Regional myocardial downregulation of the inhibitory guanosine triphosphate-binding protein (Gi alpha 2) and beta-adrenergic receptors in a porcine model of chronic episodic myocardial ischemia

Intracoronary delivery of an adenovirus encoding fibroblast growth factor-4 in myocardial ischemia: effect of serum antibodies and previous exposure to adenovirus

The presence of anti-adenoviral serotype 5 (Ad5) antibodies may limit the efficacy of Ad5-mediated gene transfer. We therefore tested the hypothesis that intracoronary delivery of an adenovirus encoding human fibroblast growth factor type 4 (Ad5.FGF4) would improve regional myocardial function in an animal model of ischemia when high antibody levels preexist or after a prior intracoronary dose of Ad5. High anti-Ad5 antibody levels were generated in pigs by subcutaneous immunization with an adenovirus encoding LacZ (Ad5.lacZ). Neutralizing antibody levels increased 648-fold (range, 82- to 2108-fold) above preimmunization levels, and persisted for the duration of the study. Myocardial function during pacing-induced ischemia was improved in the ischemic region (p<0.001) at both 2 and 4 weeks after Ad5.FGF4 administration despite the presence of preexisting antibodies to Ad5. In a second set of experiments, the efficacy of a second intracoronary administration of Ad5 was determined by exposing the animals first to an intracoronary dose of Ad5.lacZ, followed 7 weeks later by the therapeutic dose of Ad5.FGF4. After delivery of Ad5.lacZ, anti-Ad5 antibody levels increased 8-fold (range, 1- to 18-fold), but this prior exposure to Ad5 did not prevent the therapeutic effects after subsequent intracoronary dosing with Ad5.FGF4 (p<0.001). In the porcine Ameroid constrictor model of myocardial ischemia the presence of anti-Ad5 antibodies or prior intracoronary dosing with adenovirus does not prevent the ability of Ad5.FGF4, delivered by intracoronary injection, from normalizing regional myocardial function.

Regional Desensitization of β-Adrenergic Receptor Signaling in Swine With Chronic Hibernating Myocardium

Contractile reserve during submaximal beta-adrenergic stimulation is attenuated in patients and swine with hibernating myocardium. We tested the hypothesis that this arises as a regional adaptive response in beta-adrenergic adenylyl cyclase coupling. Pigs (n=8) were studied 3 months after instrumentation with a left anterior descending artery (LAD) stenosis when flow (LAD, 0.7+/-0.2 versus 1.2+/-0.1 mL/min per gram in normal remote; P<0.05) and wall thickening (LAD, 15.5 [corrected]+/-3.2% versus 40.0+/-5.5% in remote; P<0.05) were reduced in the absence of infarction. Whereas basal cAMP production was normal (LAD, 87+/-18 versus 91+/-19 pmol/mg per minute; P=NS), responses to isoproterenol were blunted (LAD, 83+/-6 versus 146+/-25 pmol/mg per minute in remote; P<0.05). beta-receptor density and subtype were unchanged, but there was a reduction in the number of high-affinity binding sites (LAD, 40+/-4% versus 53+/-7% in normal remote; P<0.05). The Gialpha2/Gsalpha ratio increased (LAD, 1.8+/-0.3 versus 0.99+/-0.3 in remote myocardium; P<0.05), although GppNHp-stimulated cAMP production was equivocally reduced. Forskolin responses were unchanged and similar to shams. These data indicate regional attenuation of beta-receptor adenylyl cyclase signaling in hibernating myocardium. This blunts the local contractile response to beta-adrenergic stimulation and may serve to protect against a myocardial supply/demand imbalance when external determinants of myocardial workload increase during sympathetic activation.

Signal Peptide Increases the Efficacy of Angiogenic Gene Transfer for Treatment of Myocardial Ischemia

We have previously shown that intracoronary delivery of recombinant adenoviruses encoding angiogenic proteins that contain signal peptides (fibroblast growth factor-4 and fibroblast growth factor-5) ameliorate myocardial ischemia. In the present paper, we test the hypothesis that the presence of the signal peptide is an important element in the favorable effects that transgene expression has on regional flow and function in an animal model of myocardial ischemia. We performed intracoronary delivery of two different recombinant adenoviruses encoding a fibroblast growth factor-2 variant, one with a signal peptide, FGF-2LI(+sp), and one without a signal peptide, FGF-2LI(-sp). In a model of stress-induced myocardial ischemia, intracoronary injection of these recombinants resulted in mRNA and protein expression of the transferred gene. Two weeks after gene transfer, regional abnormalities in stress-induced function and blood flow were improved after delivery of FGF-2LI containing the signal peptide. In the absence of the signal peptide, perfusion was not improved, and function was improved to a lesser degree than with FGF-2LI containing the signal peptide. These studies indicate that the presence of a signal peptide increases the efficacy of treatment and may reduce the required recombinant adenovirus dose for a given effect, and thereby provide an important safety margin for clinical application.

Alterations in hepatic glucagon receptor density and in Gsalpha and Gialpha2 protein content with diet-induced hepatic steatosis: effects of acute exercise

The present study was undertaken to test the hypothesis that a high-fat diet-induced liver lipid infiltration is associated with a reduction of hepatic glucagon receptor density (B(max)) and affinity (K(d)), and with a decrease in stimulatory G protein (G(s)alpha) content while enhancing inhibitory G protein (G(i)alpha(2)) expression. We also hypothesized that, under this dietary condition, a single bout of endurance exercise would restore hepatic glucagon receptor parameters and G protein expression to standard levels. Female Sprague-Dawley rats were fed either a standard (SD) or a high-fat diet (HF; 40% kcal) for 2 wk (n = 20 rats/group). Each dietary group was thereafter subdivided into a nonexercised (Rest) and an acute-exercised group (Ac-Ex). The acute exercise consisted of a single bout of endurance exercise on a treadmill (30 min, 26 m/min, and 0% slope) immediately before being killed. The HF compared with the SD diet was associated with significantly (P < 0.05) higher values in hepatic triglyceride concentrations (123%), fat pad weight, and plasma free fatty acid (FFA) concentrations. The HF diet also resulted in significantly (P < 0.05) lower hepatic glucagon receptor density (45%) and G(s)alpha protein content (75%), as well as higher (P < 0.05) G(i)alpha(2) protein content (27%), with no significant effects on glucagon receptor affinity. Comparisons of all individual liver triglyceride and B(max) values revealed that liver triglycerides were highly (P < 0.003) predictive of the decreased glucagon receptor density (R = -0.512). Although the 30-min exercise bout resulted in some typical exercise effects (P < 0.05), such as an increase in FFA (SD diet), a decrease in insulin levels, and an increase in plasma glucagon concentrations (SD diet), it did not change any of the responses related to liver glucagon receptors and G proteins, with the exception of a significant (P < 0.05) decrease in G(i)alpha(2) protein content under the HF diet. The present results indicate that the feeding of an HF diet is associated with a reduction in plasma membrane hepatic glucagon receptor density and G(s)alpha protein content, which is not attenuated by a 30-min exercise bout. It is suggested that liver lipid infiltration plays a role in reducing glucagon action in the liver through a reduction in glucagon receptor density and glucagon-mediated signal transduction.

