Presystolic calcium-loading of the sarcoplasmic reticulum influences time to peak force of contraction. X-ray microanalysis on rapidly frozen guinea-pig ventricular muscle preparations

Effects of inhibition of calcium and potassium currents in guinea-pig cardiac contraction: comparison of beta-caryophyllene oxide, eugenol, and nifedipine

1. To investigate the effects of the clove oil constituents beta-caryophyllene oxide and eugenol on the heart muscle, experiments were performed on isolated papillary muscles and on ventricular myocytes of the guinea-pig. The results were compared with those obtained with the dihydropyridine, nifedipine. 2. All three substances exerted negative inotropic effects in heart muscle although with different potencies and different influences on the time course of the contraction curve. 3. They all reduced rested-state contractions (RSCs) in the presence of isoprenaline which are thought to be due to the Ca(2+) current (I(Ca)). 4. beta-Caryophyllene oxide, eugenol and nifedipine inhibited the I(Ca) in single cells from the guinea-pig ventricle in a concentration-dependent, reversible way, but with different potencies. 5. In addition to the I(Ca)-inhibiting effect, beta-caryophyllene oxide strongly inhibited and eugenol slightly inhibited the potassium current. 6. The action potential of papillary muscles at a 1 Hz contraction frequency was greatly shortened by nifedipine, slightly shortened by eugenol, but not changed by beta-caryophyllene oxide. 7. The inhibition of the potassium current by beta-caryophyllene oxide obviously prevents a shortening of the action potential due to the diminution of the I(Ca). Accordingly, the negative inotropic effect of beta-caryophyllene oxide is closely related to the inhibition of I(Ca). In contrast, eugenol and nifedipine, which shorten the action potential, exert stronger negative inotropic effects than expected from their influence on I(Ca). 8. The results show that the negative inotropic effect of a calcium channel blocker can be attenuated by an additional inhibition of potassium channels.

Twitch-potentiation increases calcium in peripheral more than in central mitochondria of guinea-pig ventricular myocytes

1. The mitochondrial total calcium content ([Ca]mt) was studied with electron probe microanalysis (EPMA) in isolated guinea-pig ventricular myocytes in order to answer the question of whether electrical stimulation increases [Ca]mt in subsarcolemmal and central mitochondria to a different extent. 2. In unstimulated myocytes subsarcolemmal [Ca]mt was (mean +/- s.e.m.) 535 +/- 229 micromol (kg dry weight (DW))-1 and central [Ca]mt was 513 +/- 162 micromol (kg DW)-1. These values do not differ and correspond to approximately 180 micromol calcium per litre of mitochondria or 180 microM. 3. Contractions were potentiated to an optimum by stimulation with trains of 12 paired stimuli. After potentiation with 12 paired action potentials, cells were shock-frozen 120 ms after the start of the first action potential of the 13th pair. Subsarcolemmal [Ca]mt was 1.3 +/- 0.4 mmol (kg DW)-1 (433 microM) and central [Ca]mt was 227 +/- 104 micromol (kg DW)-1 (76 microM). The difference was significant. 4. After potentiation with 12 paired voltage-clamp pulses, cells were shock-frozen 120 ms after the start of the first pulse of the 13th pair. Subsarcolemmal [Ca]mt was 2.2 +/- 1.0 mmol (kg DW)-1 (733 microM) and central [Ca]mt was 630 +/- 180 micromol (kg DW)-1 (210 microM). After removal of extracellular K+, five paired voltage-clamp pulses increased subsarcolemmal [Ca]mt to 2.1 +/- 0.8 mmol (kg DW)-1 (700 microM), which was significantly higher than the central [Ca]mt of 389 +/- 88 micromol (kg DW) -1 or 130 microM. 5. In unstimulated cells, [Na] and [K] in subsarcolemmal and central mitochondria were not different. In potentiated myocytes, subsarcolemmal [Na]mt was 236 +/- 20 mmol (kg DW)-1 or 79 mM, which is significantly higher than the central [Na]mt of 50 +/- 5 mmol (kg DW)-1 or 16 mM. 6. The differences in [Ca]mt and [Na]mt are attributed to subsarcolemmal cytosolic microdomains of elevated [Ca2+] and [Na+] generated during contractile potentiation by transmembrane Ca2+ and Na+ fluxes.

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.

