Functional correlates of central arterial geometric phenotypes

We have assessed the functional correlates of common carotid artery (CCA) arterial geometry, derived by combining a measure of vascular mass (VM) with the wall-to-lumen (W/L) ratio in both untreated hypertensive (HT) and normotensive (NT; blood pressure <140/90 mm Hg) subjects of a broad age span (30 to 79 years) of both genders. Brachial systolic, diastolic, and pulse (SBP, DBP, PP) pressures; CCA SBP and PP; CCA diameter (D); intima-media thickness (IMT); relative distensibility; circumferential wall stress (MBPxW/L); fluid shear stress (FSS); strain; augmentation index (AGIh); and aortic pulse wave velocity (PWV) were measured in 680 NT and 635 untreated HT Taiwanese men and women. Carotid geometric phenotypes (CGPs) were derived from ultrasonographic measures of VM and W/L ratio. A normal CGP (CGP1) was defined as that within the 95th NT percentile of age- and gender-specific VM and W/L means. Three "deviant" CGPs were defined as follows: CGP2 or remodeling, ie, a normal VM coupled with an increased W/L; CGP3 or hypertrophy, ie, an increase in both VM and W/L; and CGP4 or hypertrophy with dilation, ie, an increased VM with normal W/L. The prevalence of specific CGPs in the total sample was 83.4% for CGP1, 5.5% for CGP2, 2.2% for CGP3, and 8.9% for CGP4. Compared with CGP1, all deviant CGPs had increased carotid resistance, had higher CCA circumferential wall stress, and varied in blood flow velocity. Compared with CGP1, CGP2 subjects were more likely to be women (69.3% versus 45.9%), were on average 10 years older, and had similar central and brachial BP levels, PWV, and AGIh but had increased strain, higher distensibility, lower flow, and a higher FSS. CGP3 subjects did not differ in age or gender but had a higher prevalence of HT; higher circumferential stress, PWV, and distensibility; and lower flow, as well as a trend toward higher SBP, PP, and AGIh and lower FSS. CGP4 subjects did not differ in age or gender but exhibited higher AGIh and aortic PWV, lower distensibility and FSS, and unchanged strain and flow. CGP4 was the only deviant CGP in which the average brachial or central arterial pressures were significantly increased. CGP4 subjects also had the highest prevalence of HT among all the CGPs (77.8% versus 45% in CGP1). CGPs exhibit some common mechanical or functional properties but each also exhibits a unique profile. Although differing quantitatively in NT and HT and at young and older age, the characteristic functional profile of a given CGP is preserved, regardless of age or BP status. A normal CGP is characterized by a low circumferential wall stress and high FSS. Each deviant CGP is characterized by a unique combination of increased circumferential wall stress, with variable FSS, strain, distensibility, central BP, and late pressure augmentation. The interplay among these factors, particularly circumferential wall and FSS, likely determines the CGP; conversely, the resultant CGP may modulate the FSS and wall stress for a given pressure and flow.

Measurement variation of aortic pulse wave velocity in the elderly

Accurate and reproducible measures are required to study arterial stiffness in human populations. The reproducibility of aortic pulse wave velocity was evaluated in 14 participants from a population-based study of cardiovascular disease in the elderly. Three data files were collected per participant by each of two sonographers and files were read by two readers. Seven of the 14 participants returned for a second visit 1 week later to assess between-visit variability. Reproducibility was evaluated with Pearson and intraclass correlations and by the absolute value of the difference between replicate values. The overall reliability coefficient was rI = 0.77. Between-sonographer, between-reader, and between-visit correlations were rP = 0.80 to 0.87, rP = 0.73 to 0.89 and rP = 0.63. The mean absolute value of the difference between replicates was 59.4 to 94.0 cm/sec and 88.7 to 112.8 cm/sec for sonographers and readers, respectively. These results indicate that the mean PWV measure is reproducible even when sonographers and readers are newly trained.

High-density lipoprotein cholesterol and vascular stiffness at baseline in the activity counseling trial

In a middle-aged patient population, age was associated with stiffer vessels and high-density lipoprotein cholesterol with more elastic vessels. High-density lipoprotein cholesterol may be an indirect indicator of aerobic capacity or of less atherosclerosis, suggesting mechanisms for preserving vascular integrity.

Cardiac synthesis, processing, and coronary release of enkephalin-related peptides

Although preproenkephalin mRNA is abundant in the heart, the myocardial synthesis and processing of proenkephalin is largely undefined. Isolated working rat hearts were perfused to determine the rate of myocardial proenkephalin synthesis, its processing into enkephalin-containing peptides, their subsequent release into the coronary arteries, and the influence of prior sympathectomy. Enkephalin-containing peptides were separated by gel filtration and quantified with antisera for specific COOH-terminal sequences. Proenkephalin, peptide B, and [Met(5)]enkephalin-Arg(6)-Phe(7) (MEAP) comprised 95% of the extracted myocardial enkephalins (35 pmol/g). Newly synthesized enkephalins, estimated during a 1-h perfusion with [(14)C]phenylalanine (4 pmol x h(-1) x g wet wt(-1)), were rapidly cleared from the heart during a second isotope-free hour. Despite a steady release of enkephalins into the coronary effluent (4 pmol x h(-1) x g wet wt(-1)), enkephalin replacement apparently exceeded its release, and tissue enkephalins actually accumulated during hour 2. In contrast to the tissue, methionine-enkephalin accounted for more than half of the released enkephalin. Chemical sympathectomy produced an increase in total enkephalin content similar to that observed after 2-h control perfusion. This observation suggested that the normal turnover of myocardial enkephalin may depend in part on continued sympathetic influences.

beta2-adrenergic cAMP signaling is uncoupled from phosphorylation of cytoplasmic proteins in canine heart

BACKGROUND

Recent studies of beta-adrenergic receptor (beta-AR) subtype signaling in in vitro preparations have raised doubts as to whether the cAMP/protein kinase A (PKA) signaling is activated in the same manner in response to beta2-AR versus beta1-AR stimulation.

METHODS AND RESULTS

The present study compared, in the intact dog, the magnitude and characteristics of chronotropic, inotropic, and lusitropic effects of cAMP accumulation, PKA activation, and PKA-dependent phosphorylation of key effector proteins in response to beta-AR subtype stimulation. In addition, many of these parameters and L-type Ca2+ current (ICa) were also measured in single canine ventricular myocytes. The results indicate that although the cAMP/PKA-dependent phosphorylation cascade activated by beta1-AR stimulation could explain the resultant modulation of cardiac function, substantial beta2-AR-mediated chronotropic, inotropic, and lusitropic responses occurred in the absence of PKA activation and phosphorylation of nonsarcolemmal proteins, including phospholamban, troponin I, C protein, and glycogen phosphorylase kinase. However, in single canine myocytes, we found that beta2-AR-stimulated increases in both ICa and contraction were abolished by PKA inhibition. Thus, the beta2-AR-directed cAMP/PKA signaling modulates sarcolemmal L-type Ca2+ channels but does not regulate PKA-dependent phosphorylation of cytoplasmic proteins.

CONCLUSIONS

These results indicate that the dissociation of beta2-AR signaling from cAMP regulatory systems is only apparent and that beta2-AR-stimulated cAMP/PKA signaling is uncoupled from phosphorylation of nonsarcolemmal regulatory proteins involved in excitation-contraction coupling.

Opposing effects of alpha 1-adrenergic receptor subtypes on Ca2+ and pH homeostasis in rat cardiac myocytes

We examined the effect of alpha 1-adrenergic receptor (AR) subtypes on contraction, cytosolic Ca2+ concentration ([Ca2+]i), and cytosolic pH (pHi) of rat ventricular myocytes loaded with the Ca2+ indicator indo 1 or the pH indicator carboxyseminaphthorhodafluor-1. Nonselective alpha 1-AR stimulation was effected with phenylephrine plus nadolol. alpha 1-AR subtype stimulation was achieved with alpha 1-AR and chloroethylclonidine (CEC) or with alpha 1-AR and WB-4101. Cells were in bicarbonate buffer with 0.5 mM Ca2+ and were electrically stimulated at 0.5 Hz. Results show that 1) nonselective alpha 1-AR stimulation increased twitch and [Ca2+]i transient amplitudes, myofilament response to Ca2+, and pHi; 2) alpha 1-AR plus CEC increased twitch and [Ca2+]i transient amplitudes and also enhanced myofilament response to Ca2+ via cytosolic alkalinization; 3) alpha 1-AR plus WB-4101 decreased twitch and [Ca2+]i transient amplitudes and also pHi; and 4) cytosolic acidification due to alpha 1-AR plus WB-4101 was abolished by protein kinase C inhibition (staurosporine pretreatment) or downregulation (prolonged exposure to phorbol esters). In summary, the net effects of alpha 1-adrenergic stimulation on contraction, [Ca2+]i, and pHi are due to opposing WB-4101- and CEC-sensitive alpha 1-AR subtype signaling pathways.

Modulation of the transient outward current in adult rat ventricular myocytes by polyunsaturated fatty acids

With the whole cell patch-clamp technique, we studied the effects of the n-3 and n-6 polyunsaturated fatty acids (PUFAs), linoleic (C18:2n-6), eicosapentaenoic (C20:4n-3), docosahexaenoic (C22:5n-3), and arachidonic (AA; C20:4n-6) acids, on K+ currents in rat ventricular myocytes. At low concentrations (5-10 microM) all PUFAs except AA inhibited, by approximately 40%, the transient outward current (I(to)) without affecting other K+ currents and markedly prolonged the action potential (AP). AA inhibited I(to) but also augmented a sustained depolarization-induced outward K+ current (Isus); the latter effect did not occur in the presence of 4-aminopyridine or with eicosatetraynoic acid, a nonmetabolizable analog of AA. Higher concentrations of PUFAs (20-50 microM) further inhibited I(to) and also inhibited Isus. Thus, at high concentrations, PUFAs have a nonspecific effect on several K+ channels; at low concentrations, PUFAs preferentially inhibit I(to) and prolong the AP.