Increased Regional Function and Perfusion After Intracoronary Delivery of Adenovirus Encoding Fibroblast Growth Factor 4: Report of Preclinical Data

This paper reports the preclinical data that were used to support clinical trials of intracoronary delivery of a replication-incompetent human adenovirus-5 vector encoding human fibroblast growth factor 4 (Ad5FGF4). Using stress-induced myocardial ischemia in pigs, intracoronary injection of Ad5FGF4 resulted in mRNA and protein expression of the transferred gene. Two weeks after gene transfer, regional stress-induced dysfunction and perfusion were ameliorated and improved function persisted for at least 12 weeks. Transgene protein was present in hearts of all animals that received gene transfer but was not found in extracardiac sites. FGF4 was undetectable in samples of plasma obtained at multiple time points after intracoronary delivery of Ad5FGF4. Adenovirus vector DNA was detected in some extracardiac tissues by polymerase chain reaction (PCR) and was dose dependent, occurring primarily after the highest dose delivered (10(12) virus particles [vp]) with much less incidence at 10(11) vp. Histologic evaluation indicated that intracoronary administration of Ad5FGF4 was not associated with abnormal findings in any organ examined. These data provide a rationale for intracoronary delivery of Ad5FGF4 to increase regional cardiac perfusion and function in patients with myocardial ischemia. Based on these preclinical studies, the method does not appear to be associated with major toxic effects.

Ischemic preconditioning prevents ischemia-induced beta-adrenergic receptor sequestration

Preconditioning enables endogenous protection to repeated myocardial ischemia. However, the effect of preconditioning on beta1 adrenergic receptor (AR) signal remains controversial. We have recently developed receptor assay system using whole cells, in which overexpressed cell surface beta ARs can be readily quantitated without disrupting the cell. Using this technique, we examined the effects of chemical/metabolic ischemia on the beta1 AR sequestration and adenylyl cyclase activity. Isoproterenol treatment, but not forskolin treatment, of HEK293T cells overexpressing beta1 ARs led to a rapid decrease (within 2 hours) in the number of the cell surface receptor, which was negated in the presence of concanavalin A. Similarly, treatment of cells with potassium cyanide and 2-deoxy-D-glucose (chemical/metabolic ischemia) induced similar receptor sequestration. When isoproterenol was superimposed on chemical/metabolic ischemia, the degree of sequestration became greater. However, when cells were pre-exposed to potassium cyanide on the preceding day (chemical preconditioning), the sequestration induced by either isoproterenol or chemical/metabolic ischemia was attenuated. Adenylyl cyclase catalytic activity as assessed by stimulation with forskolin was decreased by chemical/metabolic ischemia but fully recovered after 24 hours, suggesting that chemical/metabolic ischemia treatment did not alter cell viability. Putting together, chemical/metabolic ischemia induced beta1 AR sequestration in a similar manner to isoproterenol. In addition, preconditioning prevented the beta1 AR sequestration induced by both isoproterenol and chemical/metabolic ischemia. Pre-conditioning may play a role in preserving the cell surface beta ARs by inhibiting the sequestration that is usually induced by an ischemic event or beta adrenergic stimulation.

Hibernation, Stunning, and Preconditioning: Historical Perspective, Current Concepts, Clinical Applications, and Future Implications

Despite considerable advances, coronary artery disease is the leading cause of morbidity and mortality in the Western world. The development of effective therapeutic strategies for protecting the myocardium from ischemia would have major impact on patients with coronary artery disease. It is now accepted that patients with coronary artery disease can experience prolonged regional ischemic dysfunction that does not necessarily arise from irreversible tissue damage, and to some extent, can be reversed by restoration of blood flow. The initial stages of dysfunction are probably caused by chronic stunning that can be reversed after revascularization, resulting in rapid and complete functional recovery. On the other hand, the more advanced stages of dysfunction likely correspond to chronic hibernation. After revascularization, functional recovery will probably be quite delayed and mostly incomplete. Over the past decade, the possibility that an innate mechanism of myocardial protection might be inducible in the human heart has generated considerable excitement. In the last two decades, there was phenomenal growth in the understanding of the mechanism known as ischemic preconditioning that is responsible for the innate myocardial protection. Continued research and progress in this area may soon lead to the availability of preconditioning-mimetic treatments. The current concepts, mechanisms, and potential clinical applications of myocardial hibernation, stunning, and ischemic preconditioning are reviewed.

