Post-rest Contraction Research Articles

Remodelamento miocárdico após grandes infartos converte potenciação pós-pausa em decaimento da força em ratos

Post-rest contraction (PRC) of cardiac muscle provides indirect information about the intracellular calcium handling.Our aim was to study the behavior of PRC, and its underlying mechanisms, in rats with myocardial infarction.Six weeks after coronary occlusion, the contractility of papillary muscles (PM) obtained from sham-operated (C, n=17), moderate infarcted (MMI, n=10) and large infarcted (LMI, n=14) rats was evaluated, following rest intervals of 10 to 60 seconds before and after incubation with lithium chloride (Li(+)) substituting sodium chloride or ryanodine (Ry). Protein expression of SR Ca(2+)-ATPase (SERCA2), Na(+)/Ca(2+) exchanger (NCX), phospholamban (PLB) and phospho-Ser(16)-PLB were analyzed by Western blotting.MMI exhibited reduced PRC potentiation when compared to C. Opposing the normal potentiation for C, post-rest decays of force were observed in LMI muscles. In addition, Ry blocked PRC decay or potentiation observed in LMI and C; Li(+) inhibited NCX and converted PRC decay to potentiation in LMI. Although MMI and LMI presented decreased SERCA2 (72±7% and 47±9% of Control, respectively) and phospho-Ser(16)-PLB (75±5% and 46±11%, respectively) protein expression, overexpression of NCX (175±20%) was only observed in LMI muscles.Our results showed, for the first time ever, that myocardial remodeling after MI in rats may change the regular potentiation to post-rest decay by affecting myocyte Ca(2+) handling proteins.

Amiodarone Modulation of Intracellular Transport of Calcium Ions in Cardiomyocites

The influence of amiodarone on intracellular transport of calcium ion in cardiomyocytes of rat was investigated. The experiments were performed on isolated papillary muscles of Wistar rats. Force-frequency dependence (0.7, 1, 2, 3, 4 Hz), extrasystolic and postextrasystolic contractions and post-rest (4-60 s) reactions of rat myocardium after amiodarone treatment (1 μM) were investigated. Decay potentiation coefficient of contraction force was estimated. Results. The analyses of force-frequency dependence has shown that amiodarone prevent the decreasing of the force contraction at increasing of the stimulation frequency. Amiodarone promotes increase of the time constant t1(T50), that indicate the drug promotes acceleration of Са2+ transport inside the SR resulting increase of Са2+ in the places of its release from the sarcoplasmic reticulum (SR). Treatment of papillary muscle with amiodarone decreased amplitude of extrasystolic contractions. As known, postextrasystolic and post-rest reactions of myocardium characterize the SR function. We have found amiodarone increased potentiation of postextrasystolic and post-rest contractions. Preliminary caffeine perfusion of muscles preparations cancelled the amiodarone-induced increasing postextrasystolic and post-rest potentiation. However, potentiation decay coefficient before and after treatment with amiodarone didn’t have difference. Conclusions, amiodarone influences on intracellular calcium ions homeostasis by modulation SR functions related with most likely are stipulated either by activation of Са2+ transport from uptake sites to release sites or by prevent of Са2+ leakage from the SR.

Disfunção miocárdica e alterações no trânsito de cálcio intracelular em ratos obesos

