Sodium-calcium exchange in heart: membrane currents and changes in [Ca2+]i

Changes in cytosolic calcium monitored by inward currents during action potentials in guinea-pig ventricular cells

Action potentials were recorded from single cells isolated from guinea-pig ventricular muscle. Contraction was measured with an optical technique. Tail currents thought to be activated by cytosolic calcium were recorded when action potentials were interrupted by application of a voltage-clamp. A family of tail currents was recorded by interrupting the action potential at various times after the upstroke. The envelope of tail current amplitudes was taken as an index of changes in cytosolic calcium. Consistent with this interpretation, tail currents were negligible following intracellular loading with the calcium chelator BAPTA to suppress calcium transients. The cytosolic calcium transient estimated from the envelope of tails reached a peak approximately 50 ms after the upstroke of the action potential, and fell close to diastolic levels before repolarization was complete; 10 mM caffeine delayed the time to peak contraction, and caused a prolongation of the cytosolic calcium transient estimated from the envelope of tail currents. Caffeine also induced the appearance of a distinct late plateau phase of the action potential. Intracellular BAPTA suppressed the late plateau, contraction and tail currents in cells exposed to caffeine. Exposure to caffeine increased the time constant for decay of tail currents (from approximately 25 to 70 ms). When action potentials were greatly abbreviated by interruption with a voltage-clamp, a progressive decline occurred in the subsequent three contractions and tail currents. There was a progressive reversal of these effects over four responses when the full action potential duration was restored. None of these effects was observed in cells exposed to caffeine. Calcium-activated tail currents appear to be a useful qualitative index of changes in cytosolic calcium. The observations are consistent with the suggestion that cytosolic calcium is reduced during the plateau by a combination of calcium extrusion through Na-Ca exchange and calcium uptake into caffeine-sensitive stores. It also appears that reduction of stores loading during abbreviated action potentials reduces subsequent contraction in cells not exposed to caffeine.

Effect of palytoxin on the calcium current and the mechanical activity of frog heart muscle

1. The effect of palytoxin (PTX) on the Ca current (ICa) and the mechanical activity of frog atrial fibres was studied by use of the double sucrose gap voltage clamp technique. 2. In normal Ringer solution, PTX transiently increased the electrically-evoked peak tension which then decreased while a major contracture developed. PTX slowed the time course of the relaxation phase of the evoked tension. 3. Evidence is presented which suggests that the toxin also increased the entry of Ca and Sr via the Na-Ca exchange mechanism. It also induced the development of a Ca-dependent outward current which was inhibited by Sr. 4. In Na-free solution, PTX increased ICa and shifted the reversal potential for Ca towards more negative membrane potentials, thus suggesting that the internal Ca concentration had increased. Current-voltage, tension-voltage, time to peak-voltage and inactivation time constant-membrane potential curves were all shifted towards more negative membrane potentials in the presence of PTX. 5. These effects of PTX are similar to those caused by the increase in internal Ca concentration induced by Na ionophores by way of voltage-dependent Ca influx of the Na-Ca exchange mechanism.