Dissociation of regional adaptations to ischemia and global myolysis in an accelerated Swine model of chronic hibernating myocardium

We tested the hypothesis that an acute critical limitation in coronary flow reserve could rapidly recapitulate the physiological, molecular, and morphological phenotype of hibernating myocardium. Chronically instrumented swine were subjected to a partial occlusion to produce acute stunning, followed by reperfusion through a critical stenosis. Stenosis severity was adjusted serially so that hyperemic flow was severely reduced yet always higher than the preocclusion resting level. After 24 hours, resting left anterior descending coronary artery (LAD) wall thickening had decreased from 36.3+/-4.0% to 25.5+/-3.7% (P<0.05), whereas resting flow had remained normal (67+/-6 versus 67+/-8 mL/min, respectively). Although peak hyperemic flow exceeded the prestenotic value, resting flow (45+/-10 mL/min) and LAD wall thickening (17.0+/-5.0%) progressively decreased after 2 weeks, when physiological features of hibernating myocardium had developed. Regional reductions in sarcoplasmic reticulum proteins were present in hibernating myocardium but absent in stunned myocardium evaluated after 24 hours. Histological analysis showed an increase in connective tissue along with myolysis (myofibrillar loss per myocyte >10%) and increased glycogen typical of hibernating myocardium in the LAD region (33+/-3% of myocytes from animals with hibernating myocardium versus 15+/-4% of myocytes from sham-instrumented animals, P<0.05). Surprisingly, the frequency of myolysis was similar in normally perfused remote regions from animals with hibernating myocardium (32+/-7%). We conclude that the regional physiological and molecular characteristics of hibernating myocardium develop rapidly after a critical limitation in flow reserve. In contrast, the global nature of myolysis and increased glycogen content dissociate them from the intrinsic adaptations to ischemia. These may be related to chronic elevations in preload but appear unlikely to contribute to chronic contractile dysfunction.

Alterations of cardiac beta-adrenoceptor mechanisms due to calcium depletion and repletion

In order to understand the modification of beta-adrenoceptor linked signal transduction by changes in the intracellular Ca2+, we examined the status of beta-adrenoceptors (beta-ARs), G-proteins and adenylyl cyclase (AC) in Ca2+-deficiency and Ca2+-overload by perfusing the isolated rat heart with Ca2+-free medium for 5 min and Ca2+-containing medium for 5 min following Ca2+-free perfusion, respectively. Ca2+-depletion caused not only an increase in basal, isoproterenol-, Gpp(NH)p-, NaF- and forskolin-stimulated AC activities but also produced an increase in the beta1-AR affinity and density as well as up-regulation of G(s)-protein function and uncoupling of G(i)-protein to AC. Ca2+-repletion for 5 min following 5 min Ca2+-free perfusion reversed the increased AC activities to varying degrees. The beta1-AR affinity was further increased upon Ca2+-repletion whereas its density was decreased. Ca2+-repletion also decreased protein content for AC and beta-AR kinase but augmented the changes in G(s)- and G(i)-protein functions. Although low Na+ medium perfusion during Ca2+-depletion prevented the changes in G-proteins during both Ca2+-depletion and Ca2+-repletion periods, the increased beta1-AR affinity and density as well as changes in AC activities due to Ca2+-depletion were not affected while alterations due to Ca2+-repletion were fully prevented. These results suggest that changes in Ca2+-homeostasis may represent a mechanism for alterations in the beta-adrenergic signal transduction pathway in the heart under pathological conditions.

Attenuation of age-related declines in glucagon-mediated signal transduction in rat liver by exercise training

This study investigated alterations in glucagon receptor-mediated signal transduction in rat livers from 7- to 25-mo-old animals and examined the effects of exercise training on ameliorating these changes. Sixty-six young (4 mo), middle-aged (12 mo), and old (22 mo) male Fischer 344 rats were divided into sedentary and trained (treadmill running) groups. Isolated hepatic membranes were combined with [(125)I-Tyr(10)]monoiodoglucagon and nine concentrations of glucagon to determine maximal binding capacity (B(max)) and dissociation constant (K(d)). No alterations were found in B(max) among groups; however, middle-aged trained animals had significantly higher glucagon affinity (lower K(d); 21.1 +/- 1.8 nM) than did their untrained counterparts (50.2 +/- 7.1 nM). Second messenger studies were performed by measuring adenylyl cyclase (AC) specific activity under basal conditions and with four pharmacological stimulations to assess changes in receptor-dependent, G protein-dependent, and AC catalyst-dependent cAMP production. Age-related declines were observed in the old animals under all five conditions. Training resulted in increased cAMP production in the old animals when AC was directly stimulated by forskolin. Stimulatory G protein (G(s)) content was reduced with age in the sedentary group; however, training offset this decline. We conclude that age-related declines in glucagon signaling capacity and responsiveness may be attributed, in part, to declines in intrinsic AC activity and changes in G protein [inhibitory G protein (G(i))/G(s)] ratios. These age-related changes occur in the absence of alterations in glucagon receptor content and appear to involve both G protein- and AC-related changes. Endurance training was able to significantly offset these declines through restoration of the G(i)/G(s) ratio and AC activity.

Method for non-invasively recording electrocardiograms in conscious mice

BACKGROUND

The rapid increase in the development of mouse models is resulting in a growing demand for non-invasive physiological monitoring of large quantities of mice. Accordingly, we developed a new system for recording electrocardiograms (ECGs) in conscious mice without anesthesia or implants, and created Internet-accessible software for analyzing murine ECG signals. The system includes paw-sized conductive electrodes embedded in a platform configured to record ECGs when 3 single electrodes contact 3 paws.

RESULTS

With this technique we demonstrated significantly reduced heart rate variability in neonates compared to adult mice. We also demonstrated that female mice exhibit significant ECG differences in comparison to age-matched males, both at baseline and in response to beta-adrenergic stimulation.

CONCLUSIONS

The technology we developed enables non-invasive screening of large numbers of mice for ECG changes resulting from genetic, pharmacological, or pathophysiological alterations. Data we obtained non-invasively are not only consistent with what have been reported using invasive and expensive methods, but also demonstrate new findings regarding gender-dependent and age-dependent variations in ECGs in mice.

Diabetes reduces right atrial β-adrenergic signaling but not agonist stimulation of heart rate in swine

This study assessed the effects of streptozotocin diabetes in swine on the heart rate response to β-adrenergic stimulation the adenylyl cyclase signal transduction pathway. Diabetic animals (n = 9) were hyperglycemic compared to the control group (n = 10) (12.6 ± 1.0 vs. 3.53 ± 0.29 mM). There were no significant differences between the diabetic and nondiabetic groups in the heart rate response to isoproterenol, however, there was a significant reduction (14%) in β-adrenergic receptor density in the right atrium in the diabetic (61 ± 3 fmol/mg protein) versus the nondiabetic group (71 ± 3) (P < 0.05). The content of guanosine triphosphate binding regulatory proteins (Gs and Gi) in the right atrium was not affected by diabetes, nor was adenylyl cyclase activity under unstimulated conditions or with receptor-dependent stimulation with isoproterenol. On the other hand, adenylyl cyclase activity was 34% lower when directly stimulated with forskolin, and it was reduced by 23% when stimulated through Gs with Gpp(NH)p. In conclusion, beta-adrenergic stimulation of heart rate with isoproteronol and the receptor-dependent signal transduction pathway remained intact in the right atrium of diabetic swine despite reduced beta-adrenergic receptor density, G-protein content, and direct stimulation of adenylyl cyclase activity.Key words: diabetes, G-proteins, heart rate, receptors, signal transduction.