Opioid peptide receptor stimulation reverses beta-adrenergic effects in rat heart cells

Opioid peptide receptor (OPR) agonists are co-released with the beta-adrenergic receptor (beta-AR) agonist norepinephrine (NE) from nerve terminals in the heart during sympathetic stimulation. Whereas recent studies indicate that OPR and beta-AR coexist on the surface of cardiac myocytes, whether significant "cross talk" occurs between OPR and beta-AR signaling cascades within heart cells is unknown. In the present study we demonstrate a marked effect of delta-OPR stimulation to modulate beta-adrenergic responses in single isolated rat ventricular myocytes. Nanomolar concentrations (10(-8) M) of the OPR agonist leucine enkephalin (LE), a naturally occurring delta-opioid peptide, inhibited NE-induced increases in sarcolemmal L-type Ca2+ current, cytosolic Ca2+ transient, and contraction. The antiadrenergic effect of LE was pertussis toxin sensitive and abolished by naloxone, an opioid receptor antagonist. In contrast, LE was unable to inhibit the positive inotropic effects induced by equipotent concentrations of 8-(4 chlorophenylthio)-adenosine 3',5'-cyclic monophosphate, a cell-permeant adenosine 3',5'-cyclic monophosphate analog, or by the non-receptor-induced increase in contraction by elevated bathing Ca2+ concentration. These results indicate that an interaction of the OPR and beta-AR systems occurs proximal to activation of the adenosine 3',5'-cyclic monophosphate-dependent protein kinase of the beta-AR intracellular signaling pathway. This modulation of beta-adrenergic effects by OPR activation at the myocyte level may have important implications in the regulation of cardiac Ca2+ metabolism and contractility, particularly during the myocardial response to stress.

Initial contractile response of isolated rat heart cells to halothane, enflurane, and isoflurane

BACKGROUND

In several beating cardiac muscle preparations, a short-lived increase in twitch tension or amplitude has been observed when they were exposed abruptly to solutions containing halothane or enflurane. As exposure to the anesthetics was continued, the expected negative inotropic effect became evident after the short-lived increase in twitch. No such increase in twitch has been reported during exposure to isoflurane. It has been hypothesized that this short-lived increase in twitch is caused by an enhancement of calcium release from the sarcoplasmic reticulum, but other mechanisms have not been excluded.

METHODS

Freshly isolated, single rat ventricular cells were stimulated to beat at room temperature and abruptly exposed to solutions containing halothane (0.25-0.64 mM), enflurane (0.69-1 mM), or isoflurane (0.31-0.54 mM). During these exposures, twitch amplitude was measured and intracellular calcium concentration was followed using the calcium-sensitive dye indo-1. In some experiments, the whole-cell patch-clamp technique was used to measure membrane current. In addition, in several cells the sarcoplasmic reticulum calcium content was assessed through the response to brief pulses of caffeine.

RESULTS

Both the twitch amplitude and the intracellular calcium transient were increased temporarily in cells abruptly exposed to halothane or enflurane. No such behavior was found with isoflurane. After continued exposure to all three agents, both the twitch amplitude and the calcium transient were less than control. During the beats exhibiting an increase in twitch, no alteration in the relation between cell length (twitch amplitude) and the intracellular calcium transient was found compared with control conditions. In addition, the temporary increase in twitch amplitude occurred in cells contracting under voltage-clamp control when halothane was introduced, and it was not associated with any increase in the calcium current. The sarcoplasmic reticulum calcium content at the time of the halothane-induced increase in twitch also was not increased.

CONCLUSIONS

The short-lived increase in twitch after abrupt exposure to halothane or enflurane is related to increased intracellular calcium during the beat and not to any changes in myofilament sensitivity to calcium. Because these results eliminate most alternative explanations for this phenomenon, the authors conclude that halothane, and probably also enflurane, increases the fraction of calcium released from the sarcoplasmic reticulum with each heart beat. Isoflurane appears to lack this action.

Actomyosin interaction modulates resting length of unstimulated cardiac ventricular cells

We sought to determine whether resting or diastolic cardiac myocyte length during low stimulation rates is regulated by myofilament interaction. Cytosolic Ca2+ concentration ([Ca2+]i, via indo 1 fluorescence) and length, in the presence and absence of 2,3-butanedione monoxime (BDM), a potent inhibitor of force production in striated muscle, were measured in rat and guinea pig cardiac myocytes at rest and after electrical stimulation. In tetanized cells BDM reduced steady contraction amplitudes for a given [Ca2+]i. In an actomyosin-sliding filament assay without Ca2+ or regulatory proteins, BDM decreased actin filament velocity along myosin. BDM increased both diastolic and resting cell lengths without changes in [Ca2+]i. The resting cell length also increased when [Ca2+]i was reduced by removing extracellular Ca2+, an effect further enhanced by BDM and by loading cells with the intracellular Ca2+ chelator, 1,2-bis(2-amino-phenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethylester. Thus myofilament interaction is present in cardiac cells, both at rest or during low rates of stimulation, and this myofilament interaction is regulated, in part, by the ambient [Ca2+]i.

L- and T-type calcium currents differ in finch and rat ventricular cardiomyocytes

We compared L-type Ca current (ICaL) and T-type Ca current (ICaT) in finch and rat myocytes, using whole-cell patch clamp techniques. Cell capacitance averaged 50 +/- 4 pF in finch (n = 25) v 145 +/- 8 pF in rat (n = 38) cells, P < 0.001. In cells bathed in 1 mM Cao at 22 degrees C, peak ICaL amplitude, during a voltage clamp step (10 mM EGTA in pipette) from -45 mV to -5 mV, averaged 10.5 +/- 0.3 pA/pF in finch v 6.9 +/- 0.6 pA/pF, P < 0.001 in rat cells. ICaL inactivation kinetics were faster in finch (4.6 +/- 0.3 ms) than in rat (13.4 +/- 1.3 ms) cells. P < 0.001. ICaT was not detectable in rat cells (2 mM bathing [Ca]); but in finch cells, a large ICaT which averaged 6.8 +/- 1.4 pA/pF was activated at -30 mV and was relatively insensitive to nitrendipine (0.1 microM). The distinctive features of ICaL and ICaT in finch cells may have a role in the ability of the finch to achieve a very rapid heart rate. They may also facilitate excitation-Ca2+ release coupling in finch ventricular cells which are devoid of T tubules and have relatively few junctions between the sarcolemma and the sarcoplasmic reticulum.

Endoplasmic reticulum Ca2+ depletion unmasks a caffeine-induced Ca2+ influx in human aortic endothelial cells

Intracellular Ca2+ pools contribute to changes in cytosolic [Ca2+] ([Ca2+]i), which play an important role in endothelial cell signaling. Recently, endothelial ryanodine-sensitive Ca2+ stores were shown to regulate agonist-sensitive intracellular Ca2+ pools. Since caffeine binds the ryanodine Ca2+ release channel on the endoplasmic reticulum in a variety of cell types, we examined the effect of caffeine on [Ca2+]i in human aortic endothelial cell monolayers loaded with the fluorescent probe indo 1. Under baseline conditions, 10 mmol/L caffeine induced a small increase in [Ca2+]i from 86 +/- 10 to 115 +/- 17 nmol/L (mean +/- SEM); this effect was similar to that of 5 mumol/L ryanodine and was unaffected by buffer Ca2+ removal. After depletion of an intracellular Ca2+ store by the irreversible endoplasmic reticulum Ca(2+)-ATPase inhibitor thapsigargin (1 mumol/L), ryanodine did not affect [Ca2+]i. In contrast, caffeine induced a large rapid increase in [Ca2+]i (176 +/- 19 to 338 +/- 35 nmol/L, P < .001) after thapsigargin exposure; this effect of caffeine was only observed when extracellular Ca2+ was present. A similar increase in [Ca2+]i was induced by caffeine after depletion of ryanodine- and histamine-sensitive Ca2+ stores or after pretreatment with the endoplasmic reticulum Ca(2+)-ATPase inhibitor cyclopiazonic acid (10 mumol/L). Thus, under baseline conditions the effect of caffeine on [Ca2+]i is similar to that of ryanodine and appears to be due to the release of an intracellular store. However, after depletion of an endoplasmic reticulum Ca2+ store, caffeine, but not ryanodine, stimulates Ca2+ influx, resulting in a large increase in [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduced threshold for myocardial cell calcium intolerance in the rat heart with aging

To determine whether advancing age is accompanied by a reduced Ca2+ tolerance, we measured Ca(2+)-dependent diastolic pressure, prolonged relaxation and systolic functional deterioration, spontaneous sarcoplasmic reticulum (SR)-generated Ca2+ oscillations [detected as scattered laser light intensity fluctuations (SLIF)], aftercontractions, and ventricular fibrillation in isolated, isovolumic, atrioventricular-blocked intact hearts from 24- to 26-mo (old) and 6- to 8-mo (young) male Wistar rats. In enzymatically isolated single cardiac myocytes, the likelihood of the occurrence of spontaneous contractile waves driven by spontaneous SR Ca2+ release was also determined. In response to stepwise increase in perfusate Ca2+ concentration (Cao), a reduction in the maximum developed pressure accompanied by an elevation in end-diastolic pressure and a prolonged contraction duration was observed at lower Cao in old vs. young hearts (P < 0.01 for each parameter). Furthermore, Ca(2+)-dependent ventricular fibrillation occurred during pacing in six old but in no young hearts (P < 0.01), aftercontractions were observed in seven old vs. one young heart (P < 0.01), and SLIF increased to a greater extent in old vs. young hearts. In single cardiac myocytes, spontaneous contractile waves occurred more frequently with increasing age (P < 0.01). These results indicate that aging is associated with an increased likelihood for the occurrence of SR-generated Ca2+ oscillations and functional abnormalities that result from these oscillations.

