摘要:

To evaluate the effects of oxidative stress on cardiac membrane currents, single cells from frog ventricle were exposed totert-butyl hydroperoxide (t-BHP). Incubation of these cells with 2 mMt-BHP causes a rapid depletion of cellular glutathione, followed by a more gradual increase in the contents of malonaldialdehyde and conjugated dienes. Effects of this rapidly evolving oxidative stress were studied on sodium, calcium, and potassium currents of isolated ventricular cells.t-BHP caused a progressive decrease in the magnitude of sodium current obtained on depolarization from a holding potential of −85 mV, which was accompanied by a shift in the reversal potential toward more negative potentials. The voltage dependence of the steady-state parameters for activation and inactivation was shifted, such that in peroxide-exposed cells, there was a greater overlap of activation and inactivation parameters, which would be expected to result in an increased window current. In addition, in the presence oft-BHP, the time constant for activation was decreased at most depolarizing potentials, whereas the time constant for inactivation was increased. The resultant sodium current transients were, therefore, slower in the presence of the peroxide because of slower inactivation. Prolonged exposure of the cells tot-BHP led to a complete and selective inhibition of the Na+current. However, even when all the Na+current was inhibited, the K+and Ca2+currents remained essentially unaltered. Also, no large outward currents were observed at this stage, indicating that ATP concentration was not drastically decreased. The barrier properties of plasma membrane remained intact, as it was possible to form gigohm seals between the patch pipette and the plasma membrane of cells treated with 2-14 mMt-BHP for up to 30 minutes. These results account for the proarrhythmic effects of free radicals and oxidative stress on cardiac tissues.

ISSN:0009-7330    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

The effect of diet-induced, moderate elevation of serum cholesterol on vascular reactivity in isolated rabbit abdominal aortic rings was examined by using a series of vasoconstrictor and vasodilator agonists. Serum cholesterol of rabbits that were fed a cholesterol-free, casein-rich diet for 10 weeks was elevated approximately 4.5-fold compared with values found in control rabbits that were fed standard lab chow (223±41 versus 51±5 mg/dl, respectively). Relaxation responses to carbamylcholine chloride and (±)-isoproterenol hydrochloride in vessels from hypercholesterolemic rabbits were markedly inhibited in the presence of norepinephrine, prostaglandin F2α, 5-hydroxytryptamine, and angiotensin II but not in the presence of phorbol 12,13-dibutyrate. The depressed vasodilation in hypercholesterolemic vessels appeared to depend on the agonist initiating the contraction. Sodium nitroprusside-induced relaxations were unchanged in rings from hypercholesterolemic rabbits compared with rings from control rabbits for all contractile agonists except KCI. Isolated aortic rings from hypercholesterolemic rabbits exhibited a slight but significantly increased vasoconstrictor sensitivity to 5-hydroxytryptamine and KCI but not to norepinephrine, prostaglandin F2α, angiotensin II, or phorbol 12,13-dibutyrate compared with aortic rings from control rabbits. These results demonstrate that modest elevation of serum cholesterol is sufficient to depress vasodilator and enhance vasoconstrictor responses to certain agonists. Vasodilator effects are impaired to a greater extent by a small increase in serum cholesterol than are responses to vasoconstrictor agonists. It is postulated that the induction of differential alterations in vascular reactivity with moderate increase in serum cholesterol may represent important early events predisposing arteries to vasospasm.

ISSN:0009-7330    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

We determined the extent of convergence of preganglionic fibers from the right and left vagus nerves on postganglionic neurons that supply the sinoatrial node in chloralose-anesthetized dogs. We administered hemicholinium-3 and stimulated the right vagus nerve at a high frequency to deplete acetylcholine from the postganglionic parasympathetic neurons supplied by that nerve. We compared the effects of this “depletion regimen” with the responses in two control groups: a stimulation control group, which was subjected to high-frequency right vagus stimulation only, and a drug control group, which received a hemicholinium-3 infusion only. The effects of right vagus stimulation did not differ from those of left vagus stimulation in either of the control groups. In the animals subjected to the depletion regimen, the responses to right vagus stimulation were almost abolished. However, the left vagus nerve retained its ability to prolong cardiac cycle length in these animals. Thus, our experiments indicate that left vagus preganglionic fibers do not converge with right vagus preganglionic fibers on a substantial pool of postganglionic neurons that innervate the canine sinoatrial node.

