Calcium-activated transient outward current in calf cardiac Purkinje fibres

Modulation by calcium of the potassium permeability of dog heart sarcolemmal vesicles

The movement of K+ in heart sarcolemmal vesicles has been followed through the opposing movement of the tetraphenylphosphonium ion. Ca2+ (5-50 microM) stimulates the efflux of K+ from K+-loaded vesicles [Km(Ca2+) approximately equal to 10 microM]. and the activation requires that Ca2+ be present inside the vesicles together with K+. The efflux of 86Rb+ from K+-, Rb+-loaded vesicles is similarly stimulated by 5-50 microM Ca2+ [Km(Ca2+) approximately equal to 10 microM]. The Ca2+-induced increase of K+ permeability does not become spontaneously inactivated. The effects of some inhibitors have been tested under conditions in which Ca2+ promotes the entry of K+ into vesicles. In this system, direct interaction of the drug with the Ca2+ and K+ membrane binding site(s) was therefore studied. Tetraethylammonium ion, which inhibits the potential-dependent K+ channel, does not interfere with the effect of Ca2+ whereas quinidine (IC50 = 12 microM) and trifluoperazine (IC50 = 8 microM at 50 micrograms of sarcolemmal protein per ml) inhibit.

A voltage clamp study of the effects of AR-L 115 BS on the pacemaker current of cardiac Purkinje fibres

The new cardiotonic agent AR-L 115 BS was investigated by means of the double-micro-electrode voltage clamp technique on sheep cardiac Purkinje fibres. Clinical and pharmacological studies show that AR-L 115 BS increases heart rate as a side effect at medium to high therapeutic doses. The classical analysis of the pacemaker current was therefore performed to study the possible mechanism of this effect at a cellular level. The kinetic parameter s infinity and the reversal potential of the pacemaker current were shifted in the depolarizing direction after exposure to AR-L 115 BS. Peak values of the fully activated pacemaker current were either increased or diminished, while potassium leakage was slightly increased. These results are not related to the action of AR-L 115 BS on beta-adrenergic receptors but possibly due to enhanced intracellular calcium (see third paper in this series). Despite its tendency to increase heart rate, high concentrations of AR-L 115 BS should not be expected to promote arrhythmias in the Purkinje system since the electrophysiological effects tend to counteract each other.

Fluctuations in membrane current driven by intracellular calcium in cardiac Purkinje fibers

Spontaneous oscillatory fluctuations in membrane potential are often observed in heart cells, but their basis remains controversial. Such activity is enhanced in cardiac Purkinje fibers by exposure to digitalis or K-free solutions. Under these conditions, we find that voltage noise is generated by current fluctuations that persist when membrane potential is voltage clamped. Power spectra of current signals are not made up of single time-constant components, as expected from gating of independent channels, but are dominated by resonant characteristics between 0.5 and 2 HZ. Our evidence suggests that the periodicity arises from oscillatory variations in intracellular free Ca that control ion movements across the surface membrane. The current fluctuations are strongly cross-correlated with oscillatory fluctuations in contractile force, and are inhibited by removing extracellular Ca or exposure to D600. Chelating intracellular Ca with injected EGTA also abolishes the current fluctuations. The oscillatory mechanism may involve cycles of Ca (or Sr) movement between sarcoplasmic reticulum and myoplasm, as previously suggested for skinned cardiac preparations. Our experiments in intact cells indicate that changes in surface membrane potential can modulate cytoplasmic Ca oscillations in frequency and perhaps amplitude as well. A two-way interaction between surface membrane potential and intracellular Ca stores may be a common feature of heart, neuron, and other cell types.

Block of outward current in cardiac Purkinje fibers by injection of quaternary ammonium ions

We have studied the effects of iontophoretic injection of the quaternary ammonium compounds tetraethylammonium (TEA) and tetrabutylammonium (TBA) in cardiac purkinje fibers. We find that TBA(+) is a more effective blocker than TEA(+), but injection of either compound reduces the time-dependent outward plateau currents, transient outward current (I(to)), and the delayed rectifier (I(x)). Our findings provide evidence that these outward cardiac currents are carried by channels that in some respects are pharmacologically similar to squid axon potassium channels. We demonstrate that this procedure is a new tool that can be useful in the analysis of membrane currents in the heart.