Several mechanisms have been proposed to contribute to cardiac dysfunction in obesity models, such as alterations in calcium (Ca²⁺) handling proteins and β-adrenergic receptors. Nevertheless, the role of these factors in the development of myocardial dysfunction induced by obesity is still not clear. The purpose of this study was to investigate whether obesity induced by hypercaloric diets results in cardiac dysfunction. Furthermore, it was evaluated whether this functional abnormality in obese rats is related to abnormal Ca²⁺ handling and the β-adrenoceptor system. Male 30-day-old Wistar rats were fed with standard food (C) and a cycle of five hypercaloric diets (Ob) for 15 weeks. Obesity was defined as increases in body fat percentage in rats. Cardiac function was evaluated by isolated analysis of the left ventricle papillary muscle under basal conditions and after inotropic and lusitropic maneuvers. Compared with the control group, the obese rats had increased body fat and glucose intolerance. The muscles of obese rats developed similar baseline data, but the myocardial responsiveness to post-rest contraction stimulus and increased extracellular Ca²⁺ were compromised. There were no changes in cardiac function between groups after β-adrenergic stimulation. Obesity promotes cardiac dysfunction related to changes in intracellular Ca²⁺ handling. This functional damage is probably caused by reduced cardiac sarcoplasmic reticulum Ca²⁺ ATPase (SERCA2) activation via Ca²⁺ calmodulin kinase.

Involvement of L‐type calcium channel and serca2a in myocardial dysfunction induced by obesity

Obesity has been shown to impair myocardial performance. Nevertheless, the mechanisms underlying the participation of calcium (Ca(2+) ) handling on cardiac dysfunction in obesity models remain unknown. L-type Ca(2+) channels and sarcoplasmic reticulum (SR) Ca(2+) -ATPase (SERCA2a), may contribute to the cardiac dysfunction induced by obesity. The purpose of this study was to investigate whether myocardial dysfunction in obese rats is related to decreased activity and/or expression of L-type Ca(2+) channels and SERCA2a. Male 30-day-old Wistar rats were fed standard (C) and alternately four palatable high-fat diets (Ob) for 15 weeks. Obesity was determined by adiposity index and comorbidities were evaluated. Myocardial function was evaluated in isolated left ventricle papillary muscles under basal conditions and after inotropic and lusitropic maneuvers. L-type Ca(2+) channels and SERCA2a activity were determined using specific blockers, while changes in the amount of channels were evaluated by Western blot analysis. Phospholamban (PLB) protein expression and the SERCA2a/PLB ratio were also determined. Compared with C rats, the Ob rats had increased body fat, adiposity index and several comorbidities. The Ob muscles developed similar baseline data, but myocardial responsiveness to post-rest contraction stimulus and increased extracellular Ca(2+) was compromised. The diltiazem promoted higher inhibition on developed tension in obese rats. In addition, there were no changes in the L-type Ca(2+) channel protein content and SERCA2a behavior (activity and expression). In conclusion, the myocardial dysfunction caused by obesity is related to L-type Ca(2+) channel activity impairment without significant changes in SERCA2a expression and function as well as L-type Ca(2+) protein levels.

Severe food restriction induces myocardial dysfunction related to SERCA2 activity

Previous studies have shown that food restriction promotes myocardial dysfunction in rats. However, the molecular mechanisms that are responsible are unclear. We investigated the role of sarcoplasmic reticulum Ca2+-ATPase (SERCA2) on myocardial performance in food-restricted rats. Male Wistar-Kyoto rats, 60 days old, were fed a control or restricted diet (daily energy intake reduced to 50% of the control) for 90 days. Expression of Serca2a, phospholamban (PLB), Na+/Ca2+ exchanger (NCX), and thyroid hormone receptor (TRalpha1, TRbeta1) mRNA was determined by quantitative PCR. SERCA2 activity was measured by using 20 micromol/L cyclopiazonic acid (CPA) in a left ventricular papillary muscle preparation during isometric contraction in basal conditions and during post-rest contraction. Serum concentrations of thyroxine (T4) and thyrotropin (TSH) were also determined. The 50%-restricted diet reduced body and ventricular weight and serum T4 and TSH levels. The interaction of CPA and food restriction reduced peak developed tension and maximum rate of tension decline (-dT/dt), but increased the resting tension intensity response during post-rest contraction. PLB and NCX mRNA were upregulated and TRalpha1 mRNA was downregulated by food restriction. These results suggest that food restriction promotes myocardial dysfunction related to impairment of sarcoplasmic reticulum Ca2+ uptake as a result of a hypothyroid state.