ISSN:0009-7330    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

Increasing evidence suggests that the renin-angiotensin system modulates cardiovascular homeostasis both via its circulating, plasma-borne components and through locally present, tissue-resident systems with site-specific activity. The existence of such a system in the heart has been proposed, based on biochemical studies as well as on the demonstration of renin and angiotensinogen messenger RNA in cardiac tissue. We conducted the present study to determine whether biologically active angiotensin peptides may be cleaved within the heart from locally present angiotensinogen. Isolated, perfused rat hearts were exposed to infusions of purified hog renin; the coronary sinus effluent was collected and subsequently assayed for angiotensin I (Ang I) and angiotensin II (Ang II) by high-pressure liquid chromatography and specific radioimmunoassay. Both Ang I and II were undetectable under control conditions but appeared promptly after the addition of renin. Dose-dependent peak values for Ang I release ranged from 2.42±0.65 fmol/min to 1.38±0.18 pmol/min during renin infusions at concentrations between 10 microunits/ml and 5 milliunits/ml. Ang II levels measured in the perfusate reflected a mean fractional intracardiac conversion of Ang I to Ang H of 7.18±1.09%. Generation of Ang I and Ang II was inhibited in the presence of specific inhibitors of renin and converting enzyme, respectively. To investigate the source of angiotensinogen, we measured spontaneous angiotensinogen release from isolated perfused hearts. In the absence of renin in the perfusate, angiotensinogen was initially released in high, but rapidly declining, concentrations and subsequently at a low, but stable, rate. Prior perfusion with angiotensinogen-rich plasma resulted in enhanced early angiotensinogen release but did not alter the second, delayed phase, suggesting that, in addition to plasma-derived substrate, locally produced angiotensinogen may also participate in the intracardiac formation of angiotensin. Supporting this interpretation, hearts from animals pretreated with dexamethasone showed increased angiotensinogen messenger RNA concentrations as well as increased rates of angiotensinogen release not only during the early but also during the late phase. Our study newly demonstrates that Ang I and II may be formed within the isolated heart from locally present substrate, which appears to be derived in part from the circulating pool and in part from endogenous synthesis. These findings add support to the concept of a functionally active and locally integrated cardiac renin-angiotensin system and emphasize its potential physiological and pathological relevance.

ISSN:0009-7330    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

Developmental changes in the contributions of transsarcolemmal Ca2+influx and Ca2+release from intracellular storage sites to myocardial contraction were evaluated in isolated ventricular myocytes from neonatal (aged 1-7 days) and adult (aged 8-10 weeks) New Zealand White rabbits. Contractions ceased in one beat when extracellular Ca2+was decreased from 1 mM to micromolar levels using a rapid perfusion technique. On reperfusion with 1 mM Ca2+, recovery of control contraction amplitude occurred after significantly fewer beats in neonatal myocytes compared with adult myocytes, and after 1 minute compared with 5 minutes of reduced Ca2+. After 15 minutes of perfusion with either 1 or 10 μM ryanodine, contraction amplitude decreased in both age groups, but the decrease was significantly greater in adults than in neonates. These experiments indicate that isolated ventricular myocytes may be used in the study of developmental changes in intracellular Ca2+regulation. Results suggest that cardiac contraction in neonates is relatively more dependent on transsarcolemmal Ca2+influx. Furthermore, although Ca2+release from intracellular storage sties is present in both neonates and adults, its role in cardiac contraction is more significant in adults.