cGMP prevents delayed relaxation at reoxygenation after brief hypoxia in isolated cardiac myocytes

Previous studies in isolated cardiac myocytes suggest that impaired relaxation during reoxygenation after brief hypoxia results from abnormal Ca(2+)-myofilament interaction. Recent studies indicate that guanosine 3',5'-cyclic monophosphate (cGMP)-elevating interventions selectively enhance myocardial relaxation. We investigated the effect of 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP) on posthypoxic relaxation in single rat myocytes, with simultaneous measurement of contraction and intracellular Ca2+ (indo 1 fluorescence). In control myocytes (n = 11), reoxygenation after 10 min of hypoxia markedly prolonged time to peak shortening (+36.5 +/- 4.2%) and half-relaxation time (+75.7 +/- 11.3% cf. normoxic values; both P < 0.001) and reduced diastolic length but did not change cytosolic Ca2+. Under normoxic conditions, 50 microM 8-BrcGMP slightly reduced time to peak shortening and half-relaxation time and increased diastolic length but did not alter cytosolic Ca2+. In the presence of 8-BrcGMP, there was no posthypoxic delay in twitch relaxation nor was there a decrease in diastolic length (half-relaxation time -5.8 +/- 3.3% cf. normoxic values; P < 0.05 cf. control group; n = 11). Cytosolic Ca2+ remained unaltered. Thus, 8-BrcGMP fully prevents impaired posthypoxic relaxation in isolated cardiac myocytes, probably by altering Ca(2+)-myofilament interaction.

Effects of sarcoplasmic reticulum Ca2+ load on the gain function of Ca2+ release by Ca2+ current in cardiac cells

We studied the effects of variable sarcoplasmic reticulum (SR) Ca2+ loading on changes in the gain index of Ca2+ release from the SR, measured as the ratio of the amount of Ca2+ released to the magnitude of the Ca2+ current (ICa) integrated for the initial 20 ms of the depolarization, in whole cell voltage-clamped rat ventricular myocytes dialyzed with the Ca2+ indicator indo 1 salt at 23 degrees C. Changes in ICa were measured directly, and changes in the SR Ca2+ release were indexed by changes in the amplitudes and rates of rise of cytosolic Ca2+ (Ca2+i) transients. The SR Ca2+ load was graded by the duration of conditioning voltage-clamp steps and verified by caffeine-dependent Ca2+i transients. A train of abbreviated (from 100 to 20 ms) voltage-clamp depolarizations, which triggers SR Ca2+ release but fails to replenish the SR with Ca2+, diminished the SR Ca2+ load by 56 +/- 5%, did not alter peak ICa but reduced the amplitudes of the ICa-dependent Ca2+i transients by 52 +/- 3%, and decreased the gain index by 60 +/- 3% (SE; n = 5 or 6). Changes in the amplitudes of Ca2+i transients elicited by ICa and changes in the gain index were linearly correlated (r2 = 0.83 and 0.79, respectively; P < 0.001 for each) with changes in amplitudes of Ca2+i transients elicited by caffeine pulses applied in lieu of the respective voltage-clamp pulses.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-associated changes in beta-adrenergic modulation on rat cardiac excitation-contraction coupling

Previous studies have demonstrated that the ability of beta-adrenergic receptor (beta AR) stimulation to increase cardiac contractility declines with aging. In the present study, the control mechanisms of excitation-contraction (EC) coupling, including calcium current (ICa), cytosolic Ca2+ (Cai2+) transient and contraction in response to beta AR stimulation were investigated in ventricular myocytes isolated from rat hearts of a broad age range (2, 6-8, and 24 mo). While the baseline contractile performance and the Cai2+ transient did not differ markedly among cells from hearts of all age groups, the responses of the Cai2+ transient and contraction to beta-adrenergic stimulation by norepinephrine (NE) diminished with aging: the threshold concentration and the ED50 increased in rank order with aging; the maximum responses of contraction and Cai2+ transient decreased with aging. Furthermore, the efficacy of beta AR stimulation to increase ICa was significantly reduced with aging, and the diminished responses of the contraction and Cai2+ transient amplitudes to NE were proportional to the reductions in the ICa response. These findings suggest that the observed age-associated reduction in beta AR modulation of the cardiac contraction is, in part at least, due to a deficit in modulation of Cai2+, particularly the activity of L-type calcium channels.

8-bromo-cGMP reduces the myofilament response to Ca2+ in intact cardiac myocytes

The role of cGMP in myocardial contraction is not established. Recent reports suggest that nitric oxide, released by endothelial cells or within myocytes, modifies myocardial contraction by raising cGMP. We studied the effects of 8-bromo-cGMP (8bcGMP, 50 mumol/L) on contraction (cell shortening) and simultaneous intracellular Ca2+ transients (indo 1 fluorescence ratio) in intact adult rat ventricular myocytes (0.5 Hz and 25 degrees C) 8bcGMP reduced myocyte twitch amplitude and time to peak shortening (-19.6 +/- 4.2% and -17.6 +/- 1.3%, respectively) and increased steady-state diastolic cell length (+0.6 +/- 0.1 microns, mean +/- SEM, n = 8; all P < .05) but had no effect on shortening velocity, systolic or diastolic fluorescence ratio, or time to peak fluorescence ratio (all P = NS). In 7 of 13 myocytes, this negative inotropic effect was preceded by a transient positive inotropic effect, with small increases in twitch amplitude, shortening velocity, and cytosolic Ca2+ transient. Analysis of 8bcGMP effects on both the dynamic and steady-state relation between cell shortening and intracellular Ca2+ (during twitch contraction and tetanic contraction, respectively) indicated reduction in the myofilament response to Ca2+ in all cases. These 8bcGMP effects were inhibited by KT5823 (1 mumol/L), an inhibitor of cGMP-dependent protein kinase, or by the presence of isoproterenol (3 nmol/L). 8bcGMP had no effect on cytosolic pH in cells (n = 4) loaded with the fluorescent probe carboxyseminaphthorhodafluor-1. These data indicate that cGMP may modulate myocardial relaxation and diastolic tone by reducing the relative myofilament response to Ca2+, probably via cGMP-dependent protein kinase.

A functional ryanodine-sensitive intracellular Ca2+ store is present in vascular endothelial cells

The presence of the ryanodine receptor was recently demonstrated in vascular and endocardial endothelium, but its function has not been established. We investigated whether functional ryanodine-sensitive Ca2+ stores are present in cultured endothelial cells from rat aorta (RAECs), human aorta (HAECs), human umbilical vein (HUVECs), and bovine pulmonary artery (BPAECs) and what role these may play in intracellular Ca2+ regulation. Under resting conditions, HAECs, BPAECs, and HUVECs demonstrated a slow increase in intracellular Ca2+ (indexed by indo 1 fluorescence) on exposure to 5 mumol/L ryanodine, whereas RAECs did not. However, after an initial bradykinin exposure in RAECs, ryanodine markedly blunted the rapid increase in Ca2+ on a second exposure to bradykinin. In HUVECs, ryanodine in buffer with 1.5 mmol/L Ca2+ did not inhibit the agonist-sensitive Ca2+ increase, whereas it blunted the rapid increase in Ca2+ on histamine exposure in buffer with 5 mmol/L Ca2+, suggesting that increasing [Ca2+] enhances the binding of ryanodine to its receptor. Thus, functional ryanodine-sensitive Ca2+ stores are present in vascular endothelial cells. These appear to be involved in regulation of Ca2+ storage and release from agonist-sensitive intracellular compartments.

Stereoselective actions of thiadiazinones on canine cardiac myocytes and myofilaments

Thiadiazinones are cardiotonic agents that have potent, direct, and stereoselective actions on the myofilament response to Ca2+ in intact myocardium. Their mechanism of action is unknown. We studied the effects of racemic thiadiazinone, EMD 53998 (5-[1-(3,4-dimethoxybenzoyl)-1,2,3,4-tetrahydro-6-quinolyl]-6-meth yl-3,6- dihydro-2H-1,3,4-thiadiazin-2-one), and its enantiomers on Ca2+ signaling in myocytes, myofilaments, and myofilament proteins. Intact canine ventricular myocytes responded to the positive enantiomer, EMD 57033, with an increase in the extent of shortening during twitch contractions without increasing the peak amplitude of the Ca2+ transient. The negative enantiomer, EMD 57439, also increased the extent of shortening, but in this case there was a concentration-dependent increase in the peak amplitude of the Ca2+ transient. This is predicted from in vitro data showing that this enantiomer is a relatively potent inhibitor of phosphodiesterase activity. There was no effect of EMD 57439 on the relation between pCa and actomyosin Mg-ATPase activity of canine heart myofibrils. In contrast, EMD 57033 shifted the pCa-Mg-ATPase activity relation to the left. There was no effect of either enantiomer on Ca2+ binding to myofilament troponin C. Moreover EMD 57033, but not EMD 57439, stimulated actomyosin ATPase activity of myofilament preparations in which either troponin or troponin-tropomyosin had been extracted. EMD 57033 had no effect on Mg-ATPase activity of pure ventricular myosin. EMD 57033 also stimulated the velocity of actin filament sliding on myosin heads adhered to nitrocellulose-coated glass coverslips. We propose that the action of EMD 57033 is at the actin-myosin interface on a "receptor" that may be on actin or the crossbridge. Drug binding to this domain appears to reverse the inhibition of actin-myosin interactions by troponin-tropomyosin and also to promote transition of crossbridges from weak to strong force-generating states.

Pathological characterization of male Wistar rats from the gerontology research center

Male Wistar rats aged 6-26 months were obtained from the colony of The Gerontology Research Center of the National Institute on Aging, and pathological profiles were assessed. One hundred animals were sacrificed at 6, 12, 18, 21, 24, and 26 months and used for cross-sectional determinations; also, 150 animals were followed longitudinally and sacrificed when clinical signs of moribundity appeared. Renal disease contributed the most common pathology observed in both studies (found in over 70% of the animals examined), with neoplasms a secondary problem (pituitary tumors were by far the most prevalent, with adenomas present in approximately 20% of the animals). This analysis represents the first complete pathological characterization of this commonly used rat model for aging research, and offers an opportunity for comparison with other rat strains.