Food restriction impairs myocardial inotropic response to calcium and β-adrenergic stimulation in spontaneously hypertensive rats

Although long-term food restriction (FR) has been shown to induce cardiac remodeling and dysfunction, there are few data on the effects of FR on pressure-overloaded hearts. The aim of this study was to examine the effects of FR on cardiac muscle performance during inotropic stimulation in the myocardium of spontaneously hypertensive rats (SHRs). Male 60-day-old SHRs were subjected to FR for 90 days. Food-restricted animals received 50% of the ad libitum amount of food consumed by the control group. Myocardial function was studied in isolated left ventricular papillary muscle under isometric contraction in basal condition (1.25 mmol/L extracellular Ca(2+) concentration) and after 3 inotropic maneuvers: (1) at postrest contraction of 30 seconds, (2) at extracellular Ca(2+) concentration of 5.2 mmol/L, and (3) after beta-adrenergic stimulation with 10(-6) mol/L isoproterenol. At basal condition, time from peak tension to 50% relaxation was greater in the food-restricted group (P < .05). Inotropic stimulation with postrest contraction and isoproterenol promoted a significant lower increase of developed tension, maximum rate of tension development, and maximum rate of tension decline in the food-restricted compared to the control group. The elevation of extracellular Ca(2+) concentration induced a lower increase of developed tension, maximum rate of tension development, and time from peak tension to 50% relaxation in the food-restricted than in the control group. In conclusion, long-term FR promotes impairment of myocardial inotropic response to calcium and beta-adrenergic stimulation in SHRs.

Mechanisms Responsible for Reduced Cardiotoxicity of Mitoxantrone Compared to Doxorubicin Examined in Isolated Guinea-Pig Heart Preparations

We reported previously that doxorubicin, an anticancer agent that has an anthracycline structure, alters Ca2+ releasing and uptake mechanisms in the sarcoplasmic reticulum of myocardial cells. These effects of doxorubicin are apparently related to its cardiotoxicity. Mitoxantrone is a similar anticancer agent with an anthracenedion structure that has been shown to be significantly less cardiotoxic. In the present study, the effects of mitoxantrone on the functions of the sarcoplasmic reticulum were examined in isolated muscle preparations obtained from the guinea-pig heart. In electrically-stimulated left atrial muscle preparations, incubation in vitro for 4 hr with 30 or 100 microM mitoxantrone significantly prolonged the time to the peak of twitch tension, markedly increased the developed tension observed at lower stimulation frequencies, thereby attenuating the slope of positive force-frequency relationships, and increased the postrest contraction observed after a 60-sec quiescent period. In myocytes isolated from ventricular muscles, 30 microM mitoxantrone increased the peak and the size of intracellular Ca2+ concentrations ([Ca2+] i), and prolonged the time to peak [Ca2+]i. In skinned muscle fiber preparations obtained from the left ventricular muscle, 30 muM mitoxantrone significantly increased the caffeine-induced contraction without affecting the Ca2+ sensitivity of contractile proteins. These results suggest that mitoxantrone enhances Ca2+ release from the sarcoplasmic reticulum in isolated atrial muscle preparations obtained from the guinea-pig heart. Apparent enhancement of the sarcoplasmic reticulum functions, in contrast to anthracyclines that has been shown to suppress these functions, seems to explain the relative lack of marked cardiotoxicity of mitoxantrone.