Impact of delayed reperfusion of myocardial hibernation on myocardial ultrastructure and function and their recoveries after reperfusion in a pig model of myocardial hibernation

This study examined the effect of delayed reperfusion of myocardial hibernation from 24 hours to 7 days on myocardial ultrastructural and functional changes and their recoveries after reperfusion.

BACKGROUND

We have previously shown in pigs that after reperfusion the functional and structural alterations in short-term myocardial hibernation which was reperfused in 24 hours can recover in 7 days. The effect of delayed reperfusion of hibernating myocardium on the extent and severity of cellular and extracellular structural changes of hibernating myocardium, and their recoveries after reperfusion is not known.

METHODS AND RESULTS

A severe LAD stenosis was created in 27 pigs, reducing resting flow by 30-40% immediately after placement of the stenosis and producing acute ischemia as evidenced by regional lactate production, a decrease in regional coronary venous pH, reduced regional wall thickening (from 38.5 +/- 5.1% to 10.4 +/- 8.0%) and a 33% reduction of regional oxygen consumption. The stenosis was maintained either for 24 hours in 9 pigs (group 1) with LAD flow of 0.65 +/- 0.13 ml/min/g (38% reduction), or for 7 days in 17 pigs (group 2) with LAD flow of 0.67 +/- 0.14 ml/min/g (36% reduction). There were no differences (p = NS) in the reduction of wall thickening, rate-pressure product, lactate production, or regional oxygen consumption between group 1 and group 2. Quantitative morphometric evaluation of the ultrastructure on electromicrographs revealed a greater decrease in sarcomere volume and a higher incidence of myocytes with reduced sarcomere volume in 7-day than in 24-hour hibernating regions (53 +/- 19% versus 33 +/- 14%, p < 0.05). Patchy myocardial necrosis with replacement fibrosis was common, but 6 of the 18 pigs had no myocardial necrosis or replacement fibrosis in the 7-day hibernating group, and 4 of 9 pigs had no patchy myocyte necrosis in the 24 hour hibernating group. In 6 pigs in group 1 in which the stenosis was then released and hibernating myocardium reperfused in 24 hours, regional wall thickening recovered to 30 +/- 6% (p = NS compared to baseline) after one week of reperfusion. In 12 pigs in group 2 in which the stenosis was released and hibernating myocardium reperfused in 7 days, regional wall thickening recovered slowly, from 10.1 +/- 7.2% to 18.1 +/- 8.3% at one week (n = 5) and to 28.0 +/- 3.6% at 3-4 weeks of reperfusion (n = 7, p < 0.05 compared to baseline). Similarly, the sarcomere volume or myofilament recovered significantly (p < 0.01) and was not different compared to the normal region (p = NS) in the 24-hour hibernating region of group 1, but the recovery was much slower and was incomplete at 4 weeks (p < 0.01) compared to baseline in the 7-day hibernating region of group 2. Recovery of regional wall thickening correlated with ultrstructural recovery (p < 0.01). By multivariate stepwise regression analysis, the degree of LAD flow reduction, the extent of fibrosis, and myofilament loss were independent predictors of the extent of functional recovery.

CONCLUSIONS

In a porcine model of myocardial hibernation with myocardial hypoperfusion, systolic dysfunction, and metabolic adaptations, a longer period of myocardial hibernation with delayed reperfusion was associated with more severe abnormalities of myocytes. an increasing interstitial fibrosis, and more protracted myofibrillar and functional recoveries after reperfusion. The extent of functional recovery is related to the degree of coronary flow reduction, the severity of the ultrastructural changes, and the extent of interstitial fibrosis.

Repeated stunning precedes myocardial hibernation in progressive multiple coronary artery obstruction

OBJECTIVE

The aim of this study was to characterize a regional myocardial flow-function relationship in collateral dependent myocardium produced by multiple coronary artery obstruction.

METHODS

Ameroid constrictors were placed around the proximal right (RC) and circumflex (CX) coronary arteries and a silicon tubing cuff around the proximal LAD (left anterior descending artery) (luminal stenosis +/- 77%) in 18 dogs. Weekly two-dimensional echocardiography was performed for regional function (anterior [A], inferoposterior [IP], wall thickening [WT]), and fractional shortening (FS). Colored microspheres injected at baseline and before sacrifice, before and after dipyridamole (0.5 mg/kg) injection, determined resting flow (RF) and coronary reserve (CR), respectively.

RESULTS

Coronary angiography performed at four weeks after surgery confirmed occlusion of RC and CX with collateralization and a tight stenosis of LAD. Initially, an episodic reduction in A and IP WT was observed which became persistent later (AWT: 16 +/- 3%; IPWT: 16 +/- 4%, FS: 20 +/- 4%, p < 0.005 vs. baseline [BS]). With dobutamine a biphasic response (improvement in A and IP WT between 5-15 and dysfunction between 20-30 microg/kg/min) was observed. Seven dogs were sacrificed at eight weeks and showed normal RF but reduced transmural CR (A: 75 +/- 18%; IP: 46 +/- 22% of control). Seven dogs underwent PTCA of the LAD at eight weeks and showed gradual improvement in AWT with normalization at 12 weeks (AWT: 30 +/- 5%, p < 0.001 vs. eight weeks). At sacrifice RF and CR in the A wall were normal but there was reduced subendocardial RF in the IP region (64% of BS). Further, biopsy samples showed normal histological findings and high energy phosphate content in all dogs. Radioligand binding assays using 125I-iodocyanopindolol showed downregulation of beta-adrenergic receptor density in the dysfunctional regions compared with control.