Modulation of calcium homeostasis in cultured rat aortic endothelial cells by intracellular acidification

Acidosis produces vasodilation in a process that may involve the vascular endothelium. Because synthesis and release of endothelium-derived vasodilatory substances are linked to an increase in cytosolic calcium concentration ([Ca2+]i), we examined the effect of intracellular acidification on cultured rat aortic endothelial cells loaded either with the pH-sensitive probe carboxy-seminaphthorhodafluor-1 or the Ca(2+)-sensitive fluorescent probe indo 1. The basal cytosolic pH (pHi) of endothelial monolayers in a 5% CO2-HCO3- buffer was 7.27 +/- 0.02 and that in a bicarbonate-free solution was 7.22 +/- 0.03. Acidification was induced either by removal of NH4Cl (delta pHi = -0.10 +/- 0.02), changing from a bicarbonate-free to a 5% CO2-HCO3(-)-buffered solution at constant buffer pH (delta pHi = -0.18 +/- 0.03), or changing from a 5% to a 20% CO2-HCO3- solution (delta pHi = -0.27 +/- 0.07). Regardless of the method used, intracellular acidification increased [Ca2+]i as indexed by indo 1 fluorescence. The increase in [Ca2+]i induced by changing from a 5 to a 20% CO2-HCO3- solution was not significantly altered by removal of buffer Ca2+ either before or after depletion of bradykinin- and thapsigargin-sensitive intracellular Ca2+ stores. Thus intracellular acidification of vascular endothelial cells releases Ca2+ into the cytosol either from pH-sensitive intracellular buffer sites, mitochondria, or from bradykinin- and thapsigargin-insensitive intracellular stores. This Ca2+ mobilization may be linked to endothelial synthesis and release of vasodilatory substances during acidosis.

Repletion of sarcoplasmic reticulum Ca after ryanodine in rat ventricular myocytes

The ryanodine (R)-induced loss of sarcoplasmic reticulum (SR) Ca2+ and the abilities of the SR to accumulate Ca2+ and participate in contractile activation after R were studied in rat ventricular myocytes. Indo 1 fluorescence (IF) indexed cytosolic Ca2+, and caffeine assayed SR Ca2+. Before R, there was a negative staircase, and the SR accumulated Ca2+ at rest. During stimulation (0.5 Hz), R decreased IF and contraction, converting the negative staircase to positive. When R was pulsed onto resting cells, IF increased and cells shortened, subsequently behaving as if stimulated in R. After R, there was no caffeine-releasable Ca2+ at rest, and little accumulated during 0.5-Hz stimulation. At high rates, caffeine-releasable Ca2+ and diastolic IF increased. In isoproterenol and R, IF transients and contractions recovered at 0.5 Hz with a marked positive staircase and little diastolic IF increase. Within 10 beats, SR Ca2+ accumulated to pre-R levels. R eliminated the positive inotropic effect of paired-pulse stimulation, but isoproterenol temporarily restored it. Twitch contractions in thapsigargin, an SR Ca2+ pump blocker, and isoproterenol were slow compared with control or R + isoproterenol. R leaks SR Ca2+ into the cytosol. SR Ca2+ can be repleted in R by high-rate stimulation or by low-rate stimulation with a beta-adrenergic agonist. SR Ca2+ release in R can be temporarily restored if Ca2+ influx and SR Ca2+ pumping are increased enough to overcome the SR Ca2+ leak.

Stimulation of opioid receptors on cardiac ventricular myocytes reduces L type Ca2+ channel current

Recent studies have indicated that opioid peptide receptors are present on cardiac ventricular cells and that Leucine enkephalin (LE), a naturally occurring delta opioid peptide receptor agonist, leads to marked reductions in twitch amplitude and in the cytosolic Ca2+ transient (Ca(i)) of single adult rat ventricular myocytes. The specific mechanisms by which Ca(i) is reduced by LE have not been fully elucidated. Specifically, it is unknown whether LE affects the Ca2+ current (ICa) of L type Ca2+ channels. In the present study we determined the effect of LE on ICa of individual cardiac ventricular cells freshly isolated from adult rats. LE (10(-8) M) decreased the amplitude of ICa by 40% (during regular whole cell voltage clamp depolarizations to 0 mV at 0.5 Hz at 23 degrees C from a holding potential of -40 mV). The relative magnitude of this effect increased with the magnitude of the test potential from -20 to +50 mV. ICa inactivation was also prolonged by LE. These effects of LE on ICa were abolished by Naloxone (NAL), an opioid receptor antagonist. Thus, the effects of the opioid peptide, LE, to decrease the Ca(i) transient and contraction amplitudes in individual cardiac ventricular cells, are, in part, mediated by an LE induced reduction in ICa.

Different effects of alpha- and beta-adrenergic stimulation on cytosolic pH and myofilament responsiveness to Ca2+ in cardiac myocytes

alpha-Adrenergic stimulation (alpha-AS) and beta-adrenergic stimulation (beta-AS) of the myocardium are associated respectively with an increase and a decrease in myofilament responsiveness to Ca2+. We hypothesized that changes in cytosolic pH (pH(i)) may modulate these opposite actions of alpha-AS and beta-AS. The effects of alpha-AS (50 microM phenylephrine and 1 microM nadolol) and beta-AS (0.05 microM isoproterenol) on contraction and either cytosolic Ca2+ (Cai) or pH(i) were assessed in adult rat ventricular myocytes bathed in bicarbonate buffer (pH 7.36 +/- 0.05). In cells loaded with the ester derivative (AM form) of indo-1, the 410/490-nm ratio of emitted fluorescence indexed Cai. Myofilament responsiveness to Ca2+ was assessed by the relaxation phase of the length-indo-1 fluorescence relation during a twitch. alpha-AS and beta-AS shifted this relation in opposite directions, indicating that alpha-AS increased and beta-AS decreased myofilament responsiveness to Ca2+. In addition, the positive inotropic action of alpha-AS was associated with an increased Cai transient amplitude in 50% of the myocytes (n = 12), whereas beta-AS always increased Cai (n = 5). In cells loaded with the fluorescent pH(i) probe SNARF-1 AM, the emitted 590/640-nm fluorescence is a measure of pH(i). The effect of alpha-AS on the extent of cell shortening during the twitch (ES) was expressed as the percentage of resting cell length. Both ES and pH(i) were assessed in myocytes bathed in 1.5 mM [Ca2+] and stimulated at 0.5 Hz (control ES, 7.4 +/- 1.5%; control pH(i), 7.11 +/- 0.05; n = 10). alpha-AS enhanced both ES (delta ES, 1.8 +/- 0.6%; p less than 0.05) and pH(i) (delta pH(i), 0.06 +/- 0.01; p less than 0.005), and there was a significant correlation between delta ES and delta pH(i) (r = 0.76, p less than 0.05). A similar effect of alpha-AS on pH(i) was observed in the absence of electrical stimulation (n = 8). The alpha-AS-induced enhancement of ES and pH(i) was abolished by 10 microM ethylisopropylamiloride, a Na(+)-H+ exchange inhibitor (n = 7). In additional experiments, myocytes were preincubated either with 0.2 microM 4 beta-phorbol 12-myristate 13-acetate (n = 8) or with 5 nM staurosporine (n = 8), which have been shown to downregulate and inhibit Ca(2+)-activated phospholipid-dependent protein kinase C, respectively. In either group, alpha-AS had no effect on pH(i) and decreased ES to approximately 60% of control.(ABSTRACT TRUNCATED AT 400 WORDS)

Novel diazinone derivatives separate myofilament Ca2+ sensitization and phosphodiesterase III inhibitory effects in guinea pig myocardium

The inotropic state of the myocardium can be enhanced via an increase in cell Ca2+ loading or in myofilament responsiveness to Ca2+. Although different pharmacological agents combine these properties, no presently available drug acts predominantly as a myofilament sensitizer in situ. We have investigated the effects and the mechanism of action of novel diazinone derivatives, EMD 54622, EMD 53998, and EMD 54650 (developed by E. Merck, Darmstadt), on guinea pig myocardial preparations. Force- and ATPase-pCa relations in skinned fibers show differing potencies of these agents on myofilament sensitization: EMD 54622 greater than EMD 53998 much greater than EMD 54650. This is in contrast to their relative potencies to inhibit isolated myocardial phosphodiesterase III: EMD 54650 greater than EMD 53998 greater than EMD 54622. In isolated hearts studied at constant coronary flow, each of the three diazinone derivatives had a positive inotropic effect. In enzymatically dissociated left ventricular myocytes loaded with the Ca2+ probe indo-1, the positive inotropic effect of EMD 54622 occurred with no change in the amplitude of the cytosolic [Ca2+] (Cai) transient. In contrast, both EMD 53998 and EMD 54650 enhanced Cai transient and twitch contraction amplitudes. Length-indo-1 fluorescence relations were analyzed to determine the effects of the three substances on myofilament responsiveness to Ca2+. EMD 54622 enhanced and EMD 54650 had no effect on myofilament responsiveness to Ca2+. Less uniform results were obtained with EMD 53998 (in two of five cells the myofilament responsiveness to Ca2+ was increased, whereas in three other cells it was unaltered). Our results indicate that structural changes in the diazinone molecule shift the mechanism of action for the positive inotropic effect of the diazinone derivatives in the intact cell from a predominant myofilament sensitization (EMD 54622) to an enhancement in cell Ca2+ loading and an augmentation in the Cai transient (EMD 54650).