Existence of two transient outward currents in sheep cardiac Purkinje fibers

Voltage clamp analysis of the transient outward (positive dynamic) current was performed in sheep Purkinje fibers at a pulse frequency of 1/min. 4-aminopyridine (4-AP, 1 mM) suppressed most of the transient outward current, thus revealing the slow inward current, isi, and an associated brief outward current ibo. The long lasting component of the current suppressed by 4-AP was labelled ilo. In the presence of 4-AP, ibo was suppressed either by caffeine 10 mM or when Sr was substituted for Ca, both conditions making isi clearly detectable. Mn ions suppressed both isi and ibo. Current decay was a monoexponential process for ibo (tau = 12 ms) and a two exponential process for ilo (tau 1 = 80-100 ms, tau 2 = 250-400 ms). The peak amplitude-Em relationships were different for the two currents. It was shown that the reversal potential of ilo was not measurable by the usual method probably because of the too fast activation-deactivation kinetics of the current. It is concluded that not one but two transient outward currents with different electrophysiological and pharmacological characteristics exist in the sheep Purkinje fiber. The reason of the caffeine-sensitivity of ibo is discussed.

The effects of sodium pump activity on the slow inward current in sheep cardiac Purkinje fibres

The effects of Na pump activity on the slow inward current, Isi, magnitude and twitch tension were investigated in sheep cardiac Purkinje fibres. A two-microelectrode voltage-clamp method was used, tension being measured simultaneously. Na pump activity was lowered either by reducing the extracellular K concentration, [K]O, or by applying the cardiotonic steroid strophanthidin. Reduction of [K]O from 4 to 0 mM leads to time-dependent increases in Isi magnitude and twitch tension. The increases of Isi and tension could be reversed by adding Tl, Rb, Cs or NH4 ions to the K-free superfusate. The actions of these ions are attributed to the known ability of these cations to activate the external site of the Na pump. This conclusion is supported by the observation that such activator cations do not reverse the increases in Isi and tension produced by strophanthidin. We conclude that the effects of low [K]O on Isi are mediated by Na pump inhibition. Similarly the Na pump inhibition produced by strophanthidin increases Isi and tension, although, in this case, other mechanisms may also contribute. Measurements of the activity of the electrogenic Na pump show that elevated intracellular Na ion concentration secondary to Na pump inhibition and not the instantaneous Na pump turnover rate mediates the increase in Isi magnitude.

Effects of nystatin-mediated intracellular ion substitution on membrane currents in calf purkinje fibres

1. Calf cardiac Purkinje fibres were exposed briefly to the ionophore nystatin to promote exchange of caesium for intracellular potassium. The effects of Cs loading were stable for at least 30 min, but they could be reversed by nystatin-mediated K loading.2. After Cs loading, the resting potential shifted to about -20 mV and the current-voltage relationship showed a strong inhibition of inwardly rectifying K channels.3. Anodal break stimulation evoked normal action potential upstrokes and twitch contractions. The early repolarization (phase 1) was markedly slowed.4. Cs loading simplified the pattern of current changes evoked by step depolarizations over the plateau range. Membrane current reached an inward peak and then declined monotonically.5. The current signal showed no hint of the transient outward current found in untreated or K-loaded preparations. Furthermore, Cs loading abolished the outward tails associated with deactivation of transient outward current, and occluded the blocking effect of the K-channel inhibitor 4-aminopyridine.6. Inhibition of transient outward current revealed a maximal inward current of about 5 muA/muF in 5.4 mM-Ca(o), which is considerably larger than the net inward current without Cs loading.7. The inward current was attributed to Ca channels on the basis of its sensitivity to membrane potential, extracellular Ca, D600, Mn and Cd.8. Cs loading also reduced slow current changes associated with delayed rectification and pace-maker depolarization.9. The results support the hypothesis that the transient outward current is carried by K(+) ions, while providing a method for unmasking inward Ca current.

The effects of stimulation rate on calcium-dependent action potentials recorded from chick embryo heart cell aggregates

1. Action potentials were recorded from aggregates of heart cells prepared from 3- to 7-day chick embryos. At 3 days the maximum rate of rise (+ Vmax) was insensitive to TTX; at 7 days it was considerably reduced by TTX. 2. In the presence of TTX the action potential overshoot was dependent on [Ca]0; the results may be fitted using constant field theory and assuming that the membrane is over a hundred times more permeable to Ca than to Na or K. 3. An increase in stimulation rate in the range 0.2-2 Hz led to an increase in both overshoot and + Vmax. This effect was not seen after addition of 20 mM-tetraethylammonium ions, nor when Sr was substituted for Ca in the external medium. We suggest that these rate-dependent changes may result from partial inactivation of an outward K current.