Slow inward tail currents in rabbit cardiac cells

1. A whole-cell gigaseal suction microelectrode voltage-clamp technique has been used to study slow inward tail currents in single myocytes obtained by enzymatic dispersion of rabbit ventricle and atrium. A variety of stimulation protocols, Tyrode solutions and pharmacological agents have been used to test three hypotheses: (a) that the slow inward tail current is generated by an electrogenic Na(+)-Ca2+ exchanger; (b) that a rise in [Ca2+]i, due to release from the sarcoplasmic reticulum can modulate the activity of this exchanger; and (c) that the uptake of calcium by the sarcoplasmic reticulum is a major determinant of the time course of the tail current. 2. As shown previously in amphibian atrium and guinea-pig ventricle, slow inward tail currents can be observed consistently under conditions in which action potentials and ionic currents are recorded using microelectrode constituents which only minimally disturb the intracellular milieu. 3. In ventricular cells, the envelope of these tail currents obtained by varying the duration of the preceding depolarizations shows that (a) the tail currents are activated by pulses as short as 10 ms, and reach a maximum for pulse durations of 100-200 ms, (b) the rate of decay of the tail current gradually increases as the activating depolarizations are prolonged, and (c) the tails cannot be due to deactivation of calcium currents, in agreement with other studies in frog heart. 4. When the mean level of [Ca2+]i is raised following inhibition of the Na(+)-K+ pump by strophanthidin (10(-5) M) or reductions in [K+]o (0.5 mM), the slow inward tail grows in size prior to the onset of a contracture or other signs of calcium-induced toxicity. 5. In a number of different preparations, replacement of [Ca2+]o with BaCl2 markedly or completely inhibits the Na(+)-Ca2+ exchanger, whereas Sr2+ replacement does not have this effect. In myocytes from rabbit ventricle the slow inward tails are reduced significantly and decay more slowly in 0.5-2.2 mM-BaCl2 Tyrode solution, while in 2.2 mM SrCl2 these tails are not altered. 6. The slow inward tail also shows a dependence on [K+]o, corresponding to previous data on Na(+)-Ca2+ exchange in other tissues. Increasing [K+]o in the Tyrode solution to a final concentration of 10-15 mM results in a marked inhibition of the slow tails. This effect cannot be accounted for by changes in the inwardly rectifying potassium current, IK1. 7. The slow tail currents were changed significantly by increasing the temperature of the superfusing Tyrode solution.(ABSTRACT TRUNCATED AT 400 WORDS)

Direct measurement of changes in intracellular calcium transients during hypoxia, ischemia, and reperfusion of the intact mammalian heart

In studies of ischemia and reperfusion, a major experimental problem has been the inability to measure intracellular ionized calcium ([Ca2+]i) in the intact heart. We have developed a new approach in which the bioluminescent calcium indicator aequorin is used to measure [Ca2+]i in the isolated, coronary-perfused ferret heart. Aequorin is loaded into subepicardial myocytes of the left ventricle, and the signals are recorded simultaneously along with isovolumic left ventricular (LV) pressure at a constant pacing rate. This system shows 1) no attenuation or change of time course of LV pressure development or coronary perfusion pressure after aequorin loading; 2) consistent responses to physiological interventions and drugs; 3) individual aequorin and pressure signals that do not require signal averaging for analysis; and 4) [Ca2+]i levels comparable with those reported in tissue or isolated myocyte cell preparations. During 5 minutes of hypoxia, diastolic [Ca2+]i and LV diastolic pressure increased while the systolic values of both [Ca2+]i and pressure decreased. The peak-to-peak systolic [Ca2+]i versus LV isovolumic pressure relation remained close to the control curve. In contrast, during 3 minutes of global ischemia, LV systolic and diastolic pressures fell rapidly, while [Ca2+]i increased substantially. The [Ca2+]i versus pressure relations for both systole and diastole shifted to the right, indicating desensitization of the contractile apparatus to [Ca2+]i. These results provide evidence that different primary mechanisms determine the systolic and diastolic responses to acute hypoxia versus ischemia. During hypoxia, changes in [Ca2+]i handling probably play a major role, while during ischemia, changes in the Ca2+ sensitivity of the myofilaments appear to be of primary importance in the modulation of contractile dysfunction.

Inhibition of Na-Ca exchange by general anesthetics

General anesthetics, typically octanol, were found to inhibit the influx of calcium in isolated sodium-loaded adult rat heart cells, using 45Ca, quin 2, or indo 1. Inhibition by octanol, like inhibition by sodium, was competitive with calcium. Octanol and sodium together inhibited calcium influx synergistically. At physiological levels of extracellular calcium and sodium, the EC50 was 177 +/- 37 microM for octanol and 48 +/- 5 microM for decanol. These values are threefold to fourfold larger than those reported to cause 50% loss of righting reflex in tadpoles, a measure of their anesthetic effectiveness. We conclude that general anesthetics inhibit Na-Ca exchange at the sarcolemma. We suggest that octanol inhibits like sodium, and the synergism stems from the cooperativity of sodium inhibition at the binding and regulatory sites of the exchanger. Insofar as Na-Ca exchange may regulate inotropy, the inhibition of Na-Ca exchange by general anesthetics could contribute to their negative inotropic effect.