Myocardial contractile dysfunction contributes to the development of heart failure in rats with aortic stenosis

Objectives To analyze the potential contribution of contractility state and ventricular geometry to the development of heart failure in rats with aortic stenosis. Methods Rats were divided into three groups: compensated aortic stenosis (AS, n = 11), heart failure AS ( n = 12) and control rats (C, n = 13). Results After 21 weeks, failing AS rats presented higher systolic (C = 36.6 ± 3.1, AS = 78.6 ± 4.8 ⁎, failing AS = 104.6 ± 7.8 ⁎ †) and diastolic meridian stress (C = 6.9 ± 0.4, AS = 20.1 ± 1.1 ⁎, failing AS = 43.2 ± 3.2 ⁎ †), hydroxyproline (C = 3.6 ± 0.7 mg/g, AS = 6.6 ± 0.6 ⁎ mg/g, failing AS = 9.2 ± 1.4 ⁎ † mg/g) and cross-sectional area (C = 338 ± 25 μm 2, AS = 451 ± 32 ⁎ μm 2, failing AS = 508 ± 36 ⁎ † μm 2), in comparison with control and compensated AS animals ( ⁎ p < 0.05 vs. control, † p < 0.05 vs. AS). In the isometric contraction study, considering the time from peak tension to 50% relaxation (RT 50), the relative variation responses, following post-rest contraction and increase in Ca 2+ concentration, were higher in failing AS than compensated AS animals. In contrast, following post-rest contraction, compensated AS group presented higher values of the peak developed tension (DT) than failing AS group. Following beta-adrenergic stimulation, control animals presented higher values of + d T/d t and − d T/d t than AS animals. In addition, failing AS animals presented higher TPT values than compensated AS animals. Conclusion Myocardial contractile dysfunction contributes to the development of heart failure in rats with aortic stenosis.

Force-frequency relationship and rest potentiation in papillary muscles of Siberian ground squirrel in the period of preparation to hibernation

activity to hibernation. To date, any published data on the nature of rhythm-inotropic relationship in the myocardium of hibernating animals in this period are completely absent. Experiments were performed on the right ventricular papillary muscle isolated from ground squirrels ( C. undulatus ). The isolation, stimulation, and contraction amplitude ( A ) measurement were performed as described previously [11] at 30 ± 1°e . The steady-state force‐frequency relationships were studied in the frequency range from 0.1 to 1.0 Hz. The contraction amplitude of papillary muscles at a frequency of 0.1 Hz was taken as 100%. The maximum rest potentiation of contractility (in the range of 1 to 60 s), sometimes up to 600 s, was determined as an increment in the amplitude of the first post-rest contraction ( A 1 ) relative to the amplitude of the preceding steady-state contraction ( A 0 ) at a stimulation frequency of 0.8 Hz and calculated by the formula (( A 1 – A 0 )/ A 0 ) × 100, %) [14]. The mechanical restitution curves were obtained by plotting the increment in A 1 versus pause duration. The data are expressed as the mean and square error of the mean ( p < 0.05).

Eucalyptol, an essential oil, reduces contractile activity in rat cardiac muscle

Eucalyptol is an essential oil that relaxes bronchial and vascular smooth muscle although its direct actions on isolated myocardium have not been reported. We investigated a putative negative inotropic effect of the oil on left ventricular papillary muscles from male Wistar rats weighing 250 to 300 g, as well as its effects on isometric force, rate of force development, time parameters, post-rest potentiation, positive inotropic interventions produced by Ca2+ and isoproterenol, and on tetanic tension. The effects of 0.3 mM eucalyptol on myosin ATPase activity were also investigated. Eucalyptol (0.003 to 0.3 mM) reduced isometric tension, the rate of force development and time parameters. The oil reduced the force developed by steady-state contractions (50% at 0.3 mM) but did not alter sarcoplasmic reticulum function or post-rest contractions and produced a progressive increase in relative potentiation. Increased extracellular Ca2+ concentration (0.62 to 5 mM) and isoproterenol (20 nM) administration counteracted the negative inotropic effects of the oil. The activity of the contractile machinery evaluated by tetanic force development was reduced by 30 to 50% but myosin ATPase activity was not affected by eucalyptol (0.3 mM), supporting the idea of a reduction of sarcolemmal Ca2+ influx. The present results suggest that eucalyptol depresses force development, probably acting as a calcium channel blocker.