Intracellular [Ca2+] transients in voltage clamped cardiac Purkinje fibers

The Ca2+-activated bioluminescent protein aequorin was used to observe intracellular [Ca2+] transients in voltage clamped canine Purkinje fibers. The pattern of luminescence during a voltage clamp pulse was characterized by two components: L1, which is a rapid initial increase in luminescence and L2, which is a slower, secondary rise of variable configuration. 1. L1, L2, inward current, and contraction were abolished by D 600 (2 microM). 2. Paired clamp pulses. L1 reprimes more rapidly than L2; L1 reprimes within 100 ms, L2 does not. 3. Clamp pulse duration. Peak inward current was the same for 50 ms or 500 ms clamp pulses; L1 was either the same or slightly reduced in 50 ms clamp pulses compared to 500 ms clamp pulses. L2, however, was abolished in repetitively given 50 ms pulses compared compared to repetitively given 500 ms pulses. When 500 ms pulses were alternated with 50 ms pulses, L2 was greater in the 50 ms pulse than in the 500 ms pulse. 4. Clamp pulse potential. In the range-35 to O mV, peak L1 and peak inward current occurred at nearly the same time, had the same threshold potential, and had a similar dependence on membrane potential. In the presence of L2, contractions develop severalfold greater peak tension, time to peak tension is longer, and relaxation is more rapid than in the absence of L2. It is concluded that Ca2+ released from stores accounts for L2 and most of the 'activator calcium'. Ca2+ from another source accounts for L1 and activates a small early component of the contraction. L1 has some properties expected for a signal related to Ca2+ entering via slow inward current, but not via Na/Ca exchange.

Block and activation of the pace-maker channel in calf purkinje fibres: effects of potassium, caesium and rubidium

1. The effects of low concentrations of Cs(+) (0.01-3mM) on the fully activated I-V relation ī(f)(E) for the pace-maker current in calf Purkinje fibres have been investigated. The action of Cs(+) is two-fold: in the negative region of the I-V curve Cs(+) induces a channel blockade; on the other hand, at more positive potentials Cs(+) can produce the opposite effect, i.e. a current increase.2. Cs(+)-induced blockade is concentration- and voltage-dependent, as observed on other cation channels. Data in the far negative voltage range (about - 150 to - 50 mV) can be fitted by a simple block model (Woodhull, 1973), which gives a mean value of 0.71 for the fraction of membrane thickness (delta) crossed by Cs(+) ions before reaching the blocking site. The value of delta does not appear to be affected by either external Na or external K concentrations. Values for the dissociation constant of the blocking reaction at E = 0 mV (k(0)) are found in the range 0.5-3.7 mM. In the positive region of the ī(f)(E) relation the current depression caused by channel blockade vanishes. Unexpectedly, in this range the current can be observed to increase with Cs(+), and ī(f)(E) curves in different Cs(+) concentrations show cross-over.3. Changing external K(+) also produces similar cross-over phenomena. Investigation of this effect reveals that the increase in slope of the I-V curve on raising the external K(+) concentration follows Michaelis-Menten kinetics, and can be interpteted in terms of K(+)-induced channel activation. It is found that 44+/-6 mM-K(+) half-saturates the channel activating reaction.4. The Cs(+)-induced current increase is large in low-K(+) solutions and vanishes in high-K(+) solutions, suggesting a competition between Cs(+) and K(+) ions in their activating action. Increasing Na(+) also limits the Cs(+)-induced current increase.5. Rb(+) also blocks the i(f) channel, though less efficiently than Cs(+). The block caused by Rb(+) is, unlike that of Cs(+), nearly voltage-independent, and is explained by assuming that the blocking reaction occurs near the external mouth of the channel (mean value of delta is 0.05). The zero-voltage dissociation constant (k(0)) of the Rb(+)-blocking reaction ranges between 1.4 and 5.4 mM, and is lower in low-Na(+), high-K(+) solutions.6. A possible characterization of the i(f) channel which explains these results includes an inner ;blocking' site, to which external Cs(+) ions bind, blocking the channel, and a more external ;activatory' site, to which K(+), Cs(+), Rb(+) and possibly Na(+) ions bind. Binding of K(+) to this site induces a current increase either by modulating the channel, or actually by opening the channel itself. A similar mechanism can apply to Cs(+) and to Rb(+) binding.