Voltage dependence of force- and slow inward current restitution in ventricular muscle

This study was aimed to assess the relationship among the voltage-dependent processes underlying the excitation-contraction coupling, viz. force restitution (FR), transmembrane Ca fluxes and Ca release. The experiments (n = 22) were performed on voltage-clamped dog trabeculae in which force and slow inward current were measured. Standard steady-state was achieved by clamp driving at 0.5 Hz, 300 ms, 70 mV depolarizing pulses from holding = resting potential at 30 degrees C. Voltage and duration of single pulses and intervals in between were varied according to five protocols. The voltage dependence of Ca release was tested by varying single pulses at equal steady-state, i.e., at equal Ca availability. Contractions could be elicited in absence of ICa (20-30 mV step) and in the presence of disproportionately small ICa (above 80 mV). The voltage dependence of Ca availability for the release was tested by constant test pulses following either a variable conditioning clamp pulse or a period of rest at a variable voltage. After a low voltage pulse and, hence, depressed or absent ICa, the test contraction is diminished in presence of normal or even augmented Isi at any test interval (i.e., FR is depressed). Diminished Ca influx thus reduces the Ca availability of the subsequent beat. During prolonged depolarization (by 60 mV and more) a tonic response appears, but a phasic response cannot be elicited (FR is inhibited). Upon subsequent repolarization FR starts from zero and is significantly enhanced. It is concluded that, during depolarization, Ca release channels are in an open state, thus allowing free recirculation of Ca, but no build-up of a sufficient Ca gradient at the release site.

Binding of calcium to myoplasmic buffers contributes to the frequency-dependent inotropy in heart ventricular cells

In guinea-pig ventricular cells, the Ca2+ buffer capacity of the myoplasm was estimated from the ratio of ionized calcium (from Indo-1 fluorescence) through total calcium (ionized plus bound calcium, from x-ray microprobe analysis). During post-rest potentiation (1 Hz paired-pulses in voltage-clamp), where diastolic sarcomere length remained nearly constant, Ca2+ buffer capacity slowly fell from 5500:1 to 700:1 suggesting that slow Ca2+ binding sites became saturated. We discuss that frequency-inotropy depends not only on the replenishment of intracellular stores with Ca2+, but also on binding of Ca2+ to these slow sites; the slow Ca2+ sites could complete with the fast activator sites on troponin C for systolic Ca2+, or they could enhance the Ca2+ affinity of the fast Ca2+ sites on troponin C by cooperative interaction.

Total and free myoplasmic calcium during a contraction cycle: x-ray microanalysis in guinea-pig ventricular myocytes

1. At 36 degrees C and 2 mM [Ca2+]o single guinea-pig ventricular myocytes were voltage clamped with patch electrodes. With a paired-pulse protocol applied at 1 Hz, a first pulse to +5 mV was followed by a second pulse to +50 mV. When paired pulsing had potentiated the contraction to the maximum, the cells were shock-frozen for electron-probe microanalysis (EPMA). Shock-freezing was timed at the end of diastole (-80 mV) or at different times during systole (+5 mV). 2. The same paired-pulse protocol was applied to another group of myocytes from which contraction and [Ca2+]i was estimated by microfluospectroscopy (50 microM-Na5-Indo-1). Potentiation moderately reduced diastolic sarcomere length from 1.85 to 1.82 microns and increased diastolic [Ca2+]i from about 95 to 180 nM. In potentiated cells, during the first pulse, contraction peaked within 128 +/- 25 ms after start of depolarization. [Ca2+]i peaked within 25 ms to 890 +/- 220 nM (mean +/- S.E.M.) and fell within 100 ms to about 450 nM. 3. Sigma Camyo, the total calcium concentration in the overlapping myofilaments (A-band), was measured by EPMA in seventeen potentiated myocytes. During diastole, sigma Camyo was 2.6 +/- 0.4 mmol (kg dry weight (DW]-1 which can be converted to 0.65 mM (mmoles per litre myofibrillar space). Since [Ca2+]i was 180 nM, we estimate that 99.97% of total calcium is bound. 4. A time course for systolic sigma Camyo was determined by shock-freezing thirteen cells at different times after start of depolarization to +5 mV. Sigma Camyo was 5.5 +/- 0.3 mmol (kg DW)-1 (1.4 mM) after 15-25 ms, 4.6 +/- 0.5 mmol (kg DW)-1 (1.1 mM) after 30-45 ms, and 3.1 mmol (kg DW)-1 (0.8 mM) after 60-120 ms. The fast time course of sigma Camyo suggests that calcium binds to and unbinds from troponin C at a fast rate. Hence, it is the slow kinetics of the cross-bridges that determines the 130 ms time-to-peak shortening. 5. Mitochondria of potentiated cells contained during diastole a total calcium concentration, sigma Camito, of 1.3 +/- 0.2 mmol (kg DW)-1 (0.4 mM). During the initial 15-25 ms of systole, sigma Camito did not change, however, during 30-45 ms sigma Camito rose to 3.7 +/- 0.5 mmol (kg DW)-1 (1.2 mM). The data suggest that sigma Camito can follow sigma Camyo with some delay, thereby participating in both slow diastolic and fast systolic changes in total calcium (sigma Ca), at least under the given conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes of isolated cardiac muscle function in response to extracellular sodium reduction