Contractile response of individual cardiac myocytes to norepinephrine declines with senescence

The present study utilized individual isolated left ventricular cardiac myocytes from hearts of animals of a broad age range to evaluate the response to norepinephrine and to other stimuli that augment myocardial cell contractile performance. During electrical stimulation before drugs neither the amplitude nor the velocity of shortening normalized for resting cell length differed among cells isolated from 2-, 6- to 8-, or 24-mo-old animals. Norepinephrine augmented twitch amplitude and velocity about fourfold in cells from 2-mo-old hearts but only by 2.5-fold in cells from 24-mo-old hearts (age effect, P less than 0.001). In contrast, the contractile response to increases in bathing [Ca2+] or to the addition of the calcium channel agonist BAY K 8644 or of 8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphate (CPT cAMP) did not vary with age. These results indicate that the age-associated contractile deficit during beta-adrenergic stimulation is specific to the beta-adrenergic pathway and an age-associated deficit in the net production of cAMP. This can be attributed to a diminished cardiac myocyte response to beta-adrenergic agonists, in contrast to modulation of the beta-adrenergic response by other receptor agonists, which are present in intact tissue but absent under the conditions of the present study.

Cytosolic calcium and myofilaments in single rat cardiac myocytes achieve a dynamic equilibrium during twitch relaxation

1. Single isolated rat cardiac myocytes were loaded with either the pentapotassium salt form or the acetoxymethyl ester (AM) form of the calcium-sensitive fluorescent probe, Indo-1. The relationship of the Indo-1 fluorescence transient, an index of the change in cytosolic calcium [Ca2+]i concentration, to the simultaneously measured cell length during the electrically stimulated twitch originating from slack length at 23 degrees C was evaluated. It was demonstrated that even if the Ca2+ dissociation rate from Indo-1 was assumed to be as slow as 10 s-1, the descending limb ('relaxation phase') of the Indo-1 fluorescence transient induced by excitation under these conditions is in equilibrium with the [Ca2+]i transient. Additionally, the extent of Indo-1 loading employed did not substantially alter the twitch characteristics. 2. A unique relationship between the fluorescence transient and cell length was observed during relaxation of contractions that varied in amplitude. This was manifest as a common trajectory in the cell length vs. [Ca2+]i phase-plane diagrams beginning at the time of cell relengthening. The common trajectory could also be demonstrated in Indo-1 AM-loaded cells. The Indo-1 fluorescence-length relation defined by this common trajectory is steeper than that described by the relation of peak contraction amplitude and peak fluorescence during the twitch contractions. 3. The trajectory of the [Ca2+]i-length relation elicited via an abrupt, rapid, brief (200 ms) pulse of caffeine directly onto the cell surface or by 'tetanization' of cells in the presence of ryanodine is identical to the common [Ca2+]i-length trajectory formed by electrically stimulated contractions of different magnitudes. As the [Ca2+]i and length transients induced by caffeine application or during tetanization in the presence of ryanodine develop with a much slower time course than those elicited by electrical stimulation, the common trajectory is not fortuitous, i.e. it cannot be attributed to equivalent rate-limiting steps for the decrease of [Ca2+]i and cell relengthening. 4. The [Ca2+]i-length relation defined by the common trajectory shifts appropriately in response to perturbations that have previously been demonstrated to alter the steady-state myofilament Ca2+ sensitivity in skinned cardiac fibres. Specifically, the trajectory shifts leftward in response to an acute increase in pH or following the addition of novel myofilament calcium-sensitizing thiadiazinone derivatives; a rightward shift occurs in response to an acute reduction in pH or following the addition of butanedione monoxime.(ABSTRACT TRUNCATED AT 400 WORDS)

Spontaneous sarcoplasmic reticulum Ca2+ release leads to heterogeneity of contractile and electrical properties of the heart

The cytosolic Ca2+ (Cai) oscillation generated by the sarcoplasmic reticulum (SR) in response to an action potential (AP) occurs relatively synchronously within and among cells. The SR can also generate spontaneous Cai oscillations (S-CaOs), i.e., not triggered by sarcolemmal depolarization. The local increase in Cai due to S-CaOs is equivalent to that induced by an AP. Heterogeneity of diastolic Cai caused by asynchronous S-CaOs among cells within myocardial tissue leads to heterogeneous myofilament activation, the summation of which produces a Ca(2+)-dependent component to diastolic tone. The local increases in Cai due to S-CaOs also cause oscillatory sarcolemmal depolarizations due to Ca2+ modulation of the Na/Ca exchanger and of non-specific cation channels. Thus, inhomogeneous levels of diastolic Cai may lead to heterogeneity in cell coupling and thus may also affect the impulse conduction. The magnitude of the S-CaOs induced diastolic tonus and membrane depolarization varies with the extent to which S-CaOs are synchronized; partially synchronized S-CaOs following an AP induced SR Ca2+ release produce an aftercontraction and after depolarization. When local S-CaOs is sufficiently synchronized within the cell the resultant depolarization summates and can be sufficient to trigger spontaneous AP. Inhomogeneity of diastolic SR Ca2+ loading and sarcomere lengths within individual cardiac cells due to S-CaOs leads to inhomogeneous systolic Cai levels and sarcomere length inhomogeneities in response a subsequent AP; this heterogeneity compromises the systolic contraction amplitude. Heterogeneity of systolic Cai among cells due to diastolic S-CaOs also leads to heterogeneity of AP repolarization times, due, to heterogeneous Cai modulation of the Na/Ca exchanger, the non-specific cation channel and of the L type sarcolemmal Ca2+ channel. S-CaOs occurrence during a long AP plateau may also modulate the removal of voltage inactivation of L type Ca2+ channels and affect the likelihood of the occurrence of "early after depolarizations." Thus, as a single entity, S-CaOs may be implicated in diverse manifestations of heart failure--impaired systolic performance, increased diastolic tonus and an increased probability for the occurrence of arrhythmias.

Kappa-opioid peptide receptor stimulation increases cytosolic pH and myofilament responsiveness to Ca2+ in cardiac myocytes

Although kappa- and delta-opioid receptors on mammalian cardiac myocytes have been discovered recently, the intracellular effects that result from stimulation of these receptors remain unknown. We examine the effects of a rapid and brief exposure to a kappa-opioid receptor agonist on intracellular Ca2+, pH, and cell length in individual isolated rat ventricular cells. The specific kappa-agonist trans-dl-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]- benzene-acetamide (U-50488H) (methane sulfonate salt) caused a transient increase in cytosolic pH (pHi) measured from the change in SNARF-1 fluorescence and an increase in cytosolic [Ca2+] (Cai), indexed by a change in indo-1 fluorescence. The initial Cai increase often was followed by Cai oscillations. Both pHi and Cai effects were blocked by the specific antagonist kappa-opioid receptor l-(N-furylmethyl)-alpha-normetazocine methane-sulfonate (Mr 1452). The amplitude of contraction that accompanied the Cai increase elicited by U-50488H was greater than that associated with a similar increase in Cai elicited by electrical stimulation or by the rapid exposure of cells to caffeine. Thus an acute and brief kappa-opioid receptor stimulation of cardiac cells leads to an increase in Cai and pHi. The pHi increase was abolished by 1) blockade of the Na(+)-H+ exchanger by ethyl isopropyl amiloride and 2) inhibition of protein kinase C (PKC) activity via pretreatment with staurosporine or prolonged incubation with 4 beta-phorbol 12-myristate 13-acetate. These maneuvers did not abolish the U-50488H-induced increase in Ca.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+ dependence of alpha-adrenergic effects on the contractile properties and Ca2+ homeostasis of cardiac myocytes

alpha-Adrenergic stimulation is known to enhance myocardial contractility. Adult rat left ventricular myocytes bathed in 1 mM [Ca2+] (Ca0) and electrically stimulated at 0.2 Hz responded to alpha-adrenergic stimulation with 50 microM phenylephrine and 1 microM propranolol with an increase in twitch amplitude to 177.1 +/- 25.6% of control (mean +/- SEM). In contrast, when cell Ca2+ loading was increased by bathing cells in 5 mM Ca0, alpha-adrenergic stimulation decreased twitch amplitude to 68.6 +/- 8.2% of control. Time-averaged cytosolic [Ca2+] of cells in 1.0 mM Ca0 is enhanced via an increase in the frequency of electrical stimulation. When myocytes were stimulated at 2 Hz in 1 mM Ca0, alpha-adrenergic stimulation did not increase twitch amplitude (103.8 +/- 12.4% of control). In myocytes loaded with the Ca2+ probe into-1, alpha-adrenergic effects during stimulation at 0.2 Hz (an increase in twitch amplitude in 1 mM Ca0 and a decrease in twitch amplitude in 5 mM Ca0) were associated with similar changes in the indo-1 transient. In 5 mM Ca0, spontaneous Ca2+ releases from the sarcoplasmic reticulum (SR) occurred in the diastolic interval between twitches (2.9 +/- 1.4 spontaneous SR Ca2+ oscillations/min; n = 7); alpha-adrenergic stimulation abolished these oscillations in six of seven cells. Thus, an increase in the frequency of spontaneous diastolic SR Ca2+ release (i.e., Ca2+ overload) is not the mechanism for the negative inotropic effect of alpha-adrenergic stimulation in 5 mM Ca0. In experiments with unstimulated myocytes, we determined whether the effect of alpha-adrenergic stimulation on cell Ca2+ homeostasis and oscillatory SR Ca2+ release observed in 5 mM Ca0 occurs only during electrical stimulation, when voltage-dependent currents are operative, or also at rest. Unstimulated rat ventricular myocytes in 5 mM Cao exhibit oscillatory SR Ca2+ release; alpha-adrenergic stimulation decreased the frequency of these oscillations to 53.9 +/- 8.9% of control, and this effect was blocked by 1 microM prazosin. In unstimulated indo-1-loaded myocytes alpha-adrenergic stimulation decreased the resting indo-1 fluorescence ratio in 5 mM Ca0, whereas it had no effect in 1 mM Ca0. Additional experiments were aimed at defining a role for Ca(2+)-activated, phospholipid-dependent protein kinase C (PKC) for the negative inotropic effect of alpha-adrenergic stimulation in 5 mM Ca0. Short-term preexposure to 0.1 microM 4 beta-phrobol 12-myristate 13-acetate (PMA) has been shown to maximally activate PKC.(ABSTRACT TRUNCATED AT 400 WORDS)