Enhancement of calcium current during digitalis inotropy in mammalian heart: positive feed-back regulation by intracellular calcium?

1. Effects of digitalis compounds on slow inward Ca current I(si)) and contractile force were examined in ferret ventricular muscle (single sucrose-gap voltage clamp) and calf Purkinje fibres (two micro-electrode voltage clamp).2. In ventricular muscle, ouabain increased I(si) and inward current tails associated with I(si) conductance. The enhancement of I(si) followed a time course similar to the development of the positive inotropic effect, and it could be observed in the absence of aftercontractions or other signs of toxicit.3. The response of myocardial I(si) and twitch force to ouabain depended strongly on a previous history of driven action potentials.4. Veratridine, a toxin that promotes Na entry through tetrodotoxin-sensitive channels, also increased I(si) and twitch force in driven ventricular muscle preparations.5. The effects of ouabain, action potential stimulation and veratridine are consistent with reported effects of K-poor solutions in indicating that elevation of intracellular Na can lead to enhancement of I(si). Additional experiments suggest that the link between Na(i) and I(si) involves intracellular Ca.6. When Cs-loaded Purkinje fibres were bathed in solutions containing Sr instead of Ca, enhancement of I(si) by strophanthidin was abolished even though a positive inotropic response persisted.7. After intracellular injection of Purkinje fibres with EGTA, I(si) no longer increased with strophanthidin, although it remained responsive to adrenaline.8. Clear-cut increases in I(si) were seen in Cs-loaded Purkinje fibres even at very low concentrations of strophanthidin (20-50 nM), where the occurence of Na pump inhibition has been questioned.9. Positive regulation of Ca entry by intracellular Ca may act as a facilitory mechanism that amplifies myocardial responsiveness to digitalis and other inotropic interventions. Through changes in I(si), small rises in diastolic free Ca might lead to large increases in the activator Ca transient during contraction.

The ionic basis of concentration-related effects of noradrenaline on the action potential of calf cardiac purkinje fibres

1. Noradrenaline has paradoxical concentration-dependent effects on the Purkinje fibre action potential. At concentrations greater than 500 nM, it shortens the action potential whereas at lower concentrations it prolongs the action potential. 2. We determined the dose--response relation for Isi to noradrenaline in Purkinje fibres and found that a noradrenaline concentration of 126 nM produces a half maximal effect. 3. A maintained component of Isi was shown to have a similar response to noradrenaline. 4. The dose-dependence of noradrenaline-induced changes in Ix was determined. The half maximal noradrenaline concentration for this relationship was shown to be 133 nM. 5. The dose--response relationships for Ix and Isi were not altered by variation in extracellular Ca2+. 6. Ix appears to be approximately twice as responsive as Isi to saturating doses of noradrenaline. 7. These data were incorporated in a computer-generated reconstruction of the effects of noradrenaline on the action potential and are sufficient to account for the paradoxical effects of noradrenaline.

Activity-dependent changes of slow inward current in ventricular heart muscle

1. The relationships between membrane voltage, contractile force and slow inward current were studied in cat and dog papillary muscles or trabeculae employing the double sucrose gap voltage clamp technique. The experiments were performed at 30 degrees C and the preparations were stimulated at a frequency of 0.5 Hz. 2. The known relationships between steady state contractile force, slow inward current and membrane voltage were confirmed. 3. Under non-steady state conditions the slow inward current decreases during ascending and increases during descending contraction staircases when the clamp steps of the test train exceed about 60 mV from resting level. Depolarization clamp steps below 60 mV produce parallel changes of the slow inward current and contractile force. Those clamp conditions which increase the contractile force shift the threshold of Isi and of contraction towards more negative values. 4. During ascending staircases an increasing background outward current was regularly observed together with diminishing slow inward current. 5. The reported current transients agree with the changes of action potential configuration during mechanical transients: the prolongation of plateau during descending staircases corresponds to an increase, and the shortening of action potential during late repolarization corresponds to a decrease of slow inward current in the respective voltage ranges. 6. The slow inward current was tentatively separated into two components. The main component is inversely proportional to contractile force and it exhibits the well known current-voltage relationship for this current. The other one is directly proportional to contractile force and may be related to a regenerative response of reticular membranes.