ISSN:0009-7330    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

To investigate the role of prostaglandins I2and E2in modulating the vasoconstrictor response to sympathetic stimulation, repeated and proximate cold pressor tests were performed in 23 healthy young volunteers. Limb vascular resistance (blood flow measured by venous occlusion plethysmography), prostaglandin I2(as 6-ketoprostaglandin F1α) and prostaglandin E2plasma levels (detected by radioimmunoassay), and plasma catecholamines (detected by high-performance liquid chromatography and electrochemical detection) were measured. A progressive increase in the duration of the vasoconstrictor response was observed with repetition of cold applications (p<0.001, by analysis of variance for trends). This increase was associated with a progressive decrease in cold-induced elevation of 6-ketoprostaglandin F1αand prostaglandin E2plasma levels until, after five stimulations, neither prostaglandin was detectable. The maximum detected concentration of norepinephrine did not significantly change, but its area under the curve in time showed a trend toward an increase. Epinephrine levels did not significantly change. The increase of vascular resistance was significantly correlated with the decrease of both prostaglandins (r= 0.93,p<0.05 for prostaglandin E2andr= 0.89,p<0.05 for 6-ketoprostaglandin F1α), whereas no significant correlations were found between variations of vascular resistance and catecholamines. Prostaglandin blockade induced by diclofenac sodium administration caused, from the first cold application, a pattern of the vasoconstrictor response and plasma prostaglandin and norepinephrine changes similar to that observed at the fifth cold application in untreated subjects, when prostaglandins are no longer detectable in plasma. We conclude that an increased vasoconstrictor response to sympathetic stimulation in humans may result from a diminished inhibitory influence of prostaglandins on adrenergic transmission.

ISSN:0009-7330    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

Acute myocardial damage similar to that seen in human myocarditis occurs in BALB/c mice after infection with coxsackievirus B3 (CB3) or encephalomyocarditis virus (EMC). To investigate the role of antigen-specific T cells in the pathogenesis of this disorder, we compared CB3 disease expression in T cell-deficient, athymic nude (nu/nu) mice, in heterozygote (nu/+) mice with normal T cell function, and in nu/nu mice reconstituted with spleen cells from CB3-or EMC-infected nu/+ mice. Acute myocarditis occurred in both nu/nu and nu/+ mice. Severe myocarditis, however, developed only in nu/+ and nu/nu mice reconstituted with CB3-sensitized T cells, but not in those reconstituted with EMC-sensitized T cells. Myocardial virus titer and serum anti-CB3 antibody production were similar in nu/+ and nu/nu groups. Additionally, the presence of Thy 1.2 (pan T), Ly 1 (precursor of other T cell subsets), and Ly 2 (suppressor/cytotoxic T) positive cells was demonstrated in the myocardium in nu/+ and nu/nu mice reconstituted with CB3 -sensitized T cells, but not with T cells sensitized by another virus, EMC. These results indicate that immature but antigen-specific T cells play a role in the pathogenesis of ongoing myocarditis.

ISSN:0009-7330    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

The mechanism by which heart cells of cardiomyopathic (CM) hamsters become calcium overloaded is not known. We examined the number of slow calcium channels, calcium uptake via slow calcium channels, calcium pool sizes, and the contractile response to Bay K 8644, verapamil, and nifedipine using isolated cardiac myocytes from 8-9-month-old CM hamsters (BIO 14.6) and age-matched normal controls. The number of dihydropyridine binding sites as assessed by specific binding of [3H]PN200-110 was similar in the two groups (control hearts: Bmax=333±89 [mean±SD] fmol/mg; CM hearts: Bmmax=357±75 fmol/mg;n=5 experiments,p=0.6). Current density through L-type calcium channels was determined using the whole-cell clamp technique (at −50 mV holding potential and −10 mV test potential) and was the same in CM myocytes (17.8±1.5 [mean±SD] pA/pF) and control myocytes (18.6±2.1 pA/pF) (n=5 experiments,p=0.5). The current-voltage relation (test potentials varied from −40 to +50 mV) was also the same in CM and control cells, as was apparent threshold, peak current, and reversal potential. However, the initial rate of45Ca influx as well as the size of the rapidly exchangeable calcium pool was significantly greater in myocytes obtained from CM than from normal hamsters. In both myocyte preparations, Bay K 8644 increased the rate of45Ca uptake by 25% at 60 seconds; verapamil decreased45Ca uptake at 60 seconds by 16% and 17% in normal and CM hamsters, respectively. A similar inhibitory effect was observed with nifedipine. The amplitude of cell motion in cells driven at 1.5 Hz as assessed by an optical-video system increased progressively with increasing concentrations of extracellular calcium or Bay K 8644 in cardiac myocytes from normal or CM hamsters. However, the concentration-effect curves for the two effectors were shifted to the left in CM cells compared with cells from normal hamsters. Both preparations demonstrated similar contractile responses to verapamil and nifedipine. These findings demonstrate that single enzymatically dissociated cardiac myocytes from CM hamsters have impaired contractile properties analogous to those seen in the intact heart and thus provide a useful experimental system in which to study underlying cellular mechanisms operative in this model of heart failure. Our results further indicate that calcium overload in CM hamster cardiac myocytes may not be due to increased calcium influx via dihydropyridine-sensitive calcium channels, as suggested previously, but rather to abnormalities of intracellular calcium homeostasis.

