +/- 3%), there was no significant change in cardiac output (2.5 +/- 0.3 l/min) compared with control (2.7 +/- 0.2 l/min). Stroke volume was preserved (control 19 +/- 2 ml; CHF 18 +/- 2 ml) at a constant heart rate by a shift to the right in the relationship between LV stroke volume and EDV, indicating the importance of chronic ventricular dilatation in maintaining pump performance. In the control state, acute volume load increased LV EDV (by 17 +/- 2 ml) and stroke volume (by 11 +/- 2 ml), whereas in CHF it did not increase LV EDV or stroke volume. Scanning electron microscopy revealed areas of reduced collagen weave pattern surrounding myofibers. Myocyte cross-sectional area by transmission electron microscopy was significantly reduced, and there were multiple electron-dense expansions of the Z lines with disruption of the normal lateral sarcomere alignment. These morphological findings suggest that chronic ventricular dilatation utilized in CHF results from myocyte stretch and morphological intracellular rearrangement. Furthermore, the failing heart cannot further augment stroke volume by acutely increasing EDV in CHF, suggesting that the Frank-Starling reserve is essentially exhausted.

Impaired regional subendocardial coronary flow reserve in conscious dogs with pacing-induced heart failure

The purpose of the present investigation was to examine coronary flow and vasodilator reserve in conscious, chronically instrumented dogs with pacing-induced heart failure, a model of severe biventricular cardiomyopathy devoid of significant hypertrophy. Twelve dogs were studied after 28 days of rapid right ventricular pacing and were compared with six sham-operated controls. Left ventricular (LV) average transmural flow was significantly less in conscious dogs with pacing-induced heart failure, whereas the endocardial-to-epicardial flow ratio was not significantly different. In response to near maximal coronary vasodilation with adenosine, subepicardial coronary flow and vasodilator reserve were preserved in congestive heart failure (CHF), yet there was evidence of impaired subendocardial flow and vasodilator reserve, and the endocardial-to-epicardial ratio was significantly less. When the markedly elevated LV end-diastolic pressure was normalized in heart failure, there was restoration of subendocardial coronary flow reserve and a normalization of the endocardial-to-epicardial ratio. Thus, in CHF, there is impaired coronary flow and vasodilator reserve that is selective to the subendocardium of the LV. At this point in the heart failure process, this impairment is not associated with structural changes but is a consequence of the marked elevation in LV end-diastolic pressure.

Acute intravenous cocaine causes transient depression followed by enhanced left ventricular function in conscious dogs

BACKGROUND

Prior studies in experimental canine models have demonstrated that intravenous cocaine administration causes myocardial depression. The purpose of the present study was to establish the mechanisms of cocaine's actions on myocardial and left ventricular performance after single intravenous bolus doses in conscious, chronically instrumented dogs, in which the full autonomic influences of cocaine would be manifest.

METHODS AND RESULTS

In the intact state, cocaine (1 mg/kg) caused a transient decrease in left ventricular dP/dt (baseline; 3,086 +/- 107 mm Hg/sec; 2.5 minutes, 2,649 +/- 114 mm Hg; p < 0.05) followed by a 25 +/- 4% increase in left ventricular dP/dt that peaked at 15 minutes (left ventricular dP/dt, 3,751 +/- 127 mm Hg/sec, p < 0.01) and remained elevated during the 30-minute period of observation. Both the initial depression and the sustained increase in left ventricular contractile response were dose related. The increase in left ventricular dP/dt persisted under circumstances in which the responses were normalized for changes in heart rate and preload that accompanied cocaine administration. The positive inotropic effects were abolished by full autonomic or selective beta-adrenergic blockades. Finally, both cardiac output (baseline, 2,461 +/- 142 min/mL; peak [5 minutes], 3,434 +/- 218 mL/min; p < 0.05) and left ventricular stroke work (baseline, 39 +/- 5 g.m; peak, 49 +/- 6 g.m; p < 0.05) were increased at all times after cocaine administration, suggesting that pump performance was enhanced, despite early reductions in myocardial contractility. Similarly, indexes of early diastolic filling were enhanced despite transient early prolongation in isovolumic relaxation.

