Post-rest Contraction Research Articles

Doxorubicin-induced late cardiotoxicity: delayed impairment of Ca<sup>2+</sup>-handling mechanisms in the sarcoplasmic reticulum in the rat

Doxorubicin treatment causes delayed development of cardiotoxicity. Whether the doxorubicin-induced impairment of cardiac functions reverses or progresses with time after the cessation of the treatment was examined. The rats were injected with doxorubicin (2.5 mg/kg, i.v., once a week for 3 weeks) and sacrificed at 1 (1W), 13 (13W), or 18 (18W) weeks after the final doxorubicin administration. The time to peak of twitch contraction observed at 2-Hz stimulation was not altered in left atrial or ventricular muscle preparations isolated from 1W rats, but it was prolonged in those from 13W and 18W rats. The reduction of the magnitude of postrest contraction and the alteration of force-frequency relationships in left atrial muscle preparations in 1W rats were not significant, but were intensified in the 13W and 18W groups. Alterations in the postrest contraction and the force-frequency relationships in ventricular muscle preparations isolated from doxorubicin-treated rat hearts were weaker, but the pattern of alteration was similar to that observed in left atrial muscle preparations. Caffeine-induced contraction observed in skinned fibers that were isolated from the 1W rats was not altered, but it was reduced in the 18W rats. The Ca2+ sensitivity of contractile proteins was not altered in doxorubicin-treated rat hearts in any of the groups. The K(d) values estimated from a [3H]ryanodine binding study were not altered, but the B(max) values were significantly lower in the 13W and 18W groups than those observed in control rats. These results suggest that the dysfunction of the sarcoplasmic reticulum progresses after the completion of doxorubicin treatment and contributes to the doxorubicin-induced late cardiotoxicity.

Post-rest contraction amplitude in myocytes from failing human ventricle.

It has been reported that the balance between the two main Ca2+ removal systems in the cardiac cells, the sarcoplasmic reticulum (SR) and Na+/Ca2+ exchanger, is altered in failing human heart. We have studied post-rest contraction behaviour as a non-invasive probe of the amount of Ca2+ stored in the SR in myocytes from failing and non-failing human ventricle. The first beat following a rest interval, as a percentage of the preceding steady state (B1/SS), was larger and more variable in cells from failing heart, indicating some accumulation of Ca2+ in the SR during rest. This could be mimicked by treatment of myocytes with digoxigenin, a compound which increases intracellular Na+, suggesting that alterations in the Na+ balance of the cell might contribute to the effect. Isoprenaline, which stimulates Ca2+ uptake by the SR while the myocyte is beating, prevented SR Ca2+ accumulation during rest in susceptible myocytes. We hypothesize that loss of SR function in the failing heart is partially compensated for by increased Ca2+ extrusion via the Na+/Ca2+ exchange in the contracting myocyte, leading to increased intracellular Na+ during activity. This Na+ is lost at rest, predisposing the cells to accumulate Ca2+ in the SR. Experiments to test this hypothesis are proposed.

Comparison of the contractile performance of the hypertrophied myocardium from spontaneous hypertensive rats and normotensive infarcted rats

The sarcoplasmic reticulum (SR) exerts a key role on the excitation-contraction coupling process in the myocardium. Since the relation between the volume of cellular organelles, such as SR, and the sarcolemmal area of myocytes is not uniform in myocardial hypertrophy of different etiologies, we compared the contractile performances of hypertrophied left ventricular papillary muscles from rats with pressure overload and with volume overload. Hemodynamically compensated spontaneous hypertensive rats (SHR, 3 months old, systolic blood pressure = 189 ± 4 mmHg, n = 8) and Wistar rats with healed (30 days) myocardial infarction (MI, n = 7) produced by ligation of the left coronary artery were used. Results were compared with age-matched Wistar control (CON) rats (n = 13). Force (F), corrected to muscle cross-sectional area (g/mm2), and dF/dt were recorded in muscles contracting isometrically and stretched to Lmax. The inotropic response to increasing extracellular Ca2+ concentrations (1.25 to 5.0 mM) was compared in twitches (0.5 Hz) and during tetanic stimulation (5 Hz, 30 s) in the muscles treated with 1 µM ryanodine. F recorded in basal conditions (Ca = 1.25 mM, 0.5 Hz) in the CON group (1.34 ± 0.20 g/mm2) was higher (p < 0.05) than in the MI (0.73 ± 0.13 g/mm2) and lower (p < 0.05) than in the SHR group (2.08 ± 0.25 g/mm2). Similar differences between groups were also observed in relation to +dF/dt. Increasing extracellular Ca produced a parallel increase of F and +dF/dt in the three groups of muscles. Ryanodine treatment reduced F and +dF/dt in all groups and completely inhibited the development of force in post-rest contractions, indicating SR inhibition. SHR muscles were more sensitive to ryanodine than CON and MI (F decrease = 64 ± 7, 51 ± 5, and 22 ± 5%, respectively, p < 0.05). The tetanic tension (Ca = 1.25 mM) was similar in SHR and CON (0.82 ± 0.19 and 0.92 ± 0.18 g/mm2; p > 0.05) and depressed in the MI group (0.35 ± 0.12 g/mm2). These data suggest an increased participation of SR as source of activator Ca in the hypertrophied muscle of SHR. This adaptation likely contributes to maintain the normal cardiac function in hemodynamically compensated SHR, despite increasing afterload levels. This adaptation seems not to occur after MI, which may contribute to the depressed contractile performance of the left ventricular muscle surviving to infarction.Key words: myocardial contractility, myocardial hypertrophy, sarcoplasmic reticulum, myocardial infarction, hypertension.