Alterations in [Ca2+]i mediated by sodium-calcium exchange in smooth muscle cells isolated from the guinea-pig ureter

1. Sodium-calcium exchange was studied in single enzymatically isolated cells of the guinea-pig ureter using the Ca2(+)-sensitive fluorescent dye Indo-1 to monitor the intracellular Ca2+ concentration ([Ca2+]i). Patch pipettes containing Indo-1 were used to introduce the dye into cells, to set the intracellular Na+ concentration ([Na+]i) and control the membrane potential during experiments. 2. With [Na+]i set at 11-12 mM and a membrane potential of -60 or -70 mV, brief depolarization of ureter cells elicited typical voltage-gated inward currents associated with rapid increases in [Ca2+]i which showed a bell-shaped potential dependence. If Ca2+ currents were blocked with nifedipine, depolarization led to slower rises in [Ca2+]i. The rates and amplitudes of these increased monotonically with progressively larger depolarizations up to +120 mV. 3. The nifedipine-resistant rises in [Ca2+]i elicited by depolarization were potentiated when the extracellular sodium concentration ([Na+]o) was reduced. Basal levels of [Ca2+]i also increased as [Na+]o was reduced, although the dependence of this effect on [Na+]o was smaller than would be predicted if [Ca2+]i was set only by a Na(+)-Ca2+ exchange process. 4. The nifedipine-insensitive rises in [Ca2+]i elicited by depolarization were potentiated at higher basal levels of [Ca2+]i. 5. The ability of cells to reduce [Ca2+]i rapidly following Ca2+ loading during voltage-gated transients was markedly inhibited if the Na+ concentration gradient was reversed, but was little affected if the Na+ gradient was decreased by 25 or 50%. Recovery from a Ca2+ load caused by reversal of the Na+ gradient could be induced by removal of Cao2+ in the continuing absence of Nao+, indicating the importance of a Na(+)-independent [Ca2+]i-lowering system. 6. The results demonstrate that Na(+)-Ca2+ exchange can modulate [Ca2+]i when [Na+]i and the membrane potential are set at or near their physiological levels in these smooth muscle cells. [Ca2+]i does not, however, appear to be markedly sensitive to the Na+ concentration gradient under the conditions employed for these experiments, suggesting that a Na(+)-independent Ca2+ extrusion system is mainly responsible for regulating [Ca2+]i under normal conditions.

Relaxation of rabbit ventricular muscle by Na-Ca exchange and sarcoplasmic reticulum calcium pump. Ryanodine and voltage sensitivity