Transient and delayed potassium currents in the egg cell membrane of the coelenterate, Renilla koellikeri

1. The properties of the fast-inactivating or transient K current and the slowly inactivating or delayed K current of the membrane of immature eggs of the clonial marine coelenterate, Renilla Koellikeri, were studied by using voltage clamp and intracellular dialysis techniques. 2. The transient current is activated when the membrane potential becomes more positive than -25 approximately -20 mV (resting potential, -72 +/- 5 mV) whereas the activation potential of the delayed current is -10 approximately OmV. These potentials are independent of either [k+]o or [K+]i. 3. The inactivation of the transient current is rapid and is almost complete for membrane potentials more negative than the activation potential while it is slow for the delayed current and incomplete within a few seconds. 4. Both currents shows similar reversal potentials which are predominantly determined by the K concentration gradient across the membrane. 5. The sensitivities of the conductance upon the internal K concentration differ between the two currents, suggesting that the interaction between the site and ions in the membrane channels differ between them. 6. Neither current is a Ca-activated K current. 7. 4-AP suppresses the transient current at concentrations substantially smaller than those that suppress the delayed current while TEA shows no effect on either current. 8. Intracellular application of pronase or tannic acid at relatively high concentrations does not alter the inactivation of either current. 9. The membrane includes a voltage-dependent Ca permeability which results in action potentials under current-clamp conditions.

Potassium current suppression by quinidine reveals additional calcium currents in neuroblastoma cells

Quinine and quinidine have been evaluated with regard to their effects on the electrical activity of neuroblastoma cells. Under voltage-clamp conditions, we have found that quinine and quinidine block both the voltage-dependent and Ca2+-dependent K+ conductances. Blockage of the voltage-dependent K+ channel is manifest as an increase in the amplitude and in the duration of the action potential. Blockage of the Ca2+-dependent K+ channel in Na+-free (replaced by Tris) solutions containing 6.8 mM Ca2+ and tetraethylammonium ion or 4-aminopyridine (to block the voltage-dependent K+ current) is seen as a further prolongation of the Ca2+ action potential and diminution of the after-hyperpolarization. A critical role of the Ca2+-dependent K+ conductance in modulation of the rate and duration of trains of Ca2+ action potentials is shown by the use of low concentrations (5-40 microM) of quinine or quinidine, which diminish the Ca2+-dependent K+ conductance in a graded manner. After complete blockade of K+ currents, the peak Ca2+ currents are enhanced at all voltages, especially at values more positive than -30 mV, where a steady-state inward current appears as well. In this same voltage range, the decay of the Ca2+ current exhibits two time constants--that of the transient inward current, which is about 20 msec, and a much slower (approximately 2000 msec) component. It is suggested that neuroblastoma cells have two types of calcium channels--one which generates the Ca2+ action potential and a second, distinguished by activation at more depolarized levels and by a slow rate of inactivation, which underlies the calcium entry necessary to activate the Ca2+-dependent K+ conductance.

Changes in 42K efflux produced by alterations in transmembrane calcium movements in turtle cardiac pace-maker tissue

1. 42K efflux has been measured from small strips of turtle sinus venosus which were electrically paced. Three different procedures for altering transmembrane calcium influx have been utilized to test whether changes in 42K efflux may be modulated by changes in intracellular calcium levels. 2. No significant changes in the 42K fractional escape rate (FER) were observed when external calcium was reduced to O mM or increased to 4 x normal (10 mM). In these experiments extracellular divalent cation concentration was held constant by adding or removing magnesium ions. 3. Application of 10 mM-Ba2+ also failed to alter 42K FER consistently. In red blood cells and snail neurones addition of barium ions has been shown to reduce significantly the calcium-mediated potassium current. 4. A tenfold increase in pacing rate (0.5-5 Hz) resulted in an augmented 42K FER, but repetition of this rate change in O mM-Ca2+ indicated that this increase in 42K FER was not strongly dependent on the amount of calcium entry. 5. Attempts to load the pace-maker cells with calcium by using the ionophore A23187 (10 micrograms ml . -1 of 2.0 x 10(-5) M) consistently resulted in very large increases in 42K FER. However, this effect (i) was blocked by atropine and (ii) was markedly reduced by pretreating the tissues with hemicholinium, indicating that A23187-induced release of acetylcholine from the endogenous nerve terminals was responsible for the observed increase in 42K FER. 6. In summary, three different experimental tests indicate that the majority of the 42K efflux is not tightly linked to transmembrane calcium movement in sinus venosus pace-maker tissue.