Comparison of the contractile performance of the hypertrophied myocardium from spontaneous hypertensive rats and normotensive infarcted rats

The sarcoplasmic reticulum (SR) exerts a key role on the excitation-contraction coupling process in the myocardium. Since the relation between the volume of cellular organelles, such as SR, and the sarcolemmal area of myocytes is not uniform in myocardial hypertrophy of different etiologies, we compared the contractile performances of hypertrophied left ventricular papillary muscles from rats with pressure overload and with volume overload. Hemodynamically compensated spontaneous hypertensive rats (SHR, 3 months old, systolic blood pressure = 189 +/- 4 mmHg, n = 8) and Wistar rats with healed (30 days) myocardial infarction (MI, n = 7) produced by ligation of the left coronary artery were used. Results were compared with age-matched Wistar control (CON) rats (n = 13). Force (F), corrected to muscle cross-sectional area (g/mm2), and dF/dt were recorded in muscles contracting isometrically and stretched to Lmax. The inotropic response to increasing extracellular Ca2+ concentrations (1.25 to 5.0 mM) was compared in twitches (0.5 Hz) and during tetanic stimulation (5 Hz, 30 s) in the muscles treated with 1 microM ryanodine. F recorded in basal conditions (Ca = 1.25 mM, 0.5 Hz) in the CON group (1.34 +/- 0.20 g/mm2) was higher (p < 0.05) than in the MI (0.73 +/- 0.13 g/mm2) and lower (p < 0.05) than in the SHR group (2.08 +/- 0.25 g/mm2). Similar differences between groups were also observed in relation to +dF/dt. Increasing extracellular Ca produced a parallel increase of F and +dF/dt in the three groups of muscles. Ryanodine treatment reduced F and +dF/dt in all groups and completely inhibited the development of force in post-rest contractions, indicating SR inhibition. SHR muscles were more sensitive to ryanodine than CON and MI (F decrease = 64 +/- 7, 51 +/- 5, and 22 +/- 5%, respectively, p < 0.05). The tetanic tension (Ca = 1.25 mM) was similar in SHR and CON (0.82 +/- 0.19 and 0.92 +/- 0.18 g/mm2; p > 0.05) and depressed in the MI group (0.35 +/- 0.12 g/mm2). These data suggest an increased participation of SR as source of activator Ca in the hypertrophied muscle of SHR. This adaptation likely contributes to maintain the normal cardiac function in hemodynamically compensated SHR, despite increasing afterload levels. This adaptation seems not to occur after MI, which may contribute to the depressed contractile performance of the left ventricular muscle surviving to infarction.

Effect of doxorubicin on postrest contraction in isolated rat hearts.

Clinical use of doxorubicin is limited by its cardiotoxicity. In doxorubicin-induced cardiomyopathy, vacuolization of the sarcoplasmic reticulum (SR) has been reported. We investigated whether doxorubicin had a direct action on the sarcoplasmic reticulum (SR) in isolated perfused rat hearts. The left and right atria were trimmed to maintain heart rate (HR) <200 beats/min. Postrest contractions, which are believed to be due primarily to Ca2+ release from the SR, were evoked with a programmable stimulator after variable rest intervals. The amplitude of the postrest contractions increased markedly as the rest interval increased. Doxorubicin (0.1 mM) significantly suppressed this potentiation of the postrest contractions. Furthermore, doxorubicin slowly induced aftercontractions and an increase in left ventricular diastolic pressure (LVDP), phenomena that are usually associated with ouabain intoxication. We conclude that doxorubicin-induced cardiomyopathy may be due to SR dysfunction, leading to intracellular Ca2+ overload.