A cellular mechanism for impaired posthypoxic relaxation in isolated cardiac myocytes. Altered myofilament relaxation kinetics at reoxygenation

Single, isolated rat ventricular myocytes were made hypoxic for 10 minutes and then reoxygenated. During hypoxia, there was a marked abbreviation of the mechanical twitch, without a decrease in its amplitude. Immediately after reoxygenation, both the time to peak shortening and the duration of relaxation were markedly prolonged, and they remained prolonged for 10-50 minutes. The alterations in contraction and relaxation were not associated with any change in the time course of either the transmembrane action potential or the cytosolic calcium transient, as recorded with the fluorescent probe indo 1. Intracellular pH, measured with a fluorescent probe (carboxyseminaphthorhodofluor), showed an acid shift during hypoxia and an alkaline rebound immediately after reoxygenation. The time courses of intracellular pH and contraction duration were not parallel during hypoxia or reoxygenation, and simulation of the alkaline pH shift by lowering PCO2 or superfusing NH4Cl (in the absence of exposure to hypoxia) did not quantitatively reproduce the prolongation of relaxation seen after reoxygenation. The prolongation of contraction after reoxygenation could be overridden by the beta-adrenergic agonist isoproterenol or the nonenzymatic phosphatase butanedione monoxime. We conclude that delayed relaxation after reoxygenation exists at the single cell level and is due to an alteration of the properties of the myofilaments. Intracellular pH is not the primary mediator of this alteration. We speculate that alteration of intracellular inorganic phosphate or covalent modification of the myofilaments might be involved.

Ethanol acutely and reversibly suppresses excitation-contraction coupling in cardiac myocytes

We used adult rat cardiac myocytes to examine the acute effects of 0.1-5.0% (vol/vol) ethanol (ETOH) on 1) the cytosolic [Ca2+] (Cai) transient measured as the change in indo 1 fluorescence at 410/490 nm and contraction elicited by electrical stimulation of single cells and 2) the sarcoplasmic reticulum (SR) Ca2+ content in cell suspensions. During stimulation at 1 Hz, clinically relevant ETOH correlations (0.1-0.15% [vol/vol]) caused a 10-15% decrease in the contraction amplitude, measured by myocyte edge tracking, without decreasing the Cai transient that initiates contraction. At higher ETOH concentrations (1-5% [vol/vol]), ETOH caused profound contractile depression and also reduced the magnitude of the Cai transient. These effects were reversed within minutes of ETOH washout. Addition of norepinephrine (10 microM) to the bathing solution or an increase in bathing [Ca2+] in the continued presence of ETOH could also reverse its effects. The relation of the amplitude of the Cai transient to the contraction amplitude measured across a range of bathing [Ca2+] was shifted by ETOH, such that for a given Cai transient a marked reduction in contraction amplitude occurred. In unstimulated myocyte suspensions, ETOH (1-5% [vol/vol]) caused a concentration-dependent depletion of SR Ca2+ content, manifested as a diminution in the Cai increase elicited by caffeine in the presence of extracellular EGTA and no added Ca2+. Thus, in rat cardiac myocytes a reduction in the myofilament Ca2+ response, possibly due to a decrease in myofilament Ca2+ sensitivity, is a mechanism for contractile depression due to clinically relevant ETOH concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

The cytosolic calcium transient modulates the action potential of rat ventricular myocytes

1. The modulation of the action potential by the cytosolic Ca2+ (Cai2+) transient was studied in single isolated rat ventricular myocytes loaded with the acetoxymethyl ester form of the Ca(2+)-sensitive fluorescent dye Indo-1. Stimulation following rest and exposure to ryanodine were used to change the amount of Ca2+ released from the sarcoplasmic reticulum and thus the size of the Cai2+ transient. The Cai2+ transient was measured as the change, upon stimulation, in the ratio of Indo-1 fluorescence at 410 nm to that at 490 nm (410/490) and action potentials or membrane currents were recorded using patch-type microelectrodes. 2. When stimulation was initiated following rest, the magnitude of the Cai2+ transient decreased in a beat-dependent manner until a steady state was reached. The negative staircase in the Cai2+ transient was accompanied by a similar beat-dependent decrease in the duration of the action potential, manifested primarily as a gradual loss of the action potential plateau (approximately -45 mV). A slow terminal phase of repolarization of a few millivolts in amplitude was found to parallel the terminal decay of the Cai2+ transient. 3. The terminal portion of phase-plane loops of membrane potential (Vm) vs. Indo-1 ratio from all of the beats of a stimulus train followed a common linear trajectory even though the individual beats differed markedly in the duration and amplitude of the action potential and Cai2+ transient. 4. When the stimulation dependence of the Cai2+ transient was titrated away with submaximal exposure to ryanodine, the stimulation-dependent changes in the action potential plateau and terminal phase of repolarization were also eliminated. The same effect was noted in cells which, fortuitously, did not show a staircase in the Cai2+ transient following a period of rest. 5. When action potentials were triggered immediately following spontaneous release of Ca2+ from the sarcoplasmic reticulum, which results in a small depolarization at the resting potential, phase-plane loops (Vm vs. Indo-1 ratio) of the spontaneous events followed the same linear trajectory as the terminal phase of repolarization in the loops of the stimulated beats. 6. Following repolarization from brief voltage clamp pulses (to minimize time and voltage-dependent currents associated with depolarization), an inward current was observed that rose and fell in phase with the Cai2+ transient. This current was present at -70 mV, near the resting potential, and at -40 mV, a potential relevant to the plateau of the action potential.(ABSTRACT TRUNCATED AT 400 WORDS)

Sustained subthreshold-for-twitch depolarization in rat single ventricular myocytes causes sustained calcium channel activation and sarcoplasmic reticulum calcium release

Single rat ventricular myocytes, voltage-clamped at -50 to -40 mV, were depolarized in small steps in order to define the mechanisms that govern the increase in cytosolic [Ca2+] (Cai) and contraction, measured as a reduction in myocyte length. Small (3-5 mV), sustained (seconds) depolarizations that caused a small inward or no detectable change in current were followed after a delay by small (less than 2% of the resting length), steady reductions in cell length measured via a photodiode array, and small, steady increases in Cai measured by changes in Indo-1 fluorescence. Larger (greater than -30 and less than -20 mV), sustained depolarizations produced phasic Ca2+ currents, Cai transients, and twitch contractions, followed by a steady current and a steady increase in Cai and contraction. Nitrendipine (or Cd, verapamil, or Ni) abolished the steady contraction and always produced an outward shift in steady current. The steady, nitrendipine-sensitive current and sustained increase in Cai and contraction exhibited a similar voltage dependence over the voltage range between -40 and -20 mV. 2 microM ryanodine in the presence of intact Ca2+ channel activity also abolished the steady increase in Cai and contraction over this voltage range. We conclude that when a sustained depolarization does not exceed about -20 mV, the resultant steady, graded contraction is due to SR Ca2+ release graded by a steady ("window") Ca2+ current. The existence of appreciable, sustained, graded Ca2+ release in response to Ca2+ current generated by arbitrarily small depolarizations is not compatible with any model of Ca2(+)-induced Ca2+ release in which the releasing effect of the Ca2+ channel current is mediated solely by Ca2+ entry into a common cytosolic pool. Our results therefore imply a distinction between the triggering and released Ca2+ pools.

Phorbol ester and dioctanoylglycerol stimulate membrane association of protein kinase C and have a negative inotropic effect mediated by changes in cytosolic Ca2+ in adult rat cardiac myocytes

We used left ventricular myocytes from adult rats to investigate the effect of 4 beta-phorbol 12-myristate 13-acetate (PMA) and of sn-1,2-dioctanoylglycerol (DiC-8) on the membrane association of protein kinase C (PKC), cytosolic [Ca2+], (Cai) homeostasis, and the contractile properties of single cardiac cells. Because PKC activity is known to be highly Ca2+ sensitive, the K+ concentration of the bathing medium was raised from 5 to 30 mM in some experiments, a perturbation known to depolarize the cell and increase Cai. In cell suspensions both PMA (3 x 10(-10) and 3 x 10(-7) M) and DiC-8 (10(-5) and 10(-4) M) increased membrane association of PKC. The effect of PMA (10(-7) M) on PKC translocation was enhanced in 30 mM KCl compared with 5 mM KCl. During steady field stimulation at 1 Hz in 1 mM bathing [Ca2+], both PMA (10(-7) M) and DiC-8 (10(-5) M) decreased twitch amplitude to approximately 60% of control in 5 mM KCl, and the negative inotropic effect of either drug was more pronounced in 30 mM KCl than in 5 mM KCl. In single cardiac myocytes loaded with the Ca2+ indicator indo-1 and bathed in 5 mM KCl, we simultaneously measured cell length and Cai. The myofilament responsiveness to Ca2+ was assessed by the relation between contraction amplitude and the peak of the Cai transient. The negative inotropic effect of both PMA and DiC-8 was related to a diminished amplitude of the Cai transient and not to a decreased myofilament responsiveness to Ca2+. In the absence of electrical stimulation, PMA (10(-7) M) and DiC-8 (10(-5) M) decreased the frequency of contractile waves due to spontaneous Ca2+ release from the sarcoplasmic reticulum, and DiC-8 also decreased resting Cai. Thus, activation of PKC, which is thought to occur as part of the response of cardiac muscle to alpha 1-adrenergic stimulation, is associated with a negative inotropic action due to a smaller Cai transient rather than to a decrease in the myofilament responsiveness to Ca2+. These effects on the membrane association of PKC and on contractility are enhanced by cell depolarization achieved by raising [KCl] in the bathing medium.