Myocardial dysfunction induced by food restriction is related to calcium cycling and beta-adrenergic system changes

Food restriction (FR) has been shown to promote myocardial dysfunction in rats. The aim of this study was to verify the participation of calcium and beta-adrenergic system on myocardial mechanical alteration in rats submitted to FR. Myocardial performance was studied in isolated left ventricular papillar muscle from young Wistar-Kyoto rats (WKY) submitted to FR or to control diet. The groups subjected to FR were fed 50% less food than the control group for 90 days. Mechanical function was studied in isometric contraction at post-rest contraction of 30 seconds (PRC), calcium chloride concentration 5.20 mM, and beta-adrenergic stimulation with isoproterenol 10 -6 M. FR decreased the body weight, and left and right ventricular weight. In basal condition (1.25 mM of calcium) time to peak tension (TPT) and time from peak tension to 50% relaxation (RT 50) were greater in the FR group. Muscle function was the same in both PRC groups. TPT decrease in both high calcium groups, more in FR rats; RT 50 dropped only in FR animals. TPT decreased in both Isoproterenol groups, more intensely in the FR group. This result suggests that food restriction impairs myocardial performance and these changes may be attributed to alterations in the intracellular calcium cycling and beta-adrenergic system.

Effect of ischemia-reperfusion on the post-rest inotropy of isolated perfused rat heart.

This paper aims to study the effect of ischemia-reperfusion on the post-rest inotropy and to characterize post-rest B1:B2 ratio as an index of intracellular Ca(2+) overload. When the rest interval between the cardiac beats is increased, the magnitude of the post-rest beats is increased. First beat (B1) is maximally potentiated with exponential decline of the second (B2) and subsequent beats, thereby establishing a normal B1:B2 ratio of post- rest inotropy of the cardiac muscle. The rest potentiation of B1 and subsequent decay in the magnitude B2 is thought to develop from the time-dependent changes in the Ca(2+)-uptake and release from the sarcoplasmic reticulum (SR). Ca(2+)-kinetics of SR can be modulated by a variety of interventions which produce Ca(2+) loading of the SR. Isolated perfused (K-H buffer, 34 degrees C) rat hearts were paced at 1 Hz (steady state frequency). Interbeat intervals between 1s and 10s were introduced and the recovery in the left ventricular contractile force (Pmax) of post-rest B1 and B2 for each interval was recorded. Their relative relationship was computed and compared under control and experimental conditions. High extracellular Ca(2+) (2.50 to 7.0 mM) or low extracellular Na(+) (50% of control), and ischemia (60 min, 34 degrees C) - reperfusion (30 min, 34 degrees C) caused the reversal of the control relationship of the B1 to B2, with B2 being more potentiated than B1, accompanied by the appearance of after-contractions during the rest intervals of 4s or more. The mean (+/- SE) control B1:B2 ratio (at 4s interval) of 1.12 +/- 0.05 was significantly (P<0.001) reduced to 0.93 +/- 0.07; 0.89 +/- 0.01; and 0.96 +/- 0.02 after high Ca(2+) (6 mM) perfusion, low Na(+)(50%) perfusion and ischemia-reperfusion respectively. Simultaneous perfusion with ryanodine (1 microM) abolished the after-contractions and significantly increased the reduced ratios. The time course of changes in B1:B2 ratio after graded ischemia-reperfusion showed a significant fall in the ratio between 30 and 60 min of ischemia. A parallel change in Pmax and a significant rise in the left ventricular end-diastolic pressure, indicating an irreversible phase of the injury was recorded. No significant changes in B1:B2 ratio were detected during the reversible phase (<30 min) of the ischemia-reperfusion injury. Ischemia-reperfusion induces significant alterations in the relative ratio of the post-rest contractions of the left ventricle in isolated perfused rat heart. The altered ratios were characterized to predict the irreversibility of the reperfusion injury and to index the extent of Ca(2+)-loading of the sarcoplasmic reticulum.