Effects of pirarubicin in comparison with epirubicin and doxorubicin on the contractile function in rat isolated cardiac muscles

1. 1. We have examined the effects of pirarubicin (THP), compared with epirubicin (EPI) and doxorubicin (DXR), on the contractile function in papillary muscles isolated from rats. 2. 2. In in vivo experiments, in which the rat was treated once a week for 4 weeks with DXR (total dose 10 mg/kg) and thereafter once a week for 4 weeks with THP, EPI or DXR (total dose 10 mg/kg), a positive instead of negative force-frequency relationship was observed in the muscles treated with EPI and DXR, but not with THP, and an increase in contractile response to extracellular Ca 2+ was observed more markedly in the muscles treated with DXR than in those treated with EPI and THP. 3. 3. In in vitro experiments, in which the muscle preparations were incubated with the drugs at 100 or 200 μM for 2 hr, EPI and DXR caused a negative inotropic effect and a prolongation of tension duration, while THP caused a slight positive inotropic effect and a slight prolongation of tension duration. 4. 4. Furthermore, a decrease in the potentiated postrest contraction was observed more markedly in the muscles incubated with EPI and DXR at 200 μM than in those with THP. 5. 5. These results suggest that both EPI and DXR show a cardiotoxicity by impairing the function of cardiac sarcoplasmic reticulum, and that the switching of the treatment from DXR to THP produces less impairing effects.

Difference in excitation-contraction mechanisms between atrial and ventricular myocardia of hatched chicks

1. 1. Effects of inotropic interventions on contractile force were examined in isolated atrial and ventricular preparations from hatched chicks. 2. 2. The duration of twitch contractions were briefer in the atria than in the ventricle. 3. 3. No difference in the extracellular Ca 2+-contractile force curve was observed between atrial and ventricular preparations. 4. 4. The sensitivity to nicardipine was higher in the ventricular preparations than in atrial preparations while that to ryanodine was higher in atrial preparations. 5. 5. The magnitude of post-rest contraction was larger than the basal contractile force in atrial preparations, while it was smaller than the basal contractile force in ventricular preparations. 6. 6. These results suggested that the atrial myocardium of hatched chick was more dependent on sarcoplasmic reticulum function than its ventricular myocardium, which is similar to the case with mammalian species.

Post-rest potentiation and its decay after inotropic interventions in isolated rat heart muscle.

The effects of various inotropic interventions on post-rest potentiation and its decay were investigated in isolated cardiac muscle. The inotropic interventions studied were: reduced extracellular Na+ and elevated extracellular Ca2+ concentration; exposure to ouabain, monensin, isoprenaline, phenylephrine and cirazoline. Force of contraction (stimulation frequency 2 Hz) was measured isometrically in left atria and right ventricular strips of rat hearts. Maximum post-rest potentiation was reached after 10 sec. of rest and amounted to 245 +/- 26% of pre-rest control in ventricle and 192 +/- 15% in atria. Ca(2+)-recirculation fraction was calculated from the decay of post-rest potentiation after resumption of regular stimulation, it was 0.77 +/- 0.01 in 11 control ventricular strips. High concentrations of caffeine (3 mmol/l) completely abolished post-rest potentiation in both tissues. The development of post-rest potentiation was accelerated in the presence of most of the inotropic agents. However, with the exception of ouabain and only in atrial muscle, none of the inotropic interventions produced higher post-rest contraction amplitudes than during controls. In rat heart muscle, the inotropic interventions studied are not any more effective in augmenting force of contraction than prolonged stimulation intervals. This suggests that (1) the distribution of Ca2+ into the sarcoplasmic reticulum is at a maximum during post-rest potentiation; (2) modifications of signal transduction pathways cannot further increase post-rest potentiation; and therefore that (3) shifts in Ca2+ distribution act as a limiting factor.