The effects of low extracellular sodium concentration [( Na+]0, 76 mmol/l) on force of contraction, transmembrane action potentials and on calcium, sodium and potassium contents were studied in guinea-pig heart muscle using sucrose or lithium as substitutes for sodium. In papillary muscle, the positive inotropic response to low [Na+]0 was accompanied by shortening in action potential duration, in atrial muscle, prolongation at 90% of repolarization was observed. After 30 min. in low [Na+]0 solution (Ca2+ 1.8 mmol/l), the net uptake of calcium in left atria was larger in sucrose- than in lithium-substituted solution, i.e. 0.45 and 0.2 mmol Ca2+/kg wet weight, respectively. The net sodium content decreased monophasically; but the potassium content did not change consistently. The uptake of lithium by the atria was at least partially compensated for by a loss in potassium and by a transient decrease in sodium content. Increase in stimulation frequency from 0.1 to 1 Hz accelerated the time course of change by a factor of 2. In low [Na+]0, post-rest adaptation of twitch tension appeared abolished in atria because of high amplitude contractions. The recovery pattern of post-rest twitch amplitude was greatly accelerated in papillary muscle. In conclusion, low [Na+]0 elevates the cellular calcium content; this extra calcium is probably located in cellular stores that are involved in the regulation of twitch amplitude.

Mechanical restitution at different temperatures in papillary muscles from rabbit, rat, and hedgehog

Mechanical restitution curves, i.e., peak isometric force as a function of the duration of the preceding test interval, were investigated in papillary muscles from rabbit, rat, and hedgehog. Peak force of rabbit papillary muscle increased with prolongation of the test interval from about 0.3 s to about 1.0 s and for longer intervals peak force declined (called type I mechanical restitution). On the other hand, in rat and hedgehog, papillary muscles' force reached a maximum value at intervals of 30-120 s (called type II mechanical restitution). When temperature was decreased from 35 to 15 degrees C, maximum force of type I mechanical restitution shifted from 1.0 to 10 s, whereas maximum force of type II restitution did not change significantly. Type II mechanical restitution consisted of two different phases, designated phase A and phase B, respectively. As temperature was decreased from 35 to 0 degree C in the hedgehog preparation, the two phases became even more separated. At 35 degrees C, the rising part of mechanical restitution in the rabbit muscle could not be distinguished from phase A of the hedgehog preparation and was also very similar to phase A of the rat muscle. Phase A is thus present in both type I and type II mechanical restitution, but phase B is a special feature of type II mechanical restitution. Phase A and phase B might be a manifestation of activator calcium originating from two different sources, e.g., the sarcoplasmic reticulum and the sarcolemma.

Chronic sodium depletion increases myocardial calcium content in normotensive rats