Sarcoplasmic reticulum Ca2+ load in human heart failure.

Excitation-contraction coupling and intracellular Ca2+ homeostasis are altered in heart failure. We tested the hypothesis that these changes are related to disturbed Ca2+ handling of the sarcoplasmic reticulum (SR). Isolated, electrically stimulated trabeculae were obtained from end-stage failing (NYHA IV) and nonfailing human hearts. Isometric twitch tension, intracellular Ca2+ transients (aequorin method) and SR Ca2+ content (rapid cooling contractures) were assessed under basal conditions (1 Hz, 37 degrees C) as well as after stepwise increasing rest intervals from 2-240 s (post-rest contractions). Protein expression of SERCA2a and phospholamban (Western blot) was assessed in a subset of failing trabeculae. In addition, the effects of SERCA1 overexpression on contractile function of isolated myocytes was tested. On average, post-rest twitch tension continuously increased with increasing rest intervals in nonfailing, but declined with rest intervals longer than 15 s in failing myocardium. The rest-dependent contractile changes were accompanied by parallel changes in intracellular Ca2+ transients. Failing trabeculae (n = 40) were grouped (group A: post-rest potentiation (force of contraction > pre-rest twitch force) after 120 s rest interval; group B: post-rest decay (force of contraction < pre-rest twitch force) after 120 s rest interval), and post-rest contractile function was related to SERCA2a and PLB expression. While PLB protein expression was not different, SERCA2a protein expression as well as SERCA2a/PLB ratio was significantly higher in group A vs. group B. Transfection of SERCA1 increased shortening amplitude and enhanced relaxation kinetics in failing human myocytes. In conclusion, SR Ca2+ handling is severely altered in human heart failure. Reduced SR Ca2+ release is due to diminished SR Ca2+ content directly related to a depressed expression of SERCA2a protein. Enhancing SERCA function or expression may improve SR Ca2+ handling in failing human myocardium.

Effect of manganese on guinea pig ventricle: initial depression and late augmentation of contractile force.

Effects of Mn2+ on isolated guinea pig ventricular myocardia were examined. In isolated papillary muscles, Mn2+ produced a transient decrease in contractile force followed by a late sustained augmentation. Mn2+ markedly increased the amplitude of post-rest contractions; the time course of potentiation was almost the same as that of the late augmentation of contractile force after Mn2+ application. Mn2+ also increased the amplitude of rapid-cooling contractures. The negative inotropic effect of diltiazem and nicardipine was not affected by the presence of Mn2+. Mn2+ shortened the action potential duration under normal condition whereas it prolonged the duration under Ca2+ free conditions. Mn2+, when applied to fura-2-loaded ventricular myocytes, markedly quenched the cytoplasmic fluorescence excited at 360 nm wavelength. We concluded that Mn2+ not only causes a decrease in contractile force by blocking the L-type Ca2+ channel, but also enters the cytoplasm through the channel and produces late augmentation of the contractile force through enhancement of sarcoplasmic reticulum function.

Impairment of myocardial contractility by anticancer anthracyclines: role of secondary alcohol metabolites and evidence of reduced toxicity by a novel disaccharide analogue.