Effect of Thiopental on Ca2+ Release from Sarcoplasmic Reticulum in Intact Myocardium

While thiopental is known to inhibit the myocardial transsarcolemmal influx of Ca2+, the effect of thiopental on sarcoplasmic reticular Ca2+ release has not been characterized. Isolated papillary muscles of rabbits were used. We measured postrest contractions to assess the Ca2+ release by sarcoplasmic reticulum in response to electrical stimulation. Contractures induced by rapid cooling were used as an index of Ca2+ content of sarcoplasmic reticulum. The effect of thiopental on the availability of extracellularly derived Ca2+ was evaluated from measurements of contractions at 0.1 Hz in the presence of 1 microM ryanodine. Thiopental sodium (10, 20, and 30 mg/l; 38, 76, and 113 microM) inhibited the postrest contraction but not the contracture induced by rapid cooling. In the presence ryanodine, thiopental inhibited the postrest contraction elicited after 10 s of rest after 2-Hz stimulation much less than the steady-state contraction at 0.1 Hz (beat interval 10 s). Thiopental inhibited the postrest contraction (no ryanodine present) more strongly than did Ni2+ (an inhibitor of the transsarcolemmal Ca2+ influx) when the contraction at 0.1 Hz in the presence of ryanodine was inhibited to the same extent. These results suggest that thiopental decreases sarcoplasmic reticulum Ca2+ release induced by electrical stimulation and inhibits the ryanodine-induced efflux of sarcoplasmic reticulum Ca2+.

Mercury Effects on the Contractile Activity of Isolated Heart Muscle

The toxic effects of HgCl 2(1, 2.5, 5, and 10 μM) were studied in isolated, isometrically contracting rat papillary muscles and frog ventricular strips. In rat papillary muscles 1 μM Hg 2+ produced a small increase in the force of contraction. Higher concentrations of HgCl 2 produced a dose-dependent decrease in contractile force. The rate of force development was affected differently, increasing at 1 and 2.5 μM Hg 2+ and decreasing to control levels at 5 and 10 μM Hg 2+. This was the result of a progressive reduction in the time to peak tension observed when HgCl 2 concentrations increased. This effect probably reflects the binding of Hg 2+ to SH groups inducing Ca 2+ release from the sarcoplasmic reticulum. The relative potentiation of postrest contractions was used as an index of sarcoplasmic reticulum activity. It was measured after pauses of increasing duration and was reduced at concentrations of 1 μM Hg 2+ when compared to that of the control. A further decrement in the relative potentiation was observed with higher Hg 2+ concentrations, indicating that the activity of the sarcoplasmic reticulum was depressed by mercury in a dose-dependent manner. Tetanic contractions were also studied in the rat myocardium. The tetanic tension did not change during treatment with 1 μM Hg 2+ hut decreased with 5 μM Hg 2+, suggesting a toxic effect on the contractile proteins only at high Hg 2+ concentrations. Frog ventricu; lar strips were studied using the same HgCl 2 concentrations and no effects on either force or relative potentiation were observed. These findings suggest that Hg 2+ promotes dose-dependent toxic effects on heart muscle via actions on the sarcolemma, the sarcoplasmic reticulum, and contractile proteins.

Temperature modulates calcium homeostasis and ventricular arrhythmias in myocardial preparations.

The aim was to evaluate the effect of temperature on reoxygenation induced ventricular arrhythmias in isolated hearts, on delayed afterdepolarisations and Iti current in Purkinje fibres, and on sarcoplasmic reticular function and Ca2+ handling of single cardiac myocytes. Isolated guinea pig hearts were retrogradely perfused at 37 degrees C with a hypoxic medium for 15 min and reoxygenated for 10 min either at 33 degrees C or at 37 degrees C. Intracellular microelectrodes were used to assess the presence of delayed afterdepolarisations and triggered activity in sheep Purkinje fibres exposed to strophanthidin at different temperatures. Iti current was evaluated in voltage clamp experiments. In rat cardiomyocytes, loaded with the fluorescent Ca2+ dye, indo-1, the sarcoplasmic reticular Ca2+ content was assessed at 30 degrees C and at 37 degrees C, either by a caffeine spritz puffed onto a cell from a patch pipette or by a post-rest contraction. Hypothermic reoxygenation reduced the incidence of ventricular arrhythmias in isolated hearts (30%, n = 10, at 33 degrees C and 75%, n = 30, at 37 degrees C, p < 0.05). In Purkinje fibres, hypothermia decreased the amplitude of delayed afterdepolarisations. Moreover, at 32 degrees C, the amplitude of Iti current was decreased to 59.2(SEM 2.6)% of the normothermic value [27.5(6.7) nA, n = 4, p < 0.005] and time to peak increased to 159.7(10.2)% [value at 37 degrees C = 470(41) ms, n = 4, p < 0.01]. In cardiac cells, sarcoplasmic reticular Ca2+ release induced by caffeine spritz or by post-rest contraction was increased at 30 degrees C. However, following a pacing period at 1 Hz, hypothermia prolonged the time to onset of the first spontaneous Ca2+ oscillation [59(14) s at 30 degrees C and 27(9) s at 37 degrees C, n = 5, p < 0.05] and reduced the oscillation frequency [1.1(0.4) min-1 at 30 degrees C and 3.1(0.9) min-1 at 37 degrees C, n = 5, p < 0.05]. Mild hypothermia increases sarcoplasmic reticular Ca2+ content but decreases the likelihood of spontaneous Ca2+ release. This may explain the reduction of delayed afterdepolarisations and Iti current amplitude in Purkinje fibres and it could represent a mechanism for the protection provided by hypothermia against ventricular arrhythmias.