Increased myocardial contractility was found in the perfused heart isolated from sodium depleted Sprague-Dawley rats. Previously, it was reported that in vitro exposure of different cardiac preparations to low Na+ buffers was associated with both an increased contractility and an increased Ca2+ content in the cells. Therefore, this study was designed to examine increases in ventricular Ca2+ content in the hearts of chronically sodium depleted rats. Two groups of 12-week-old Sprague-Dawley rats were studied. One group (n = 5) received furosemide (5 mg/kg/day IP for 4 days), a low Na+ diet and distilled drinking water for 6 weeks (low sodium plus diuretics group = LSD). The other group (n = 5) received the same low Na+ diet, but 0.5% NaCl was supplemented in drinking water (regular sodium group = RS). Body weight and blood pressure were measured weekly during the dietary period in all rats. At the end of the 6 weeks, heart weight as well as water and electrolyte contents of the heart were measured in all animals. Results showed that both body weight and heart weight were significantly lower in LSD than in RS. Moreover, ventricular Na+ content was reduced while ventricular Ca2+ content was doubled in LSD compared to RS (8.2 +/- 0.2 units vs. 9.2 +/- 0.3 units, p less than .05 and 0.45 +/- 0.13 units vs. 0.23 +/- 0.01 units, p less than .01, respectively). We conclude that in vivo sodium depletion induces an increase in ventricular calcium content; this increased myocardial calcium may be related to the increased in vitro cardiac contractility observed after chronic in vivo sodium depletion, but its distribution between intracellular and extracellular compartments needs to be determined.

Rest- and stimulation-dependent changes in exchangeable calcium content in rabbit ventricular myocardium

Ca2+ shifts in the isolated, perfused ventricular muscle of rabbit hearts were investigated with the aid of 45Ca under the conditions of complete equilibration of preparations with 45Ca containing solution. The "cellular" 45Ca content was calculated by subtraction of 45Ca2+ dissolved in the free water of extracellular space from the total tissue 45Ca2+ content. The "cellular" content of 45Ca in stimulated (60 per min) preparation was 0.887 +/- 0.067 mmol/kg wet weight (w.w.). Six minutes of rest resulted in the drop of this content to 0.503 +/- 0.054 mmol/kg w.w. despite continued perfusion with 45Ca containing solution. Contractile force (CF) decreased at that time to 23% of control. The first post-rest contraction (RSC) resulted in a gain of 0.073 mmol45Ca/kg w.w. Both the content of 45Ca and CF returned to the pre-rest values when stimulation was resumed. The difference between the 45Ca content of post-rest stimulated and rested preparations (0.384 mmol/kg w.w.) is equivalent to Ca2 fraction (Ca2), previously described in guinea pig ventricular myocardium (17, 12). However, the volume of rabbit Ca2 is only about 42% of that in guinea pig. Caffeine in concentration 12.5 mM, which did not displace Ca2 from guinea pig ventricular muscle, decreased Ca2 in the rabbit ventricle by 44%. CCCP, a protonofore destroying the mitochondrial protone gradient essential for Ca2+ uptake and maintainability, displaced Ca2 completely from rabbit ventricles. These results, although far from conclusive, do suggest that both the mitochondria and sarcoplasmic reticulum might be the site of the rate-dependent Ca2 fraction. The physiological meaning of differences in Ca2 content between rabbit, guinea pig, and rat ventricular myocardium is discussed.

Two structurally distinct calcium storage sites in rat cardiac sarcoplasmic reticulum: an electron microprobe analysis study

The elemental composition of subcellular organelles in resting rat papillary muscle was measured by electron probe x-ray microanalysis of cryosections of flash-frozen tissue. Nonmitochondrial electron-dense structures (50-100 nm in diameter) with a phosphorous concentration larger than 375 mmol/kg dry wt were identified in the interfibrillar spaces of the I band region. They were not visible in the proximity of transverse tubules. The sodium, magnesium, phosphorus, sulfur, chlorine, and potassium content of the electron dense structures showed a normal distribution, consistent with a uniform composition of a specific subcellular organelle. However, the distribution of the calcium concentrations in these electron-dense structures was bimodal, suggesting that they are composed of at least two subpopulations. One subpopulation had relatively high calcium (up to 53 mmol/kg dry wt) content with a mean value of 12.5 +/- 1.1 mmol/kg dry wt, while the other one had a relatively low calcium content with a mean value of 2.8 +/- 0.3 mmol/kg dry wt. The mean calcium concentration in the junctional sarcoplasmic reticulum (j-SR) in rat papillary muscle with calcium concentrations larger than 6 mmol/kg dry wt was 14.6 +/- 2.0 mmol/kg dry wt. We propose that the electron-dense structures described above correspond to nonjunctional sarcoplasmic reticulum and that the population containing relatively high calcium concentrations is calsequestrin-containing corbular sarcoplasmic reticulum (c-SR) confined to the I band region, while the population containing relatively low calcium concentrations corresponds to anastomosing regions of the network sarcoplasmic reticulum that lack calsequestrin.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ouabain and low-Na+ perfusion on rest-decay and post-rest recovery of cellular Ca content in ventricular muscle of guinea-pig heart