CONCLUSIONS

In this canine model of viable, collateral dependent and reversibly dysfunctional myocardium, there was early episodic dysfunction followed by persistent dysfunction which was initially associated with normal RF and later with subendocardial hypoperfusion.

Large animal models of heart failure

Congestive heart failure (HF) is a major focus of medical research. Its incidence has greatly increased in recent decades because of an aging population base and the increasingly successful treatment of other forms of chronic cardiac disease. Relevant large animal models of HF should reflect the complex interactions of cardiac dysfunction, neurohumoral dynamics and peripheral vascular abnormalities found in human HF. A number of large animal models have been developed, especially in dogs, sheep and swine, using naturally occurring HF, or single or combinations of interventions, as instruments to trigger the development of HF. Naturally occurring HF models are not commonly used because of ethical or perceived ethical grounds, however, King Charles Cavalier Spaniel and Yucatan Mini Pig models have been described. Tachycardia induced HF is the most commonly used HF model. Ventricular pacing at 220-240 bpm results in profound low output, biventricular, oedematous failure in two to three weeks. Lower pacing rates result in a more stable, sustainable, lesser degree of failure. Positive features of this model include 'acceptance', aetiological relevance to patient tachycardia induced HF, neurohumoral and functional profile similar to most human HF, relatively low cost simple preparation, ability to manipulate the degree of failure with pacing rate, reversibility, reliability and a large amount of published multi species data. Limitations to the use of the model are the rapid onset, the fact that reversibility is only relevant to the tachycardia induced patient HF, the absence of hypertrophy in failure, the diminished plasma atrial natriuretic peptide (ANP) levels, absence of ANP of ventricular origin, and the interference between rapid pacing and therapeutic interventions. Myocardial damage models of HF include those models induced by ischaemia, eg due to coronary occlusion (ligation or aneroid) or intracoronary microembolism, transmyocardial DC shock, toxic cardiomyopathy from adriamycin, doxorubicin or catecholamines. Overload models of HF may be induced by high pressure from aortic constriction, aortic regurgitation, renal artery constriction, pulmonary stenosis or aortocaval shunts, or by induction of mitral regurgitation from chordae or leaflet damage. No single, all-encompassing, large animal model of HF exists to date. Selection of the type of model to be used should be based primarily on the hypotheses to be tested and secondarily on the available resources and facilities.

Effect of chronic hypoxia on adrenoceptor responses of ovine foetal umbilical vessels

1. The effects of chronic hypoxia on alpha1-adrenoceptor-mediated contractions were investigated in foetal umbilical vessels obtained from near-term (approximately 140 day gestation) pregnant sheep maintained near sea level ( 300 m) and at high altitude (3820 m) from 30 day gestation. 2. Chronic hypoxia significantly decreased contractile sensitivity of the umbilical vein to noradrenaline (pD2: 6.22+/-0.19 vs 5.67+/-0.09) and reduced the maximum response by 43%. Noradrenaline-induced contraction of the umbilical artery was abolished. In contrast, contractions to KCI were not affected by chronic hypoxia. 3. In umbilical vein, the apparent dissociation constant (KA) of noradrenaline to alpha1-adrenoceptors was increased from 0.54+/-0.06 microM in control animals to 1.35+/-0.14 microM in chronically hypoxic animals. In accordance, radioligand binding of agonist showed high and low affinity binding sites for noradrenaline in both normoxic and chronically hypoxic tissues. Addition of GTPgammaS (100 microM) abolished apparent high affinity binding sites. Whereas proportional binding sites were not changed by chronic hypoxia, the apparent high affinity of noradrenaline was significantly decreased (pKi: 7.80+/-0.17 vs 7.20+/-0.16). 4. Chronic hypoxia significantly decreased alpha1-adrenoceptor density (fmol mg protein(-1)) in umbilical vein (24.6+/-3.2 vs 12.3+/-3.1) and the artery (7.1+/-0.4 vs 3.1+/-0.9) with no change in [3H]-prazosin binding affinity. There was a linear correlation of the maximum contractions to noradrenaline and alpha1-adrenoceptor density. 5. We conclude that chronically hypoxic-induced depression in contractions of ovine foetal umbilical vessels to noradrenaline is mediated predominantly by decreases in alpha1-adrenoceptor density and the agonist binding affinity.

Adaptation of pharmacomechanical coupling of vascular smooth muscle to chronic hypoxia

Hypoxia is one of the most common stresses that affect an organism's homeostasis. Although much is known of the mechanisms of the cellular and biochemical responses to acute hypoxia, relatively little is known of the mechanisms of the responses to prolonged or chronic hypoxia. Chronic hypoxia suppresses vascular smooth muscle contractility in many vascular beds. While the endothelium is likely to play a role, part of the mechanisms underlying chronic hypoxic-induced changes in vascular responses resides in the changes in receptor-mediated excitation-contraction coupling and/or signal transduction in the vascular smooth muscle. Recent studies have demonstrated that chronic hypoxia attenuates both receptor-second messenger and second messenger-contraction coupling efficiencies in the vascular smooth muscle. This suppression of pharmacomechanical coupling is likely to represent one of the adaptive mechanisms of vascular smooth muscle and to play an important role in an adjustment of vascular tone and blood flow under the stress of moderate chronic hypoxia.

Alterations in myocardial signal transduction due to aging and chronic dynamic exercise