1. The anticancer anthracycline doxorubicin (DOX) causes cardiotoxicity. Enzymatic reduction of a side chain carbonyl group converts DOX to a secondary alcohol metabolite that has been implicated in cardiotoxicity. We therefore monitored negative inotropism, assessed as inhibition of post-rest contractions, in rat right ventricle strips exposed to DOX or to analogues forming fewer amounts of their alcohol metabolites (epirubicin, EPI, and the novel disaccharide anthracycline MEN 10755). 2. Thirty microM EPI exhibited higher uptake than equimolar DOX, but formed comparable amounts of alcohol metabolite due to its resistance to carbonyl reduction. MEN 10755 exhibited also an impaired uptake, and consequently formed the lowest levels of alcohol metabolite. Accordingly, DOX and EPI inhibited post-rest contractions by approximately 40-50%, whereas MEN 10755 inhibited by approximately 6%. 3. One hundred microM EPI exhibited the same uptake as equimolar DOX, but formed approximately 50% less alcohol metabolite. One hundred microM MEN 10755 still exhibited the lowest uptake, forming approximately 60% less alcohol metabolite than EPI. Under these conditions DOX inhibited post-rest contractions by 88%. EPI and MEN 10755 were approximately 18% (P<0.05) or approximately 80% (P<0.001) less inhibitory than DOX, respectively. 4. The negative inotropism of 30-100 microM DOX, EPI, or MEN 10755 correlated with cellular levels of both alcohol metabolites (r=0.88, P<0.0001) and carbonyl anthracyclines (r=0.79, P<0.0001). Nonetheless, multiple comparisons showed that alcohol metabolites were approximately 20-40 times more effective than carbonyl anthracyclines in inhibiting contractility. The negative inotropism of MEN 10755 was therefore increased by chemical procedures, like side chain valeryl esterification, that facilitated its uptake and conversion to alcohol metabolite but not its retention in a carbonyl form. 5. These results demonstrate that secondary alcohol metabolites are important mediators of cardiotoxicity. A combination of reduced uptake and limited conversion to alcohol metabolite formation might therefore render MEN 10755 more cardiac tolerable than DOX and EPI.

The differential effect of propofol on contractility of isolated myocardial trabeculae of rat and guinea-pig.

1. The effects of propofol on myocardial contractility were studied in rat, in which the contractile activation mainly depends on calcium derived from the sarcoplasmic reticulum (SR), and guinea-pig, in which transsarcolemmal influx of calcium plays a major role. 2. Intact and chemically skinned trabeculae from the right ventricle were studied. Intact trabeculae were electrically stimulated and force development during steady state and post rest contractions was measured. In saponin skinned trabeculae Ca(2+) uptake and release by the SR was studied. In Triton skinned trabeculae the influence of propofol on calcium sensitivity of the myofilaments was studied. 3. In intact rat trabeculae propofol in concentrations of 28, 112 and 280 microM did not change peak force development nor the pattern of post rest contraction. In guinea-pig trabeculae propofol significantly reduced peak force to respectively 64, 40 and 23% of control values and the post rest contractions were potentiated. In skinned trabeculae propofol did not affect Ca(2+) handling by the SR, nor did it change force production and Ca(2+) sensitivity of the myofilaments. 4. This study shows that, in contrast to rat, in guinea-pig propofol directly depresses myocardial contractility, probably by decreasing transsarcolemmal Ca(2+) influx. There is no significant influence of propofol on Ca(2+) handling by the SR, nor on the contractile proteins.

Postrest contraction in the ventricular papillary muscle of spontaneously diabetic WBN/Kob rat.

In the present study, we investigated the characteristics of the postrest contraction (PRC) in chronic diabetic ventricular muscle. We used WBN/Kob rats of 7-8 weeks as the spontaneously diabetic animal and Wistar rats of 7-8 weeks as the control. We found: (1) No significant differences were seen in the amplitude, the contracting speed, and the relaxing speed of electrically stimulated twitch tension between control and WBN/Kob rats. In addition, the relationship between amplitude of twitch tension and stimulus cycle lengths (0.2-5 sec) was very similar in both animals. (2) The ratios of the first twitch tension (T1) of PRC with various rest intervals (5-600 sec) to the steady-state tension (Tss) were significantly smaller in the diabetic rats than in the controls. (3) When the preparation was stimulated at shorter cycle lengths, the recovery process of PRC was separated into at least two components (fast and slow components). In the diabetic rats, the time constant (tau) of both components was significantly longer than in controls. (4) After caffeine (10(-3) M) treatment, tau of the fast component in the control rats became longer, whereas it remained unchanged in diabetic rats. These findings suggest a dysfunction of the intracellular calcium handling system in spontaneously diabetic heart that is likely to include impaired calcium sequestration and/or extrusion.