The hyperthyroid heart. An analysis of systolic and diastolic properties in single rat ventricular myocytes

Single ventricular myocytes were isolated by collagenase digestion from the hearts of 6-8-month-old male Wistar rats in either the control (euthyroid) state or after 7 days of daily injection of 0.64 mg/kg thyroxine (hyperthyroid). Myocytes were field-stimulated from slack length, and contraction was measured with an inverted microscope-photodiode array-computer apparatus. The effect of pacing rate and ouabain administration on systolic and diastolic function was examined. Single myocytes isolated from hyperthyroid hearts maintain the properties of bulk muscle, because maximal twitch velocity is augmented 98% and the time course of contraction as measured by the time to peak shortening, relaxation time, or contraction duration is abbreviated 39%. Spontaneous sarcoplasmic reticulum calcium release, as measured by the occurrence of contractile waves, is increased in the hyperthyroid myocytes. This increased frequency of spontaneous sarcoplasmic reticulum calcium release is most marked under conditions known to be associated with high intracellular calcium, such as low pacing rates or digitalis glycoside administration. It can account for the hypoperformance of the hyperthyroid myocytes noted under these conditions because it is associated with depletion of sarcoplasmic reticulum calcium stores and diminution of subsequent twitch amplitude. These observations may help explain, in part, the cellular basis of the altered cardiac performance in the hyperthyroid state.

Simultaneous measurement of Ca2+, contraction, and potential in cardiac myocytes

A system is described that can simultaneously record cytosolic Ca2+ concentration ([Ca2+]i), cell length, and either membrane potential or current in single cardiac myocytes loaded with the fluorescent Ca2+ indicator indo-1. Fluorescence is excited by epi-illumination with 3.8-microsecond flashes of 350 +/- 5 nm light from a xenon arc. Indo-1 fluoresence is measured simultaneously in spectral windows of 391-434 nm and 457-507 nm, and the ratio of indo-1 emission in the two bands is computed as a measure of [Ca2+]i for each flash. With cells loaded with the permeant acetoxymethyl ester of indo-1, quantitation of [Ca2+]i is not precise, owing to subcellular compartmentation of indo-1; however, the instrument would allow full quantitation if indo-1 free acid was introduced by microinjection. Simultaneously, cell length is measured on-line from the bright-field image of the cell. Because fluorescence collection is time gated during the brief flash, and red light (650-750 nm) is used for the bright-field image, cell length and [Ca2+]i measurements are obtained simultaneously without cross talk. Membrane potential or current can be recorded simultaneously with indo-1 fluorescence and cell length via standard patch-clamping techniques.

Optimum spectral windows to minimize quantum noise of ratiometric intracellular fluorescent probes

When fluorescent indicators are used to measure intracellular ligands in single cells, the quality of the data is usually limited by quantum (shot) noise. For indicators which shift excitation or emission wavelengths upon ligand binding, a ratiometric method is usually employed. In choosing the spectral windows for excitation or collection of fluorescence, there is a trade-off between maximum sensitivity to ligand binding, and maximum collection of light. We show that there is a well-defined optimum choice of windows which minimizes the error caused by quantum noise in the estimated ligand concentration. An algorithm for determining these optimum windows is presented. As an example, we consider the measurement of intracellular calcium by indo-1 fluorescence emission ratio in cardiac myocytes. The optimum wavelength bands for collection of fluorescence are considerably wider than those commonly employed. The use of these windows in a pulsed-excitation time-resolved calcium measurement instrument resulted in improved signal to noise ratio of the calcium signal.

Milrinone enhances cytosolic calcium transient and contraction in rat cardiac myocytes during beta-adrenergic stimulation

We have investigated the mechanism that underlies the absence of a positive inotropic effect of milrinone on rat myocardium. The twitch characteristics of enzymatically dissociated left ventricular myocytes from the adult rat and guinea pig were assessed by edge tracking during field stimulation. In some rat myocytes loaded with the ester derivative of the Ca2+ probe Indo-1 we simultaneously measured changes in cell length and in the associated cytosolic Ca2+ (Cai) transient. Our results show that in guinea pig myocytes bathed in 0.5 mM [Ca2+] and field stimulated at 1 Hz, milrinone (10 microM) had a positive inotropic effect. In contrast milrinone had no effect on the contractile properties of rat myocytes studied under similar conditions and field stimulated at 0.2 Hz. In rat myocytes bathed in 0.5 mM [Ca2+] and stimulated at 0.2 Hz isoproterenol (1 nM) increased the amplitude and shortened the duration of the contraction and of the associated Cai transient; these effects of beta-adrenergic stimulation were further enhanced by the addition of milrinone (10 microM) in the presence of isoproterenol. Under conditions of higher cell Ca2+ loading achieved by raising bathing [Ca2+] to 1 mM and isoproterenol to 3 nM the positive inotropic effect of milrinone (10 microM) in rat myocytes saturated when spontaneous oscillatory Ca2+ release appeared in the diastolic intervals between electrically stimulated twitches. Our results suggest that an enhancement in the baseline beta-adrenergic stimulation is required for milrinone to exercise a positive inotropic action on rat myocardial tissue.

Diminished cardiac hypertrophy and muscle performance in older compared with younger adult rats with chronic atrioventricular block

The combined effect of advancing age and hemodynamic overload on cardiac muscle function has received little attention. In male, Sprague-Dawley rats, we studied the interaction of chronic atrioventricular heart block induced by transvenous electrocautery for 4-12 months (mean, 7 months) and age at study (12, 19 +/- 0.7, and 24 +/- 0.2 months) on cardiac hypertrophy and muscle function compared with age-matched, sham-operated controls. Hypertrophy was determined by the ratio of heart weight to tibial length. Muscle function was first determined from the mechanical variables of the isometric contraction of an excised, thin, left ventricular trabecular muscle bathed at 29 degrees C under a variety of calcium concentrations and stimulation patterns. Then, in the same muscles after disruption of membranes with Triton X-100, the force-pCa curve of the myofibrils was obtained. No hypertrophy occurred with aging in the control group, but alteration in hypertrophy with age occurred in the block group such that the youngest animals with block had the most hypertrophy (170%) and the oldest animals with block the least hypertrophy (120%). The tension developed by cardiac muscle and the duration of the isometric contraction were not affected by age in the control group but were significantly affected by age in the block group. The young animals with block had a markedly prolonged contraction duration and almost twice the developed tension compared with the older animals with block or with controls. The age-related difference in muscle contraction duration in the block group was associated with, and may have only been secondary to, the age-related difference in the extent of cardiac hypertrophy. For developed tension, the age-related difference in the block group could not be explained by differences in the extent of cardiac hypertrophy. Rather, this difference was attributable to both an increased myofibrillar force-generating capacity in the young block and to an impairment in excitation-contraction coupling in the old block. The results show that during long-term block, age exerted not only a significant effect on the extent of cardiac hypertrophy but also an independent effect on the developed tension of cardiac muscle.

Extracellular ATP has a potent effect to enhance cytosolic calcium and contractility in single ventricular myocytes

The effect of extracellular ATP on the contraction of single rat cardiac myocytes was investigated, together with the effect on the transient change in cytosolic Ca2+ (Cai) elicited by excitation and on the relationship between these two parameters. In unstimulated single myocytes, ATP caused a small increase in Cai (measured as the ratio of fluorescence of Indo-1 at 410 to that at 490 nm. In myocytes bathed in a medium containing 1.0 mM [Ca2+] at 23 degrees C and stimulated at 1 Hz, ATP (1 microM) resulted in a two-threefold increase in amplitude of contraction, as measured by video cinemicrographic techniques. The duration of the Cai-transient was not altered but its amplitude was markedly enhanced, as was the amplitude of contraction. The relation between Cai and contraction-amplitude was not altered by ATP, when measured over a range of extracellular [Ca2+], suggesting that ATP does not affect the myofilament-Ca2+ interaction. The primary site of action of ATP in increasing Cai is at the sarcolemma since the addition to suspensions of myocytes of caffeine (10 mM), which depletes the sarcoplasmic reticulum Ca2+ load, does not prevent the subsequent increase of Cai due to ATP. Further, lowering of the extracellular [Ca2+] to less than 1 microM with EGTA abolishes the response of Cai to ATP, though not the response to caffeine. Thus in rat cardiac myocytes ATP stimulates trans-sarcolemmal influx of Ca2+: ADP, AMP and adenosine are ineffective. ATP markedly augments the amplitude of the Cai transient elicited by electrical stimulation thus rendering it a potent inotropic agent.

Spontaneous Ca2+ release from the sarcoplasmic reticulum limits Ca2+-dependent twitch potentiation in individual cardiac myocytes. A mechanism for maximum inotropy in the myocardium

We hypothesized that the occurrence of spontaneous Ca2+ release from the sarcoplasmic reticulum (SR), in diastole, might be a mechanism for the saturation of twitch potentiation common to a variety of inotropic perturbations that increase the total cell Ca. We used a videomicroscopic technique in single cardiac myocytes to quantify the amplitude of electrically stimulated twitches and to monitor the occurrence of the mechanical manifestation of spontaneous SR Ca2+ release, i.e., the spontaneous contractile wave. In rat myocytes exposed to increasing bathing [Ca2+] (Cao) from 0.25 to 10 mM, the Cao at which the peak twitch amplitude occurred in a given cell was not unique but varied with the rate of stimulation or the presence of drugs: in cells stimulated at 0.2 Hz in the absence of drugs, the maximum twitch amplitude occurred in 2 mM Cao; a brief exposure to 50 nM ryanodine before stimulation at 0.2 Hz shifted the Cao of the maximum twitch amplitude to 7 mM. In cells stimulated at 1 Hz in the absence of drugs, the maximum twitch amplitude occurred in 4 mM Cao; 1 microM isoproterenol shifted the Cao of the maximum twitch amplitude to 3 mM. Regardless of the drug or the stimulation frequency, the Cao at which the twitch amplitude saturated varied linearly with the Cao at which spontaneous Ca2+ release first occurred, and this relationship conformed to a line of identity (r = 0.90, p = less than 0.001, n = 25). The average peak twitch amplitude did not differ among these groups of cells. In other experiments, (a) the extent of rest potentiation of the twitch amplitude in rat myocytes was also limited by the occurrence of spontaneous Ca2+ release, and (b) in both rat and rabbit myocytes continuously stimulated in a given Cao, the twitch amplitude after the addition of ouabain saturated when spontaneous contractile waves first appeared between stimulated twitches. A mathematical model that incorporates this interaction between action potential-mediated SR Ca2+ release and the occurrence of spontaneous Ca2+ release in individual cells predicted the shape of the Cao-twitch relationship observed in other studies in intact muscle. Thus, the occurrence of spontaneous SR Ca2+ release is a plausible mechanism for the saturation of the inotropic response to Ca2+ in the intact myocardium.