Normal aging without disease leads to diminished chronotropic and inotropic responses to catecholamine stimulation, resulting in depressed cardiac function with stress. The purpose of this study was to determine molecular mechanisms for decrements in adrenergic responsiveness of the left ventricle (LV) due to aging and to study the effects of chronic dynamic exercise on signal transduction. We measured beta-adrenergic receptor (beta-AR) density, adenylyl cyclase (AC) activity, and G-protein content and distribution in LV from 66 male Fischer 344 rats from three age groups that were either sedentary or treadmill trained (60 min/days, 5 days/wk, 10 wk at 75% of the maximal capacity). Final ages were 7 mo (young), 15 mo (middle-age), and 25 mo (old). There was no significant difference in beta-AR density among groups as a function of age or training. AC production of adenosine 3',5'-cyclic monophosphate (cAMP) with the use of five pharmacological stimulations revealed that old sedentary myocardium had depressed basal, receptor-dependent, G-protein-dependent, and AC catalyst stimulation (30-43%) compared with hearts from young and middle-age sedentary rats. Training did not alter AC activity in either middle-age or old groups but did increase G-protein-dependent cAMP production in young myocardium (12-34%). Immunodetectable concentrations of stimulatory and inhibitory G proteins (Gs and Gi, respectively) showed 43% less total Gs with similar Gi content in hearts from old sedentary compared with middle-age sedentary rats. When compared with young sedentary animals, Gi content was 39 and 50% higher in middle-age sedentary and old sedentary myocardium, respectively. With age, there was a significant shift in the alpha-subunit of Gs distribution from cytosolic fractions of LV homogenates to membrane-bound fractions (8-12% redistribution in middle-age sedentary vs. old sedentary). The most significant training effect was a decrease in Gi content in hearts from old trained rats (23%), which resulted in values comparable with young sedentary rats and reduced the Gi/Gs ratio by 27% in old-rat LV. We report that age-associated reductions in cardiovascular beta-adrenergic responsiveness correspond with alterations in postreceptor adrenergic signaling rather than with a decrease in receptor number. Chronic dynamic exercise partially attenuates these reductions through alterations in postreceptor elements of cardiac signal transduction.

Chronic hypoxia suppresses pharmacomechanical coupling of the uterine artery in near-term pregnant sheep

1. The role of inositol 1,4,5-trisphosphate (InsP3) in the reduced vascular responsiveness to 5-hydroxytryptamine (5-HT) caused by chronic hypoxia was examined in uterine arteries obtained from normoxic (control) and chronically hypoxic pregnant sheep (approximately 140 days gestation) maintained at high altitude (3820 m; arterial PO2, 60 mmHg) from 30 days gestation. 2. Chronic hypoxia significantly decreased uterine artery contractile sensitivity in that pD2 (-logEC50) for the contractile response to 5-HT was 7.19 +/- 0.15 and 6.62 +/- 0.12 (P < 0.05) in uterine arteries from normoxic and chronically hypoxic sheep, respectively. The intrinsic efficacy of the agonist was reduced by 75%. Although 5-HT2A receptor density (Bmax) in the uterine artery was not changed in chronically hypoxic sheep compared with normoxic sheep (32.0 +/- 9.8 vs. 31.9 +/- 5.9 fmol (mg protein)-1, respectively) as assessed from the saturation binding of [3H]ketanserin, the agonist binding affinity (pKA, -log of dissociation constant) was decreased from 6.25 +/- 0.07 in normoxic sheep to 5.85 +/- 0.08 in chronically hypoxic sheep (P < 0.05). 3. Chronic hypoxia did not change the time course of 5-HT-induced InsP3 synthesis but decreased its potency in inducing InsP3 synthesis, with the pD2 being 6.09 +/- 0.11 and 5.51 +/- 0.08 (P < 0.05) in uterine arteries from normoxic and chronically hypoxic sheep, respectively. The maximal response of 5-HT-induced InsP3 generation in the uterine artery was decreased from 251.3 +/- 24.2 pmol (mg protein)-1 in normoxic sheep to 146.6 +/- 11.3 pmol (mg protein)-1 in chronically hypoxic sheep (P < 0.05). Furthermore, the ability of the activated 5-HT receptors to couple InsP3 synthesis was significantly decreased in chronically hypoxic compared with normoxic sheep (280 +/- 10 vs. 450 +/- 20 fmol InsP3 (fmol receptor)-1, P < 0.01). In addition, for a given amount of InsP3 generated, the contractile force of the uterine artery was significantly less in chronically hypoxic sheep (0.82 +/- 0.08 g tension (pmol InsP3)-1) than that in normoxic sheep (1.28 +/- 0.05 g tension (pmol InsP3)-1) (P < 0.05). 4. These results suggest that chronic hypoxia suppresses pharmacomechanical coupling of the ovine uterine artery by inhibiting the efficiency of receptor-effector-contraction coupling. This suppression of the InsP3 pathway may play an important role in the adjustment of vascular tone and uterine blood flow in response to the stress of chronic hypoxia in late pregnancy.

Chronic hypoxia increases beta 1-adrenergic receptor mRNA and density but not signaling in neonatal rat cardiac myocytes

BACKGROUND

It is well recognized that the beta-adrenergic receptor-adenylylcyclase system is altered during myocardial ischemia/hypoxia. However, there are no data regarding either regulation of beta-adrenergic receptors, particularly at the mRNA level, or adenylylcyclase activity in isolated cardiac myocytes exposed to chronic hypoxia.

METHODS AND RESULTS

In a chronic hypoxia model in which neonatal rat ventricular myocytes were exposed to a 1% O2 environment for 72 hours, we investigated (1) beta 1-mRNA and receptor expression and adenylylcyclase activity and (2) beta 1-mRNA and receptor downregulation and adenylylcyclase desensitization induced by prolonged norepinephrine incubation. We found that hypoxia for 72 hours increased myocardial membrane beta 1-adrenergic receptor density by 44%. This increase was not associated with a corresponding decrease in cytosolic beta 1-adrenergic receptors. RNase protection assays demonstrated that hypoxia increased the steady-state levels of beta 1-mRNA by 109%. Adenylylcyclase activity stimulated by isoproterenol, sodium fluoride, guanyl-5'-imidodiphosphate, and forskolin in hypoxic membranes was not altered compared with normoxic controls. Hypoxia for 72 hours also did not affect norepinephrine-induced beta 1-mRNA and receptor downregulation and adenylylcyclase desensitization in response to isoproterenol, guanyl-5'-imidodiphosphate, or forskolin.

CONCLUSIONS

In neonatal rat cardiac myocytes, chronic hypoxia (1) increases beta 1-mRNA and receptor expression but does not alter adenylylcyclase activity stimulated at either the receptor or the postreceptor level and (2) does not affect agonist-induced beta 1-mRNA and receptor downregulation and desensitization of the adenylylcyclase response.