Alpha-Adrenoceptor stimulation-mediated negative inotropism and enhanced Na(+)/Ca(2+) exchange in mouse ventricle.

Mechanisms underlying the negative inotropic response to alpha-adrenoceptor stimulation in adult mouse ventricular myocardium were studied. In isolated ventricular tissue, phenylephrine (PE), in the presence of propranolol, decreased contractile force by approximately 40% of basal value. The negative inotropic response was similarly observed under low extracellular Ca(2+) concentration ([Ca(2+)](o)) conditions but was significantly smaller under high-[Ca(2+)](o) conditions and was not observed under low-[Na(+)](o) conditions. The negative inotropic response was not affected by nicardipine, ryanodine, ouabain, or dimethylamiloride (DMA), inhibitors of L-type Ca(2+) channel, Ca(2+) release channel, Na(+)-K(+) pump, or Na(+)/H(+) exchanger, respectively. KB-R7943, an inhibitor of Na(+)/Ca(2+) exchanger, suppressed the negative inotropic response mediated by PE. PE reduced the magnitude of postrest contractions. PE caused a decrease in duration of the late plateau phase of action potential and a slight increase in resting membrane potential; time courses of these effects were similar to that of the negative inotropic effect. In whole cell voltage-clamped myocytes, PE increased the L-type Ca(2+) and Na(+)/Ca(2+) exchanger currents but had no effect on the inwardly rectifying K(+), transient outward K(+), or Na(+)-K(+)-pump currents. These results suggest that the sustained negative inotropic response to alpha-adrenoceptor stimulation of adult mouse ventricular myocardium is mediated by enhancement of Ca(2+) efflux through the Na(+)/Ca(2+) exchanger.

Impaired sarcoplasmic calcium release inhibits myocardial contraction in experimental sepsis

Purpose: In septic shock, myocardial dysfunction develops over the course of illness, but the mechanism of this depression is not clear. In this study, mechanisms of myocardial dysfunction were examined in a porcine model of Escherichia coli sepsis. Materials and Methods: Animals were subjected to 4 hours of bacteria infusion (n = 5) (septic group) or saline infusion (n = 5) (nonseptic group), after which trabeculae were removed from the right ventricle and placed into a recirculating water bath. Measurements of steady-state contraction (SSC) were obtained at 0.5, 1, and 2 Hz. Indirect indices were used to assess abnormalities in myocardial calcium metabolism in sepsis. Extrasystoles (ES) were used to assess transsarcolemmal (TSL) calcium flux and were measured at 300 milliseconds, 400 milliseconds, and 500 milliseconds after the preceding stimulus. Postrest contraction (PRC) is an indicator of SR recirculation from the uptake to the release site and was obtained after interposing intervals of rest between steady-state beats at 0.5 Hz. Rapid-cooling contracture (RCC) is an indicator of sarcoplasmic reticulum (SR) content and was obtained at 0.5, 1, and 2 Hz and after interposing intervals of rest at 0.5 Hz. Results: SSC was not different between groups at 0.5 Hz, but compared with the nonseptic group, SSC decreased at 1 and 2 Hz in the septic group (P < .05). PRC and TSL were not different between groups. During rest intervals, calcium leaks out of SR through the ryanodine channel (ie, SR calcium release channel). In the septic group, as assessed by RCC, SR calcium leak was less than that found in the nonseptic group. Conclusion: These results indicate that myocardial dysfunction in sepsis is frequency dependent, and that the mechanism is most likely caused by inhibition of SR calcium release owing to blockade of the ryanodine channel. Copyright © 2000 by W.B. Saunders Company

Doxorubicin-induced late cardiotoxicity: delayed impairment of Ca2+-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.