CONCLUSIONS

Acute intravenous cocaine administration (0.1-2 mg/kg) has a biphasic effect on myocardial and left ventricular function with a transient depression followed by significant sustained increases in left ventricular contractility. The results are in keeping with an early local effect followed by significant adrenergic stimulation, which may be obscured by anesthesia or masked by changes in loading conditions.

Cholinergic modulation of the coronary vasoconstriction induced by cocaine in conscious dogs

BACKGROUND

The effects of cocaine on the coronary circulation were examined in conscious dogs chronically instrumented to measure arterial and left ventricular pressures, coronary blood flow, and arterial and coronary sinus oxygen content.

METHODS AND RESULTS

With heart rate held constant, the peak effects of cocaine (1 mg/kg i.v.) occurred within 2 minutes, when mean arterial pressure increased by 42 +/- 5 mm Hg, coronary blood flow increased by 13 +/- 3%, and coronary vascular resistance increased by 24 +/- 3%. The arterial oxygen content increased significantly (by 2.8 +/- 0.3 vol%), the arterial-coronary sinus oxygen difference increased by 2.5 +/- 0.6 vol%, and myocardial oxygen consumption increased by 41 +/- 9%. The increase in coronary vascular resistance induced by cocaine was attenuated (p < 0.05) in the presence of cholinergic blockade (12 +/- 3%) despite a similar increase in MVO2 (49 +/- 8%). The increase in coronary vascular resistance was enhanced (p < 0.05) in the presence of beta-adrenergic receptor blockade (46 +/- 8%), whereas the MVO2 response was less (28 +/- 3%). Again, the addition of cholinergic blockade to beta-blockade attenuated the increase in coronary vascular resistance (23 +/- 6%) without affecting the increase in MVO2 (25 +/- 4%). Combined alpha-, beta-, and cholinergic blockades abolished the systemic hemodynamic and coronary vasoconstrictor response to cocaine.

CONCLUSIONS

In conscious dogs, cocaine induces coronary vasoconstriction, which competes with coronary vasodilator responses to increases in myocardial oxygen consumption. The mechanisms of cocaine's coronary vascular effects are mediated via adrenergic stimulation, and the intensity of the vasoconstrictor effects was reduced significantly by cholinergic blockade, in both the presence and absence of beta-adrenergic receptor blockade.

Myocardial beta-adrenergic receptor function during the development of pacing-induced heart failure

The development of pacing-induced heart failure was studied in chronically instrumented, conscious dogs paced at a rate of 240 beats/min for 1 d (n = 6), 1 wk (n = 6), and 3-4 wk (n = 7). Left ventricular (LV) dP/dt was decreased (P < 0.0125) at 1 d, LV end-diastolic pressure and heart rate were increased (P < 0.0125) at 1 wk, but clinical signs of heart failure were only observed after 3-4 wk of pacing. Plasma norepinephrine rose (P < 0.0125) after 1 d of pacing, whereas LV norepinephrine was reduced (P < 0.0125) only after 3-4 wk of pacing. Both the fraction of beta-adrenergic receptors binding agonist with high affinity and adenylyl cyclase activity decreased (P < 0.0125) after 1 d of pacing. Total beta-adrenergic receptor density was not changed at any time point, but beta 1-adrenergic receptor density was decreased (P < 0.0125) after 1 wk. The functional activity of the guanine nucleotide binding protein, Gs, was not reduced, but the Gi alpha 2 isoform of the alpha subunit of the GTP-inhibitory protein rose after 3-4 wk of pacing. Thus, myocardial beta-adrenergic signal transduction undergoes change shortly (1d) after the initiation of pacing, before heart failure develops. The mechanism of beta-adrenergic receptor dysfunction in pacing-induced heart failure is characterized initially by elevated plasma levels of catecholamines, uncoupling of beta-adrenergic receptors, and a defect in the adenylyl cyclase catalytic unit. Selective down-regulation of beta 1-adrenergic receptors, increases in Gi alpha 2, and decreases in myocardial catecholamine levels occur as later events.