The guanine nucleotide-binding regulatory proteins (G proteins) in myocardium with ischemia

Guanine nucleotide-binding regulatory proteins (G proteins) play a major role in the regulation of a number of physiological processes, such as stimulation or Inhibition of adenylate cyclase activity or gaiting of ionic channels. Myocardial ischemia could induce the changes in receptor-G protein signal transduction system in the heart. Therefore, this article will focus on the role and alterations of G proteins (especially, Gs and Gi) in myocardial ischemia. The Gi protein rapidly loses functional activity during very early myocardial ischemia. In contrast to Gi protein, the function of Gs protein during this phase has not been evaluated. Moreover, the changes in Gs protein after 30 min of ischemia are contradictory. However, the sensitization of the adenylate cyclase activity in the very early phase of acute ischemia is gradually replaced by a decrease in adenylate cyclase activity with prolonged ischemia. The decrease in the function and amount of Gs protein may be one of the factors that induce these changes. The function of Gs protein was also decreased in the canine hearts with ischemia and reperfusion. In contrast to ischemia and reperfusion, there are no significant alterations in G proteins and modulation of adenylate cyclase in the stunned myocardium. It has become increasingly evident that Gi protein may play an important role in the cardioprotective effects of preconditioning. When beta-adrenoreceptor densities are reduced in chronic myocardial ischemia, decreased in the amount and function of Gi protein and increased amount of Gs protein may play the role in preservation of the adenylate cyclase activity. These alterations in G proteins may play the important role in the myocardial function during myocardial ischemia.

Intracoronary gene transfer of fibroblast growth factor-5 increases blood flow and contractile function in an ischemic region of the heart

Increased coronary blood vessel development could potentially benefit patients with ischemic heart disease. In a model of stress-induced myocardial ischemia, intracoronary injection of a recombinant adenovirus expressing human fibroblast growth factor-5 (FGF-5) resulted in messenger RNA and protein expression of the transferred gene. Two weeks after gene transfer, regional abnormalities in stress-induced function and blood flow were improved, effects that persisted for 12 weeks. Improved blood flow and function were associated with evidence of angiogenesis. This report documents, for the first time, successful amelioration of abnormalities in myocardial blood flow and function following in vivo gene transfer.

Quantification of signalling components and amplification in the beta-adrenergic-receptor-adenylate cyclase pathway in isolated adult rat ventricular myocytes

We have investigated the stoichiometric relationship of proteins involved in beta-adrenergic-receptor-mediated signal transduction in isolated rat cardiac myocytes. These cells contain about 2.1 x 10(5) beta-adrenergic receptors per cell, as determined by radio-ligand-binding assays. We have assessed the amount of Gs alpha present in myocyte membranes by immunoblotting using a purified glutathione S-transferase-Gs alpha fusion protein as a standard for quantification. By this method, we determined that cardiac myocytes contain about 35 x 10(6) and 12 x 10(6) molecules per cell of the 45 and 52 kDa forms of Gs alpha, respectively. [3H]Forskolin binding assays were used to assess the formation of high-affinity forskolin binding sites representing Gs alpha-adenylate cyclase complexes occurring in response to Gs alpha activation. Quantification of the adenylate cyclase complexes was facilitated by the permeabilization of cells with saponin. The addition of isoprenaline (isoproterenol) and guanosine 5'-[gamma-thio]trisphosphate to saponin-permeabilized myocytes results in the formation of 6 x 10(5) Gs alpha-adenylate cyclase complexes. Taken together, the data presented here demonstrate that, in a physiologically relevant setting, G-protein is present in large stoichiometric excess relative to both receptor and effector. In addition, we show that, overall, only modest signal amplification occurs between receptor and adenylate cyclase. Thus adenylate cyclase (rather than Gs) is the component distal to receptor that limits agonist-mediated increases in cyclic AMP production. Although limited data are as yet available for other G-protein-regulated effectors, we hypothesize that the stoichiometry of signalling components and the extent of signal amplification described for the beta-adrenergic response pathway will be applicable to other G-protein-coupled hormone receptor systems.

Adrenergic desensitization in left ventricle from streptozotocin diabetic swine

Patients with diabetes mellitus that exhibit cardiac pump failure display compromised stroke volume, ejection fraction, and slower rates of rise and fall of left ventricular (LV) dP/dt in the absence of ischemic injury. We hypothesized that diabetic cardiomyopathy may involve decrements in adrenergic sensitivity, with specific molecular alterations in the beta-adrenergic receptor (beta AR)- G protein- adenylyl cyclase (AC) signal transduction system. We assessed the effects of 3 months of streptozotocin-induced diabetes (125 mg/kg i.v.; DIAB, n = 10) on myocardial signal transduction in mini-pigs. DIAB were hyperglycemic compared to controls (CON, n = 10; 20.92 +/- 2.64 v 5.24 +/- 0.35 mM glucose), and had lower fasting insulin levels (6.46 +/- 0.97 v 13.68 +/- 3.91 microU/ml). Transmural LV free wall homogenates from DIAB exhibited similar beta AR density as CON, but decreased cAMP production (pmol cAMP/mg prot.min) using these pharmacological stimulators: 10 microM Isoproterenol plus 100 microM GTP (74 +/- 5 v 97 +/- 11); 100 microM Gpp(NH)p (116 +/- 7 v 161 +/- 17); 10 mM fluoride ion (266 +/- 16 v 324 +/- 25). No differences between DIAB and CON were observed when stimulated by 100 microM forskolin (440 +/- 20 v 429 +/- 33), suggesting no alterations in the catalytic subunit of AC. In DIAB, quantitative immunoblotting indicated slightly depressed levels of Gs (552 +/- 44 v 630 +/- 59 pmol/g ww; NS), but a significant redistribution of alpha s from the sarcolemma to the cytosol (32.7 +/- 0.82% v 25.9 +/- 1.7%). Significantly elevated levels of cardiac Gi were seen in DIAB homogenates compared to CON ventricles (2326 +/- 145 v 1522 +/- 181 pmol/g ww), with no alpha i subunit redistribution. We conclude that despite maintained beta AR density, receptor-dependent and G protein-dependent stimulation of AC is depressed so that streptozotocin-induced diabetic LV is affected by increased cardiac Gi, redistribution of Gs alpha to the cytosol, and an increase in the Gi/Gs ratio. These results help explain depressed catecholamine responsiveness and cardiac performance exhibited by diabetic patients.