Changes in myosin isoenzymes, ATPase activity, and contraction duration in rat cardiac muscle with aging can be modulated by thyroxine

To determine whether the relative decline in cardiac myosin isoenzyme V1 with maturation continues progressively into senescence and whether thyroxine could reverse age-associated changes in the myosin isoenzyme profile and contraction, rats 2, 8, and 24 months old were treated with thyroxine, 6.4 mg/kg, for 7 days. Myosin isoenzymes, Ca2+-myosin ATPase activities, and isometric contractile function were measured in cardiac preparations from thyroxine-treated animals and age-matched controls. Right ventricular hypertrophy did not occur with aging in controls. Thyroxine increased right ventricular weight in each age group compared to the control group. Body weight decreased by 10% in all thyroxine-treated rats. The relative right ventricular V1 isoenzyme content progressively decreased from 75 +/- 1% to 54 +/- 1% and 14 +/- 1% in controls at 2, 8, and 24 months, respectively, and was associated with a reciprocal increase in V3 myosin isoenzyme. Ca2+-myosin ATPase activity also progressively declined monotonically with age in the control rats from 854 +/- 28 nmol Pi/mg prot/min at 2 months to 529 +/- 28 nmol Pi/mg prot/min at 24 months. Thyroxine administration increased right ventricular V1 at each age to 97 +/- 2%, 73 +/- 2%, and 59 +/- 2% at 2, 8, and 24 months, respectively. A thyroxine induced increase in the Ca2+-myosin ATPase activity could be detected only in the 24-month-old animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Single adult rabbit and rat cardiac myocytes retain the Ca2+- and species-dependent systolic and diastolic contractile properties of intact muscle

The systolic and diastolic properties of single myocytes and intact papillary muscles isolated from hearts of adult rats and rabbits were examined at 37 degrees C over a range of stimulation frequencies and bathing [Ca2+]o (Cao). In both rabbit myocytes and intact muscles bathed in 1 mM Cao, increasing the frequency of stimulation from 6 to 120 min-1 resulted in a positive staircase of twitch performance. During stimulation at 2 min-1, twitch performance also increased with increases in Cao up to 20 mM. In the absence of stimulation, both rabbit myocytes and muscles were completely quiescent in less than 15 mM Cao. Further increases in Cao caused the appearance of spontaneous asynchronous contractile waves in myocytes and in intact muscles caused scattered light intensity fluctuations (SLIF), which were previously demonstrated to be caused by Ca2+-dependent spontaneous contractile waves. In contrast to rabbit preparations, intact rat papillary muscles exhibited SLIF in 1.0 mM Cao. Two populations of rat myocytes were observed in 1 mM Cao: approximately 85% of unstimulated cells exhibited low-frequency (3-4 min-1) spontaneous contractile waves, whereas 15%, during a 1-min observation period, were quiescent. In a given Cao, the contractile wave frequency in myocytes and SLIF in intact muscles were constant for long periods of time. In both intact rat muscles and myocytes with spontaneous waves, in 1 mM Cao, increasing the frequency of stimulation from 6 to 120 min-1 resulted, on the average, in a 65% reduction in steady state twitch amplitude. Of the rat myocytes that did not manifest waves, some had a positive, some had a flat, and some had a negative staircase; the average steady state twitch amplitude of these cells during stimulation at 120 min-1 was 30% greater than that at 6 min-1. In contrast to rabbit preparations, twitch performance during stimulation at 2 min-1 saturated at 1.5 mM Cao in both intact rat muscles and in the myocytes with spontaneous waves. We conclude that the widely divergent, Ca2+-dependent systolic and diastolic properties of intact rat and rabbit cardiac muscle are retained with a high degree of fidelity in the majority of viable single myocytes isolated from the myocardium of these species, and that these myocytes are thus a valid model for studies of Ca2+-dependent excitation-contraction mechanisms in the heart.

Effects of pulmonary function on mortality

Survivorship data from a 24 year longitudinal study of 874 male volunteers in the Baltimore Longitudinal Study of Aging were used to assess the role of pulmonary function on total mortality. Even when age and smoking were considered, the ratio of forced expiratory volume in 1 sec to its predicted value was significantly associated with mortality from all causes. Individuals with poorer pulmonary function showed greater mortality during the follow-up period of this study. This relationship was also seen among never smokers in this sample, further supporting the hypothesis that impaired pulmonary function is itself a predictor of total mortality and may contribute to a number of disease processes.

Cell size and capillary supply of the hypertensive rat heart: quantitative study

Cardiac mass, cell size and capillary supply were studied in the hearts of spontaneously hypertensive rats (SHR) and compared to genetically similar non-hypertensive Wistar-Kyoto (WKY) of three adult ages: 5, 15 and 23 months. The left ventricular weight of SHR was not significantly greater than that of WKY at 5 months, but was by 15 months and became even more so by 23 months. This increase could be attributed to hypertrophy of the individual cardiac muscle cells and therefore, the estimated total number of myocytes per left ventricle was essentially the same in all experimental groups. Various indices of the myocardial capillary supply were also investigated. Cardiac hypertrophy in older hypertensive rats was characterized by greater and more variable intercapillary spacing, which may have importance in myocardial oxygen supply.

Excitation-contraction in rat myocardium: alterations with adult aging

Simultaneous transmembrane action potential (TAP) and isometric contraction measurements were made in thin right ventricular papillary muscles isolated from senescent (S, 24-26 mo) and young adult (YA, 6-8 mo) rat hearts. At the peak of the length-tension curve in the steady state at 24 min-1 at 29 degrees C in perfusate [Ca2+] [( Ca2+]e) of 0.375 or 2.5 mM, contractile tension developed in response to excitation (DT) and the maximum rate of tension development (dT/dt) were not age-related; contraction duration (CD) was 17 and 27% greater in S than YA in the low and high [Ca2+]e, respectively (P less than 0.001); resting membrane potential was not age related. TAP time above zero mV, integrated area above zero mV, and times to 75% (T75) and 90% (T90) repolarization were approximately twofold greater in S than in YA in both [Ca2+]e (P less than 0.001). Changes in steady-state T75 and T90 due to an increase in [Ca2+]e from 0.375 to 2.5 mM were significantly correlated with those in CD, DT, and dT/dt in S but not in YA. We conclude that in the senescent myocardium a prolonged and greater extent of depolarization is related to the prolonged CD and may also be a determinant of the peak force developed in response to excitation under some conditions.

Age-associated alterations in viscoelastic properties of canine aortic strips

Many studies have delineated the changes in the elastic properties of arterial tissue as a function of age. Despite the fact that viscoelasticity is a prominent feature of these tissues, there is little information or characterization of age-associated changes in viscoelastic properties, over a wide range of smooth muscle activation, particularly in nonhuman tissue where atherosclerosis is not a confounding factor. In the present study, using small sinusoidal length perturbations, we determined the dynamic stiffness properties across a wide range of lengths (stretch ratios from 100 to 135%) and frequencies (from 0.25 to 35 Hz) in strips excised from ascending and descending aortas from six young (2 to 4-year-old) and 12 senescent (10- to 13-year-old) beagles. Studies were performed with the smooth muscle fully activated with calcium and norepinephrine, as well as fully inactivated with cyanide, iodoacetate, and dinitrophenol. There was a cubic nonlinear dependence of stiffness modulus on length only in senescent tissue and, surprisingly, little frequency dependence in tissue of either age. Compared to the young aortas, the three-dimensional surface representing the dependence of stiffness modulus on length and frequency from both the ascending and descending regions of aged aortas was displaced higher on the stiffness axis both with the muscle fully activated and inactivated. This age difference was accentuated at longer lengths. The phase lag between force and length was greater in the young vs. the old strips only in the activated, ascending aortic tissue. We found no age differences in the content of elastin, collagen, or in the collagen/elastin ratio, to account for these mechanical property differences.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of tibial length to quantify cardiac hypertrophy: application in the aging rat

Fluctuations in body weight as occur with aging make body weight an unreliable reference for normalizing heart weight. We compared heart weight normalized by tibial length, which remains constant after maturity, with that normalized by body weight in 5- to 28-mo-old male Wistar rats. When normalized by tibial length or body weight, relative to the 5-mo heart, the senescent left ventricle undergoes 17 vs. 38% hypertrophy, respectively, and the right ventricle undergoes 0 vs. 28% hypertrophy, respectively. Histological measurements in the 25- compared with the 5-mo-old left ventricles reveal 6% larger myocyte diameters and 12% larger cellular cross-sectional areas, indicating about 15% hypertrophy; this value agrees more closely with the estimates based on tibial length than with those based on body weight. To allow prediction of left ventricular weight in a living rat, a regression equation using body weight, age, and tibial length was derived. This enabled us to perform a longitudinal aging study that verified that the above results were not biased by selective survival. Thus, in conditions in which body weight changes, cardiac hypertrophy can be more accurately quantified by relating heart weight to tibial length than to body weight. This approach may have applicability for assessing relative sizes of other organs as well.