We studied relaxation during rapid rewarming of rabbit ventricular muscles that had been activated by rapid cooling. Rewarming from 1 degree to 30 degrees C (in less than 0.5 second) activates mechanisms that contribute to the reduction of intracellular calcium concentration and thus relaxation (e.g., sarcoplasmic reticulum [SR] calcium pump and sarcolemmal Na-Ca exchange and calcium pump). Rapid rewarming in normal Tyrode's solution induces relaxation with a half-time (t1/2) of 217 +/- 14 msec (mean +/- SEM). During cold exposure, changing the superfusate to a sodium-free, calcium-free medium with 2 mM CoCl2 (to eliminate Na-Ca exchange) slightly slows relaxation upon rewarming in the same medium (t1/2 = 279 +/- 44 msec). Addition of 10 mM caffeine (which prevents SR calcium sequestration) to normal Tyrode's solution during cold superfusion slows relaxation somewhat more (t1/2 = 376 +/- 31 msec) than sodium-free, calcium-free solution. However, if both interventions are combined (sodium-free + caffeine) during the cold exposure and rewarming, the relaxation is greatly slowed (t1/2 = 2,580 +/- 810 msec). These results suggest that either the SR calcium pump or, to a lesser extent, sarcolemmal Na-Ca exchange can produce rapid relaxation, but if both systems are blocked, relaxation is very slow. If muscles are equilibrated with 500 nM ryanodine before cooling, relaxation upon rewarming is not greatly slowed (t1/2 = 266 +/- 37 msec) even if sodium-free, calcium-free solution is applied during the cold and rewarming phases (t1/2 = 305 +/- 66 msec). This result suggests that ryanodine does not prevent the SR from accumulating calcium to induce relaxation.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium-calcium exchange in guinea-pig cardiac cells: exchange current and changes in intracellular Ca2+

1. Membrane currents and changes in [Ca2+]i attributable to the operation of an electrogenic Na-Ca exchange mechanism were recorded in single isolated guinea-pig ventricular myocytes under voltage clamp and internal perfusion with the Ca2+ indicator Fura-2. 2. Ionic currents that interfere with the measurement of Na-Ca exchange current were blocked through the use of caesium (Cs+), verapamil and tetrodotoxin (TTX). Entry of Ca2+ through surface membrane Ca2+ channels and release of Ca2+ from sarcoplasmic reticulum were blocked with verapamil and ryanodine, respectively. 3. In the presence of the blockers listed above, depolarization to positive membrane potentials elicited slow increases in [Ca2+]i and, after an instantaneous increase, a declining outward current. Repolarization elicited a decline in [Ca2+]i and, after an instantaneous increase, a declining inward current. The changes in [Ca2+]i and a major component of the current were abolished by nickel ions (Ni2+; 5 mM). 4. The reversal potential of the current abolished by Ni2+ (Ni2+-sensitive current) was determined at different levels of [Ca2+]i by ramp repolarizations from +80 to -80 mV (1-5 mV/ms). The reversal potential of the current increased linearly with log [Ca2+]i. As a result of the foregoing and other data, the Ni2+-sensitive current was taken to be Na-Ca exchange current (INaCa). 5. The relation between INaCa and [Ca2+]i (less than 1 microM) at constant voltage over the range of -80 to +60 mV was approximately linear. No evidence of saturation could be found; small deviations from linearity at high [Ca2+]i were in the direction expected for a minor contribution from Ca2+-activated non-specific cation current (Ehara, Noma & Ono, 1988). 6. When measured at the same [Ca2+]i, the peak INaCa upon repolarization to -80 to -140 mV seemed to approach a limiting value at very negative potentials. 7. Over the range of +40 to +160 mV INaCa (measured soon after depolarization and thus at the same [Ca2+]i) increased exponentially with clamp-pulse potential. These pulses (to potentials up to +160 mV) elicited a slow rise in [Ca2+]i with the peak at the end of the pulse also increasing exponentially with pulse potential. 8. Inward membrane currents with properties similar to those described above were also recorded in association with physiological [Ca2+]i transients, when Ca2+ channels and the sarcoplasmic reticulum were not blocked. 9. Some of the results are not consistent with certain predictions of a sequential step model, or with those of a simultaneous step model in which the internal binding site for Ca2+ is saturated, or with those of a model based only on thermodynamics.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of buffer magnesium on positive inotropic agents in guinea pig cardiac muscle

Experiments examined effects of extracellular Mg2+ concentration (Mgo2+) on dose-dependent actions of strophanthidin, norepinephrine, Bay K-8644 and extracellular Ca2+ (Cao2+) in electrically stimulated atrial and ventricular muscle isolated from guinea pig heart. Mgo2+ itself elicited a concentration-dependent negative inotropic effect. Elevation of Mgo2+ between 0.6 and 12 mM increased the concentration of strophanthidin necessary to produce its toxic effects without affecting the maximum developed tension prior to toxicity. Similarly, Mgo2+ did not alter the maximum contractile force elicited by cumulative addition of norepinephrine, Bay K-8644 or Cao2+, but increased their ED50 values. These data suggest that interactions between Mgo2+ and the four positive inotropic agents were not mediated by effects on receptor binding or Na+,K+-ATPase, but rather by alterations at one or more steps involved in excitation-contraction coupling.