Mechanisms for hypothermia-induced increase in contractile force studied by mechanical restitution and post-rest contractions in guinea-pig papillary muscle.

Lowering myocardial temperature increases contractile force, presumably by increasing intracellular calcium content. To study the mechanisms behind this, we compared the effects of some known inotropic interventions with hypothermia on mechanical restitution and post-rest contractile force in isolated guinea-pig papillary muscles. In four groups (n = 6 per group), the effects of: (1) reducing the ability for Na/Ca exchange to extrude Ca2+ (a) by increasing [Na+]i with ouabain or (b) by increasing [Ca2+]o; and (2) activation of calcium channels with Bay-K 8644, were compared with lowering temperature from 37 to 27 degrees C. Normally (at 37 degrees C and 2 mM CaCl2), mechanical restitution could be described by a rapid recovery phase with a time constant between 180 and 220 ms, followed by a slowly decaying phase with a time constant between 5000 and 8000 ms and post-rest contractions (1-10 min rest) were markedly depressed compared to steady-state contractions. Steady-state developed force was markedly increased at 27 degrees C, after 1 microM ouabain, 6 mM CaCl2 or 0.1 microM Bay-K 8644. At 27 degrees C the rapid recovery phase of restitution was delayed while the slowly decaying phase was not affected. Ouabain and increased [Ca2+]o caused elevation of the slowly decaying phase of restitution and markedly attenuated the post-rest depression of developed force, which may be attributed to a reduced diastolic extrusion of Ca2+ via the Na/Ca exchanger. Hypothermia and Bay-K 8644 on the other hand, augmented this post-rest depression. Hence, this study suggests that increased Ca2+ influx due to delayed inactivation of calcium channels may account for the increased developed force during hypothermia rather than reduced diastolic extrusion of Ca2+ via the Na/Ca exchanger.

Effects of E. coli sepsis and myocardial depressant factor on interval-force relations in dog ventricle.

We examined whether depressed left ventricular (LV) contractility during Escherichia coli sepsis in dogs was due to a decrease in the fractional release of calcium from the sarcoplasmic reticulum (SR) or a reduction in calcium content in this organelle. To indirectly assess SR calcium availability in a right ventricular (RV) trabecular muscle preparation, we utilized functional indexes of cellular myocardial calcium metabolism, which included rapid-cooling contracture (RCC), an indicator of SR calcium content, and postrest contraction (PRC), an index of calcium availability from the release compartment of the SR. Measurements were made during steady-state stimulation at 0.5 and 1.5 Hz, during which time rest intervals of 30-240 s were periodically imposed. SR calcium availability was measured in RV trabeculae of dogs subjected to 4 h of E. coli sepsis and was compared with calcium availability measured in nonseptic dogs. We further characterized a filterable cardiodepressant substance (FCS), which has been previously shown to be associated with LV depression in this model, to determine whether it produced changes in calcium metabolism similar to those found in sepsis. The results showed that calcium availability from the SR of septic dogs was not impaired. Furthermore, FCS was found in the 10,000- to 30,000-mol wt fraction of plasma and produced changes in PRC in canine trabeculae that were similar to those produced during sepsis. We conclude that, as assessed by PRC and RCC, SR calcium content and release are not impaired in sepsis.