Reduced beta-adrenergic receptor activation decreases G-protein expression and beta-adrenergic receptor kinase activity in porcine heart

To determine whether beta-adrenergic receptor agonist activation influences guanosine 5'-triphosphate-binding protein (G-protein) expression and beta-adrenergic receptor kinase activity in the heart, we examined the effects of chronic beta 1-adrenergic receptor antagonist treatment (bisoprolol, 0.2 mg/kg per d i.v., 35 d) on components of the myocardial beta-adrenergic receptor-G-protein-adenylyl cyclase pathway in porcine myocardium. Three novel alterations in cardiac adrenergic signaling associated with chronic reduction in beta-adrenergic receptor agonist activation were found. First, there was coordinate downregulation of Gi alpha 2 and Gs alpha mRNA and protein expression in the left ventricle; reduced G-protein content was also found in the right atrium. Second, in the left ventricle, there was a twofold increase in beta-adrenergic receptor-dependent stimulation of adenylyl cyclase and a persistent high affinity state of the beta-adrenergic receptor. Finally, there was a reduction in left ventricular beta-adrenergic receptor kinase activity, suggesting a previously unrecognized association between the degree of adrenergic activation and myocardial beta-adrenergic receptor kinase expression. The heart appears to adapt in response to chronic beta-adrenergic receptor antagonist administration in a manner that would be expected to offset reduced agonist stimulation. The mechanisms for achieving this extend beyond beta-adrenergic receptor upregulation and include alterations in G-protein expression, beta-adrenergic receptor-Gs interaction, and myocardial beta-adrenergic receptor kinase activity.

Alterations in muscarinic K+ channel response to acetylcholine and to G protein-mediated activation in atrial myocytes isolated from failing human hearts

BACKGROUND

A variety of previous studies have demonstrated reduced diastolic potential and electrical activity in atrial specimens from patients with heart disease. Although K+ channels play a major role in determining resting membrane potential and repolarization of the action potential, little is known about the effects of preexisting heart disease on human atrial K+ channel activity.

METHODS AND RESULTS

We characterized the inwardly rectifying K+ channel (IKI) and the muscarinic K+ channel [IK(ACh)] in atrial myocytes isolated from patients with heart failure (HF) and compared electrophysiological characteristics with those from donors (control) by the patch-clamp technique. Resting membrane potentials of isolated atrial myocytes from HF were more depolarized (-51.1 +/- 9.7 mV, mean +/- SD, n = 30 patients) than those from donors (-73.0 +/- 7.2 mV, n = 4 patients, P < .001). The action potential duration in HF was longer than that in donors. Although acetylcholine (ACh) shortened the action potential, reduced the overshoot, and hyperpolarized the atrial cell membrane in HF, these effects were attenuated compared with those observed in donors. The whole-cell membrane current slope conductance in HF was small, the reversal potential was more positive, and the sensitivity to ACh was less compared with donors. In single-channel recordings from cell-attached patches, IK1 channel conductance and gating characteristics were the same in HF and donor atria. When ACh was included in the pipette solution, IK(ACh) was activated in both groups. Single-channel slope conductance of IK(ACh) averaged 42 +/- 3 pS (n = 28) in HF and 44 +/- 2 pS (n = 4) in donors, and mean open lifetime was 1.3 +/- 0.3 milliseconds (n = 24) in HF and 1.5 +/- 0.4 milliseconds (n = 4) in donors. These values were virtually identical in the two groups (not significantly different, NS), although both single IK1 and IK(ACh) channel densities were less in HF. Channel open probability of IK(ACh) was also less in HF (4.0 +/- 1.2%, n = 24) than in donors (6.8 +/- 1.1%, n = 3, P < .01). The concentration of ACh at half-maximal activation was 0.11 mumol/L in HF and 0.03 mumol/L in donors. In excised inside-out patches, IK(ACh) from HF required higher concentrations of GTP and GTP gamma S to activate the channel compared with donors. These results suggest a reduced IK(ACh) channel sensitivity to M2 cholinergic receptor-linked G protein (Gi) in HF compared with donors.

CONCLUSIONS

Atrial myocytes isolated from failing human hearts exhibited a lower resting membrane potential and reduced sensitivity to ACh compared with donor atria. Whole-cell and single-channel measurements suggest that these alterations are caused by reduced IK1 and IK(ACh) channel density and reduced IK(ACh) channel sensitivity to Gi-mediated channel activation in HF.

Effects of dobutamine and arbutamine on regional myocardial function in a porcine model of myocardial ischemia

OBJECTIVES

The present study was performed to determine the mechanisms for catecholamine-induced wall motion abnormalities and to compare the diagnostic efficacy of two catecholamines: arbutamine and dobutamine.

BACKGROUND

Catecholamine stress echocardiography is used to induce regional wall motion abnormalities for the detection of coronary artery disease, but the mechanism by which these abnormalities occur is unknown.

METHODS

Ten pigs were instrumented with left circumflex coronary artery ameroid constrictors, sonomicrometers to measure transmural wall thickening in the left circumflex (ischemic) and left anterior descending (control) coronary artery beds and a pressure gauge to measure left ventricular pressure and its first derivative (dP/dt). Myocardial blood flow was measured by microspheres.

RESULTS

At 38 +/- 6 days (mean +/- SEM) after surgery, percent wall thickening was normal at rest in both beds but abnormal in the left circumflex coronary artery bed during atrial pacing. These findings were associated with reduced myocardial blood flow in the ischemic bed during atrial pacing. Dobutamine infusion increased percent wall thickening, with no differences between the two beds (p = 0.63). In contrast, arbutamine infusion increased percent wall thickening only in the nonischemic bed, with no effect on percent wall thickening in the ischemic bed (p = 0.03). Although the endocardial/epicardial blood flow ratio tended to be reduced in the left circumflex artery bed during catecholamine infusion (p = 0.07), both agents were similar in this effect. Despite differences in function between the beds, there was no difference in transmural myocardial blood flow between the two beds during catecholamine infusion. When examined at matched metabolic demands, arbutamine elicited greater differences in percent wall thickening than dobutamine between the two beds (p < 0.01).

CONCLUSIONS

Arbutamine was able to provoke regional differences in function in a manner superior to dobutamine. This occurred independently of altered transmural myocardial blood flow or differences in hemodynamic effects between the agents. Differences in their inotropic properties may be important in explaining their different effects on ischemic myocardium.