Sodium-calcium exchange current. Dependence on internal Ca and Na and competitive binding of external Na and Ca

Na-Ca exchange current was measured at various concentrations of internal Na [( Na]i) and Ca [( Ca]i) using intracellular perfusion technique and whole-cell voltage clamp in single cardiac ventricular cells of guinea pig. Internal Ca has an activating effect on Nai-Cao exchange beginning at approximately 10 nM and saturating at approximately 50 nM with a half maximum [Ca]i (Km[Ca]i) of 22 nM (Hill coefficient, 3.7). Measurement of Nai-Cao exchange current at various concentration of [Na]i revealed an apparent Km[Na]i of 20.7 +/- 6.9 mM (n = 14) with imax of 3.5 +/- 1.2 microA/microF. For [Ca]i transported by the exchange, a Km[Ca]i of 0.60 +/- 0.24 microM (n = 8) with an imax of 3.0 +/- 0.54 microA/microF was obtained by measuring Nao-Cai exchange current. These values are apparently different from the values for the external binding site which have been reported previously. Whether Na and Ca compete for the external binding site, and if so, how it affects the binding constants was then investigated. Outward Nai-Cao exchange current became larger by reducing [Na]o. The double reciprocal plot of the current magnitude and [Ca]o at different [Na]o revealed a competitive interaction between Na and Ca. In the absence of competitor [Na]o, an apparent Km[Ca]o of 0.14 mM was obtained. When comparing internal and external Km values, the external value is markedly larger than the internal one and thus we conclude that binding sites of the Na-Ca exchange molecule are at least apparently asymmetrical between the inside and outside of the membrane.

Sodium-calcium exchange during the action potential in guinea-pig ventricular cells

1. Slow inward tail currents attributable to electrogenic sodium-calcium exchange can be recorded by imposing hyperpolarizing voltage clamp pulses during the normal action potential of isolated guinea-pig ventricular cells. The hyperpolarizations return the membrane to the resting potential (between -65 and -88 m V) allowing an inward current to be recorded. This current usually has peak amplitude when repolarization is imposed during the first 50 ms after the action potential upstroke, but becomes negligible once the final phase of repolarization is reached. The envelope of peak current tail amplitudes strongly resembles that of the intracellular calcium transient recorded in other studies. 2. Repetitive stimulation producing normal action potentials at a frequency of 2 Hz progressively augments the tail current recorded immediately after the stimulus train. Conversely, if each action potential is prematurely terminated at 0.1 Hz, repetitive stimulation produces a tail current much smaller than the control value. The control amplitude of inward current is only maintained if interrupted action potentials are separated by at least one full 'repriming' action potential. These effects mimic those on cell contraction (Arlock & Wohlfart, 1986) and suggest that progressive changes in tail current are controlled by variations in the amplitude and time course of the intracellular calcium transient. 3. When intracellular calcium is buffered sufficiently to abolish contraction, the tail current is abolished. Substitution of calcium with strontium greatly reduces the tail current. 4. The inward tail current can also be recorded at more positive membrane potentials using standard voltage clamp pulse protocols. In this way it was found that temperature has a large effect on the tail current, which can change from net inward at 22 degrees C to net outward at 37 degrees C. The largest inward currents are usually recorded at about 30 degrees C. It is shown that this effect is attributable predominantly to the temperature sensitivity of activation of the delayed potassium current, iK, whose decay can then mask the slow tail current at high temperatures. 5. Studies of the relationship between the tail current and the membrane calcium current, iCa, have been performed using a method of drug application which is capable of perturbing iCa in a very rapid and highly reversible manner. Partial block of iCa with cadmium does not initially alter the size of the associated inward current tail. When iCa is increased by applying isoprenaline, the percentage augmentation of the associated tail current is much greater but occurs more slowly.(ABSTRACT TRUNCATED AT 400 WORDS)