Effects of inhibition of transsarcolemmal calcium influx by nickel on force of postrest contraction and on contracture induced by rapid cooling

The aim was to characterise the influence of a decrease in transsarcolemmal calcium influx on the calcium content of the sarcoplasmic reticulum and on the force of postrest contraction, which is largely activated by calcium released from the sarcoplasmic reticulum. Isometric force of postrest contractions in isolated rabbit papillary muscles was measured in the absence and in the presence of ryanodine (1 microM). The force measured in the presence of ryanodine was used as an index of transsarcolemmal calcium influx. The calcium content of the sarcoplasmic reticulum was estimated by rapid cooling induced contracture measurements. Nickel (Ni2+) was used to decrease the calcium influx, and the results were compared to those obtained by lowering the extracellular calcium concentration. The addition of Ni2+ or lowering extracellular Ca2+ markedly decreased the force of postrest contraction measured in the presence of ryanodine, and moderately decreased the force of postrest contraction measured in the absence of ryanodine. Neither the addition of Ni2+ nor lowering extracellular Ca2+ decreased maximum rapid cooling induced contracture; this was actually increased in the presence of Ni2+ even though a larger number of stimuli was required in the presence than in the absence of Ni2+ for the force of the subsequently elicited contracture to reach the maximum level. The results suggest: (1) that a decrease in the calcium influx reduces the force of the postrest contraction by decreasing the amount of calcium required to trigger the release of calcium from the sarcoplasmic reticulum, without decreasing its maximum calcium content; and (2) that Ni2+ decreases the rate of filling of sarcoplasmic reticular calcium stores even though the calcium content of the sarcoplasmic reticulum after a sufficiently large number of beats is higher in the presence of Ni2+ than in its absence.

Lack of inotropic effect of cyclosporin A in isolated guinea pig left atria

Cyclosporin A is being widely used to prevent graft rejection in organ transplantation and to treat autoimmune diseases. Since various toxic side effects have been observed, the aim of this study was to look for even a subtle deleterious effect of cyclosporin A on cardiac inotropy in electrically stimulated guinea pig left atria. The left atrial muscles of guinea pigs, in Tyrode's solution containing 2.7 or 5.4 mM potassium, were electrically stimulated by one of two methods: (1) continuously at 3 Hz, during which cyclosporin A was applied cumulatively (from 10(-9) to 10(-5) M); or (2) stimulated intermittently at 2.5 Hz in 5 mM cyclosporin A, with rest periods of 4 s duration interposed every 4 min. The effects of cyclosporin A on contractile force were observed for 150 min in the first stimulation method, and the effects on the steady state contractile force and amplitude of post-rest contraction were observed for 240 min in the second method. The steady state contractile force of the atria declined within the 4 h period at 2.7 mM potassium in Tyrode's solution both in the cyclosporin A group (n = 10) and in the control group (n = 5) to 68(SD 11)% and to 63(4)%, respectively. After 4 h the amplitudes of the post-rest contraction were 101(16)% and 101(4)% in cyclosporin A and control groups, respectively. At 5.4 mM potassium, the following values were obtained (cyclosporin A v control): steady state force 70(8)% (n = 11) v 69(8)% (n = 5); post-rest force 105(9)% v 102(7)%. Cyclosporin A does not influence the steady state contractile force or the amplitude of the post-rest contraction, suggesting the absence of inotropic effects on isolated guinea pig left atria.

Mechanisms for hypothermia-induced increase in contractile force studied by mechanical restitution and post-rest contractions in guinea pig papillary muscle

Mechanisms for hypothermia-induced increase in contractile force studied by mechanical restitution and post-rest contractions in guinea pig papillary muscle

Effect of haemodynamic pressure overload of the adult ferret right ventricle on inotropic responsiveness to external calcium and rest periods