The relationship between altered load and impaired diastolic function in conscious dogs with pacing induced heart failure

The complexities of diastolic dysfunction are being recognized with increasing frequency. Prior studies of diastolic dysfunction in both clinical disease states and experimental models have overlooked the major role which altered loading conditions may play in the evolution of diastolic dysfunction. In order to determine the relative importance of the multiple determinants of diastolic dysfunction in disease states requires a large animal model, suitable for chronic study in which the determinants can be controlled independently. This manuscript reviews recent work from our laboratory in which we investigated the mechanisms of diastolic dysfunction in conscious dogs with dilated cardiomyopathy induced by rapid ventricular pacing.

Alterations in left ventricular diastolic function in conscious dogs with pacing-induced heart failure

We investigated in conscious dogs (a) the effects of heart failure induced by chronic rapid ventricular pacing on the sequence of development of left ventricular (LV) diastolic versus systolic dysfunction and (b) whether the changes were load dependent or secondary to alterations in structure. LV systolic and diastolic dysfunction were evident within 24 h after initiation of pacing and occurred in parallel over 3 wk. LV systolic function was reduced at 3 wk, i.e., peak LV dP/dt fell by -1,327 +/- 105 mmHg/s and ejection fraction by -22 +/- 2%. LV diastolic dysfunction also progressed over 3 wk of pacing, i.e., tau increased by +14.0 +/- 2.8 ms and the myocardial stiffness constant by +6.5 +/- 1.4, whereas LV chamber stiffness did not change. These alterations were associated with increases in LV end-systolic (+28.6 +/- 5.7 g/cm2) and LV end-diastolic stresses (+40.4 +/- 5.3 g/cm2). When stresses and heart rate were matched at the same levels in the control and failure states, the increases in tau and myocardial stiffness were no longer observed, whereas LV systolic function remained depressed. There were no increases in connective tissue content in heart failure. Thus, pacing-induced heart failure in conscious dogs is characterized by major alterations in diastolic function which are reversible with normalization of increased loading condition.

Abnormalities in intracellular calcium regulation and contractile function in myocardium from dogs with pacing-induced heart failure

24 d of rapid ventricular pacing induced dilated cardiomyopathy with both systolic and diastolic dysfunction in conscious, chronically instrumented dogs. We studied mechanical properties and intracellular calcium (Ca2+i) transients of trabeculae carneae isolated from 15 control dogs (n = 32) and 11 dogs with pacing-induced cardiac failure (n = 26). Muscles were stretched to maximum length at 30 degrees C and stimulated at 0.33 Hz; a subset (n = 17 control, n = 17 myopathic) was loaded with the [Ca2+]i indicator aequorin. Peak tension was depressed in the myopathic muscles, even in the presence of maximally effective (i.e., 16 mM) [Ca2+] in the perfusate. However, peak [Ca2+]i was similar (0.80 +/- 0.13 vs. 0.71 +/- 0.05 microM; [Ca2+]o = 2.5 mM), suggesting that a decrease in Cai2+ availability was not responsible for the decreased contractility. The time for decline from the peak of the Cai2+ transient was prolonged in the myopathic group, which correlated with prolongation of isometric contraction and relaxation. However, similar end-diastolic [Ca2+]i was achieved in both groups (0.29 +/- 0.05 vs. 0.31 +/- 0.02 microM), indicating that Cai2+ homeostasis can be maintained in myopathic hearts. The inotropic response of the myopathic muscles to milrinone was depressed compared with the controls. However, when cAMP production was stimulated by pretreatment with forskolin, the response of the myopathic muscles to milrinone was improved. Our findings provide direct evidence that abnormal [Ca2+]i handling is an important cause of contractile dysfunction in dogs with pacing-induced heart failure and suggest that deficient production of cAMP may be an important cause of these changes in excitation-contraction coupling.