Na+-Ca2+ exchange in cardiac sarcolemma: modulation of Ca2+ affinity by exercise

The high activity of the cardiac Na+-Ca2+ exchanger has led to the suggestion that it plays an important role in the regulation of myocardial contractility. We have proposed that exercise training increases stroke volume as a consequence of an enhanced contractility caused by an adaptation in Ca2+ transport across the cardiac plasma membrane (sarcolemma). The present study examined the possibility that the Na+-Ca2+ exchanger in heart muscle is modified in response to training. Sprague-Dawley rats (female, n = 72) were randomly divided into exercise-trained (T) and sedentary control (C) groups. As a result of the 11-wk treadmill-training paradigm, group T had a 7.6% higher (P less than 0.005) heart-to-body weight ratio and a 36% increase (P less than 0.01) in gastrocnemius mitochondrial enzyme activity. Na+-Ca2+ exchange was studied in highly purified sarcolemmal vesicles using rapid-quenching techniques. The absolute initial rate of uptake was significantly higher in T vs. C at calcium concentrations [( Ca2+]) ranging from 10 to 80 microM. This increased uptake appears to be due solely to the fact that the apparent Km of the myocardial Na+-Ca2+ exchanger for Ca2+ was significantly lower in T vs. C (15.7 +/- 1.1 vs. 36.1 +/- 2.6 microM), since the maximum velocity was unchanged. The observed increase in the affinity of the exchanger for Ca2+ is not attributable to group differences in vesicular purity, cross-contamination, or passive Ca2+ efflux. This observation is consistent with observed alterations in sarcolemmal composition in response to exercise training. We propose that the modification of the Na+-Ca2+ exchanger may play an important role in the adaptation of the heart to exercise.

A fluorescent indicator for measuring cytosolic free magnesium

The previously developed chelator O-aminophenol-N,N,O-triacetic acid (APTRA) (L. A. Levy, E. Murphy, B. Raju, and R. E. London. Biochemistry 27: 4041-4048, 1988) has been modified to yield a fluorescent analogue which can be utilized as an intracellular probe for ionized Mg2+. The fluorescent analogue, FURAPTRA, with a magnesium dissociation constant of 1.5 mM, is structurally analogous to the calcium chelator fura-2 and exhibits a similar excitation shift on magnesium complexation. Hence, data on the intracellular Mg2+ concentration can be obtained using an analogous ratio method. The acetoxymethyl form of the chelator is readily loaded into cells and has been used to determine a cytosolic free Mg2+ concentration of 0.59 mM for isolated rat hepatocytes. As a consequence of the relatively high levels of cytosolic Mg2+, the problem of ion buffering is much less severe than for the analogous calcium indicators.

Depolarization of macrophage polykaryons in the absence of external sodium induces a cyclic stimulation of a calcium-activated potassium conductance