Ca2+ shifts in isolated, perfused ventricular muscle of guinea-pig hearts were investigated with the aid of 45Ca under the conditions of complete equilibration of preparations with isotope-containing containing solutions. The content of 45Ca in stimulated preparations (rate 60/min) was 1.30 +/- 0.12 mmol/kg of wet weight (w.w.). 6 min rest resulted in the drop of this content to 0.37 +/- 0.05 mmol/kg w.w. despite continued perfusion with 45Ca containing solution. The difference of 0.93 mmol/kg w.w. is equivalent to fraction Ca2 (15) and is labelled accordingly. Ouabain (1 microM) increased the 45Ca content to 1.53 +/- 0.15 mmol/kg w.w. in the stimulated and to 1.12 +/- 0.23 mmol/kg w.w. in the rested muscle. The respective values after low (50 mM) sodium perfusion were 1.70 +/- 0.11 and 1.07 +/- 0.13 mmol/kg w.w. The differences between the stimulated and rested preparations (Ca2 fraction) were 0.41 and 0.63 mmol/kg, respectively. In the control experiments the force of the first post-rest beat dropped to 20 +/- 5% of the force of steady-state beats. During ouabain and low-sodium perfusion, the force of the first contraction increased markedly and its peak was larger than that of the few subsequent beats. It is concluded that Na-Ca exchange is the important factor in the rate-dependent control of Ca2 fraction content and of contractile force.

Electron probe x-ray microanalysis of sarcolemma and junctional sarcoplasmic reticulum in rabbit papillary muscles: low sodium-induced calcium alterations

This project was undertaken to determine whether electron probe x-ray microanalysis (microprobe analysis) could be utilized to determine the subcellular sites responsible for low sodium-induced calcium accumulation in myocardium. Ultrathin cryosections of rabbit papillary muscles were analyzed using microprobe analysis, and the concentrations (mmol/kg dry wt) of Na, Mg, I, S, Cl, K, and Ca were compared against low sodium (36 mM) and control (139 mM NaCl) muscle groups. Visual resolution of junctional sarcoplasmic reticulum in freeze-dried myocardial sections was achieved, and systematic analysis of junctional sarcoplasmic reticulum and sarcolemma was performed. Myofibrils and mitochondria were also analyzed. Reductions in Na and Cl concentration were measured in virtually all compartments of muscles bathed in low sodium. In addition, low sodium produced a doubling of junctional sarcoplasmic reticulum and sarcolemmal calcium concentrations (p less than 0.01). No significant changes in calcium were observed at other analyzed sites. The increased calcium at the junctional sarcoplasmic reticulum and sarcolemma correlates with (but may not completely account for) the threefold increase in contractility measured after 40 minutes in low sodium concentrations. This work demonstrates that elemental changes associated with the myocardial junctional sarcoplasmic reticulum and sarcolemma are amenable to direct, in situ microprobe analysis and further defines these structures as primary sites of calcium accumulation in low sodium concentrations.

Ca-pools involved in the regulation of cardiac contraction under positive inotropy. X-ray microanalysis on rapidly-frozen ventricular muscles of guinea-pig

Electron probe microanalysis of rapidly-frozen small ventricular trabeculae of guinea-pig demonstrates that the distribution of total intracellular calcium varies under positive inotropy depending on the type of inotropic intervention. The sarcoplasmic reticulum (SR) (or part of it) localized at the level of the z-lines reveals high calcium accumulation at the end of diastole whenever a stimulus is followed by a contraction with a short time to peak of force. After paired pulse stimulation, only this cell compartment accumulates calcium at the end of diastole. Since this cell compartment is "Ca-empty" in muscles frozen during contraction, SR is considered to be the source of activator Ca. In several cases of inotropy (after application of ARL, caffeine or after lowering the extracellular Na+ concentration), calcium is also detectable on the mitochondria, suggesting that these organelles participate in slow regulation of cytosolic calcium. In some cases, total calcium located on the sarcomeres is increased. The interpretation of this finding is intriguing and requires the assumption of supplementary cytosolic Ca-sinks as yet unknown.