ISSN:0009-7330    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

The secretion of atrial natriuretic peptide by the heart is not simply the arterial–coronary sinus concentration difference times coronary blood flow, because only a small fraction of total coronary blood flow passes through the atria. We measured coronary sinus and arterial plasma atrial natriuretic factor (ANF) concentrations and blood flow to each part of the heart using the radioactive microsphere technique. Before acute volume expansion, the arterial-coronary sinus ANF difference was 305±23 pg/ml and rose to 1,009±220 pg/ml during volume expansion, whereas total coronary blood flow rose from 167 to 465 ml/min. Atrial blood flow rose from 2.9% to 4.6% of total coronary blood flow during volume expansion. ANF secretion rate increased from 51 to 469 ng/min. When divided by atrial weight, ANF secretion rate increased from 4.0±0.3 to 56±12 ng/min/g atrial tissue—in other words, from 0.3% to 3.7% of tissue ANF content each minute. Dividing by atrial blood flow indicated that the concentration of ANF leaving atrial tissue was 10,000 to 29,651 pg/ml, and the additional secretion of ANF was determined by the increase in coronary blood flow. Therefore, at least two mechanisms are responsible for altering coronary sinus ANF and circulating ANF: the release rate from atrial myocytes and the washout via changes in atrial blood flow.

ISSN:0009-7330    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

A prolongation of the ventricular effective refractory period in response to cholinergic agonists or vagal stimulation has been demonstrated in a number of in vivo animal models. However, exposure of isolated myocardial tissues obtained from these hearts to as much as 10−4M acetylcholine has been shown to produce essentially no change in action potential duration or effective refractory period. The discrepancy between the in vivo and in vitro findings generally has been explained on the basis of accentuated antagonism, whereby parasympathetic agonists exert their influence through antagonism of the effects of β-adrenergic tone in vivo. The fact that acetylcholine exerts little if any direct effect on the electrical activity of ventricular myocardium, although well accepted, is based exclusively on studies performed using endocardial preparations. Our recent demonstration of major electrophysiological differences between canine ventricular endocardium and epicardium prompted us to examine the effects of acetylcholine and the role of accentuated antagonism in these two tissue types. Using standard microelectrode techniques, we show that acetylcholine (10−7-10−5M) has little if any effect in canine ventricular endocardium but a pronounced effect to either prolong or markedly abbreviate action potential duration and effective refractory period in epicardium. These effects of acetylcholine on epicardium are attended by an accentuation of the spike and dome morphology of the action potential, are readily reversed with atropine, fail to appear when epicardium is pretreated with the transient outward current blocker 4-aminopyridine, are accentuated in the presence of isoproterenol (10−7to 5×10−6M), and persist in the presence of propranolol. Isoproterenol-induced abbreviation of action potential duration and effective refractory period is also shown to be more pronounced in epicardium than in endocardium; equimolar concentrations of acetylcholine completely antagonize the effects of isoproterenol in endocardium and epicardium. We conclude that acetylcholine exerts important direct effects on the electrical response of canine ventricular myocardium, which are accentuated in the presence of β-adrenergic agonists. Our findings suggest the differential response of epicardium and endocardium to acetylcholine is due to the presence of a transient outward current-mediated spike and dome morphology in the epicardial action potential. Finally, the data suggest that acetylcholine may exert antiarrhythmic as well as arrhythmogenic effects through its actions to alter conduction and refractoriness.

ISSN:0009-7330    

出版商:OVID    

年代:1990

数据来源: OVID