Macrophage polykaryons associated with the foreign body granuloma display several electrophysiological properties when studied with intracellular microelectrodes. One of the most evident properties is the slow hyperpolarization (2-5 s long, 10-60 mV amplitude), due to transient openings of Ca2+-dependent K+ channels, that is similar to those observed in macrophages. How this oscillation of membrane potential is triggered is not well known and the only way to repeatedly activate it under experimental control is through the intracellular injection of Ca2+. Although this technique is important for understanding the properties of the K+ channels, no information has been obtained about the way Ca2+ levels are raised and controlled in the cytosol. Slow hyperpolarizations can also be triggered by electrical stimulation, but reproducibility is low with cells bathed in physiological solutions. We then decided to investigate the effect of depolarization on the electrophysiological properties of macrophage polykaryons exposed to bathing solutions of several ionic compositions. We show in this paper that cell membrane depolarization induced by a long current pulse can trigger several patterns of membrane potential changes and that, in the absence of extracellular Na+, repetitive oscillations of decaying amplitudes are observed in almost all the cells. They are very similar to the slow hyperpolarizations, are dependent on the presence of extracellular Ca2+, and are blocked by quinine and D-600. Whole-cell patch clamp recording under voltage clamp conditions showed an outward current that oscillates and that also exhibits decaying amplitudes. The data presented here indicate that these oscillations are a consequence of the cyclic opening of the Ca2+-activated K+ channels and support the hypothesis that favors the participation of Ca2+ channels and of the Ca2+/Na+ exchange system in their triggering. These two mechanisms are not enough to explain either why the K+ channels close or why the membrane potential returns to the original level at the end of each cycle. The possibility of using these oscillations as a model to study the slow hyperpolarization is discussed.

Mechanism of release of calcium from sarcoplasmic reticulum of guinea-pig cardiac cells

1. The mechanisms that control release of Ca2+ from the sarcoplasmic reticulum (SR) of guinea-pig ventricular cells were studied by observing intracellular calcium concentration ([Ca2+]i transients) and membrane currents in voltage-clamped guinea-pig ventricular myocytes perfused internally with Fura-2. 2. Sarcolemmal Ca2+ current was identified through the use of tetrodotoxin (TTX) and Ca2+ channel antagonists (verapamil) and agonists (Bay K 8644). 3. Changes in [Ca2+]i attributable to release of Ca2+ from the SR were identified through the use of ryanodine, which abolishes the ability of the SR to release Ca2+. Ryanodine-sensitive increases in [Ca2+]i could be elicited either by depolarization or by repolarization (from depolarizing pulses to relatively positive membrane potentials). 4. At appropriate voltages, it is the initial fast change in [Ca2+]i elicited by either depolarization or repolarization that is abolished by ryanodine, and is defined here as ryanodine sensitive. 5. The amplitude of the ryanodine-sensitive [Ca2+]i transient elicited by depolarization had a bell-shaped dependence on membrane potential with a maximum of about 500 nM at 10 mV, and with the upper minimum between 60 and 70 mV. Verapamil-sensitive current activated over approximately the same potential range as the [Ca2+]i transient, with a peak amplitude at 10 mV, and a reversal potential of 65 mV. 6. When a holding potential of -68 mV and TTX (30 microM) were used, the most negative pulse potential at which activation of an inward current occurred was -49 mV while changes in [Ca2+]i occurred at -43 mV. 7. Ryanodine-sensitive increases in [Ca2+]i elicited by repolarization (tail transients) were maximal for repolarization to 0 mV. Smaller changes in [Ca2+]i than maximal were elicited by repolarization to both more positive and more negative potentials than 0 mV. The peak amplitude of the verapamil-sensitive tail currents elicited by repolarization increased continuously as the membrane was repolarized to potentials more negative than 60 mV. 8. Increasing depolarizing pulse duration beyond 10-20 ms did not increase the amplitude of the [Ca2+]i transient, but prolonged it. 9. The experimental results are compared to the predictions of two theories on the mechanism of excitation-contraction coupling: Ca2+-induced release of Ca2+ (CICR), as it has been formulated from data in skinned cardiac cells, and a charge-coupled release mechanism (CCRM), as it has been formulated to explain excitation-contraction coupling in skeletal muscle. 10. Some of the results are clearly not consistent with certain features of a charge-coupled release mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of intracellular sodium in the regulation of intracellular calcium and contractility. Effects of DPI 201-106 on excitation-contraction coupling in human ventricular myocardium