The inotropic effects of external calcium concentration ([Ca2+]o] and rest periods have been compared in papillary muscles isolated from control (n = 4) and pressure-overloaded right (n = 5) ventricles of adult ferrets. Hypertrophy was induced by pulmonary artery clipping for 30-45 days. Under control conditions (3 mM [Ca2+]o, 0.1 Hz), the isometric twitch force of hypertrophied muscles was decreased by 75%, time to peak was increased by 30% and time to half-relaxation was increased by 50% compared with non-hypertrophied preparations. The sensitivity of contraction to [Ca2+]o was decreased in hypertrophied muscles compared with control ([Ca2+] required for half-maximal contraction: 4.1 mM vs 1.7 mM) and the maximal contraction reached at high [Ca2+]o was smaller in pressure-overloaded muscles compared with control (8.3 +/- 2.0 mN mm-2 vs 19.0 +/- 2.1 mN mm-2 respectively). In both groups, rest periods longer than the steady-state interval were initially accompanied by a potentiation of the first post-rest contraction compared with steady-state. Peak potentiation occurred after a rest of 120 s in hypertrophied muscles and after a rest of 60 s in control. The maximal relative potentiation, i.e. compared with the steady-state twitch, was higher in hypertrophied muscles (+75%) than in control (+20%). After peak potentiation, the amplitude of the first post-rest contraction progressively decreased with increasing periods of rest, although at a slower rate in hypertrophy compared with control. The time constants of post-rest decay were 1203 +/- 99 s and 528 +/- 24 s respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Rat atrial muscle responses with caffeine: dose-response, force frequency, and postrest contractions.

Caffeine has been reported to have a positive and (or) a negative inotropic effect on cardiac muscle. In this study, the force-frequency and postrest characteristics of rat atrium were studied in the presence of caffeine (1.0-10 mM) to see if the interval between beats affected the response of cardiac muscle to caffeine. When stimulation frequency was 0.5 or 2.0 Hz, there was a positive followed by a negative inotropic response with 1, 5, or 10 mM caffeine. Incomplete relaxation occurred under these circumstances, giving rise to contracture. At low frequency of stimulation (0.1 Hz) caffeine had only a negative inotropic effect, and this effect was greater with 1 mM caffeine than with 5 mM caffeine. In the absence of caffeine, when stimulation at 0.5 or 3 Hz was interrupted, a pause of 2-20 s resulted in potentiation. When caffeine was present (2.0 mM), postrest potentiation was severely attenuated, but the steady-state contraction amplitude within the range 0.5-3.0 Hz was not different. These results are consistent with the hypothesis that caffeine induces a leak of Ca2+ from the sarcoplasmic reticulum, and this Ca2+ is extruded from the cell, possibly by Na+/Ca2+ exchange. Sarcoplasmic reticular uptake of Ca2+ and the translocation to release sites appear not to be affected by caffeine within 1-5 mM concentrations.

Myocardial depression produced by sustained tachycardia in rabbits

Much recent attention has been focused on the tachycardia-induced heart failure model. We hypothesized that sustained tachycardia would lead to myocardial depression in rabbits, as it does in dogs and swine. We evaluated the passive and active length-tension relations and postrest contraction behavior in right ventricular papillary muscles from 22 New Zealand White rabbits, 11 controls, and 11 subjected to ventricular pacing at a rate of 400 beats/min for 29.4 +/- 10.6 days. Studies were performed in oxygenated buffer at 22 degrees C. Active tension was significantly reduced at muscle lengths of 0.95.Lmax and above; at Lmax it was 4.7 +/- 0.2 g/mm2 for the control group and 3.3 +/- 0.2 g/mm2 for the paced group (P less than 0.005). Both groups showed increased force development when the concentration of calcium in the buffer was increased. There were no differences between the groups in the passive length-tension relations. Of note, postrest contraction data showed that the second postrest beat was smaller for the paced animals for rest intervals up to 2 min, suggesting that beat-to-beat trans-sarcolemmal calcium handling may differ from normal in this model. We conclude that sustained tachycardia will lead to myocardial depression in rabbits; the extension of this model to a small animal species may offer new ways to explore its causative mechanisms.

Negative inotropic effects of caffeine related to leak of Ca2+ from the sarcoplasmic reticulum.

Caffeine has been reported to have either a positive or a negative inotropic effect on cardiac muscle, and it is not known what circumstances are necessary for each of these effects. In this study, rat atria were exposed to caffeine (1–5) mM with regular stimulation at 0.1 – 3 Hz. Postrest contractions were also observed to determine if inhibition of contractions was always seen when stimulation was interrupted (Endoh and Iijima, 1981). As expected, addition of caffeine to the superfusate resulted in a transient positive inotropic response, when stimulation was 0.5 or 2.0 Hz. In contrast with this, when stimulation was 0.1 Hz, there was only a negative inotropic response. This decrease in developed tension was greater with 1 mM than with 5 mM caffeine, (see Table 1).KeywordsSarcoplasmic ReticulumStimulation FrequencyNegative Inotropic EffectInotropic ResponseDevelop TensionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.