Experiments were performed to investigate the mechanism of action of DPI 201-106 on human heart muscle. In both control and myopathic muscles, DPI produced concentration-dependent increases in action potential duration, resting muscle tension, peak isometric tension, and duration of isometric tension. These changes were associated with increases in resting intracellular calcium and peak calcium transients as measured by aequorin. At higher concentrations of DPI, a second delayed Ca2+ transient (L') appeared. L' was inhibited by tetrodotoxin and ryanodine, suggesting that DPI acts at both the sarcolemma and the sarcoplasmic reticulum. DPI toxicity was manifested by after-glimmers and after-contractions reflecting a Ca2+-overload state: DPI effects were mimicked by veratridine, a Na+ channel agonist, and reversed by tetrodotoxin, yohimbine, and cadmium, Na+ channel antagonists. These results suggest that DPI acts primarily as a Na+ channel agonist. DPI may produce an increase in intracellular Ca2+ by increasing intracellular Na+ and altering Na+-Ca2+ exchange across the sarcolemma. DPI may also increase intracellular Ca2+ by directly altering sarcoplasmic reticulum Ca2+ handling.

Acidosis during early reperfusion prevents myocardial stunning in perfused ferret hearts

Cellular calcium overload figures prominently in the pathogenesis of the contractile dysfunction observed after brief periods of ischemia (myocardial stunning). Because acidosis is known to antagonize Ca influx and the intracellular binding of Ca, we reasoned that acidosis during reperfusion might prevent Ca overload and ameliorate functional recovery. We measured developed pressure (DP) and 31P-nuclear magnetic resonance spectra in 26 isovolumic Langendorff-perfused ferret hearts. After 15 min of global ischemia, hearts were reperfused either with normal solution (2 mM [Ca]o, Hepes-buffered, pH 7.4 bubbled with 100% O2; n = 6) or with acidic solutions (pH 6.6 during 0-3 min, pH 7.0 during 4-6 min) before returning to the normal perfusate (n = 7). Ventricular function after 30 min of reperfusion was much greater in the acidic group (105 +/- 5 mmHg at 2 mM [Ca]o) than in the unmodified reperfusion group (79 +/- 7 mmHg, P less than 0.001); similar differences in DP were found over a broad range of [Ca]o (0.5-5 mM, P less than 0.001) and during maximal Ca2+ activation (P less than 0.001). Intramyocardial pH (pHi) was lower in the acidic group than in the unmodified group during early reperfusion, but not at steady state. Phosphate compounds were comparable in both groups. To clarify whether the protective effect of acidosis is due to intracellular or extracellular pH, we produced selective intracellular acidosis during early reperfusion by exposure to 10 mM NH4Cl for 6 min just before ischemia (n = 6). For the first 12 min of reperfusion with NH4Cl-free solution (pH = 7.4), pHi was decreased relative to the unmodified group. Recovery of DP was practically complete, and maximal Ca2+-activated pressure was comparable to that in a nonischemic control group (n = 5). These results indicate that transient intracellular acidosis can prevent myocardial stunning, presumably owing to a reduction of Ca influx into cells and/or competition of H+ for intracellular Ca2+ binding sites during early reperfusion.

Relaxation of isolated ventricular cardiomyocytes by a voltage-dependent process

Cell contraction and relaxation were measured in single voltage-clamped guinea pig cardiomyocytes to investigate the contribution of sarcolemmal Na+-Ca2+ exchange to mechanical relaxation. Cells clamped from -80 to 0 millivolts displayed initial phasic and subsequent tonic contractions; caffeine reduced or abolished the phasic and enlarged the tonic contraction. The rate of relaxation from tonic contractions was steeply voltage-dependent and was significantly slowed in the absence of a sarcolemmal Na+ gradient. Tonic contractions elicited in the absence of a Na+ gradient promptly relaxed when external Na+ was applied, reflecting activation of Na+-Ca2+ exchange. It appears that a voltage-dependent Na+-Ca2+ exchange can rapidly mechanically relax mammalian heart muscle.