Potential and tension changes induced by sodium removal in dog Purkinje fibres: role of an electrogenic sodium-calcium exchange

1. Isolated dog Purkinje fibres were bathed in K-free media or in the presence of ouabain 10(-4) M in order to depress the electrogenic sodium pump activity. Membrane potential and mechanical tension were recorded in the presence of normal external sodium concentration and during lowering or removal of external Na. 2. Lowering or removal of external Na (Na being replaced by choline, Tris, sucrose or Li) induced a hyperpolarization and a contracture which reached a maximum after 1 or 2 min and then decreased progressively. Using Tris, Em increased from -40 +/- 3 to -72 +/- 10 mV (n = 39). The Na-free contracture and hyperpolarization did not occur in the absence of Na pump depression. 3. Tetrodotoxin (1.2 x 10(-5)M), Mn (4 mM), verapamil (1-4 x 10(-5) M) tetraethylammonium (5 mM), 4-aminopyridine (5 mM) and Cs (20 mM, in the presence of ouabain) did not alter the Na-free contracture and hyperpolarization. On the other hand Mn (20 mM), acid media (external pH less than 6.0) and low temperatures depressed or suppressed both the hyperpolarization and contracture. Lanthanum (0.4 mM) did not suppress the hyperpolarization and the contracture. On the contrary the Na-free contracture was generally increased in the presence of La. 4. Caffeine (10 mM) induced strong contractures with no changes in Em, thus demonstrating the possibility for the Purkinje fibers of developing contractures without concomitant hyperpolarizations. 5. It can be concluded that the Na-free contracture and hyperpolarization are not due to changes in passive conductances but are related to the functioning of an electrogenic Na-Ca exchange mechanism which carries inwardly 1 Ca and outwardly 3 or more Na.

A study of the effect of the rate of stimulation on the transient outward current in sheep cardiac Purkinje fibres

1. The transient outward current, Ito, of sheep Purkinje fibres has been recorded using the two micro-electrode voltage clamp technique. 2. Ito is strongly rate-dependent: the magnitude of Ito activated during a test voltage clamp pulse after a train of action potentials is less at higher rates of stimulation. 3. After an increase or decrease in rate there is an abrupt change in Ito in the first response followed by slower changes over the next several hundred responses. 4. When a preparation is rested after repetitive activity Ito recovers in two phages: there is an initial rapid, approximately exponential phase of recovery in the first 10 s which is probably due to reactivation; this is followed by a slower phase of recovery lasting several hundred seconds. 5. Curves showing the time course of reactivation of Ito have been obtained at different rates. At high rates the curves approach smaller values of Ito and the steady-state control values of the current occur on the shoulder of the curves, i.e. before reactivation is complete. 6. It is proposed that the reduction of Ito at high rates is due to two factors: incomplete reactivation which accounts for the rapid changes of Ito and a second unknown factor which accounts for the slower changes in the current. 7. Inspection of current-voltage relationship for Ito suggests that the reduction of Ito at high rates is mainly due to a decrease of conductance rather than to a reduction of the reversal potential. 8. Replacement of the calcium in the bathing solution by strontium does not abolish Ito in sheep Purkinje fibres, suggesting that the current is distinct from the transient outward current in calf Purkinje fibres described by Siegelbaum & Tsien (1980).

Effect of rate-dependent changes in the transient outward current on the action potential in sheep Purkinje fibres

1. The rate of membrane potential change during the initial phase of rapid repolarization of the action potential in sheep Purkinje fibres has been measured by electronic differentiation. 2. Phase-plane analysis has revealed that the potential dependence of rapid repolarization corresponds to the potential range over which the transient outward current , Ito, is recorded in voltage clamp experiments. 3. The initial rate of repolarization is strongly rate-dependent and it is markedly reduced at high rates. 4. The effect of high rates of stimulation on the phase-plane diagram is consistent with a reduction in I. 5. After an increase or decrease in rate there is an abrupt change in the initial rate of repolarization in the first response followed by slower changes over several hundred responses. 6. Recovery of the initial rate of repolarization occurs in two distinct phases after repetitive to activity: there is a rapid, approximately exponential phase of recovery in the first 10 s which is followed by a slower phase of recovery lasting several hundred seconds. 7. The rate-dependent changes in the initial rate of repolarization are abolished by 4-aminopyridine, 0.5-1.0 mmol/l. 8. It is concluded that the rate-dependent changes in the initial phase of repolarization are due to the similar changes in Ito described in a companion paper (Boyett, 1981). 9. Rate-dependent changes in peak tension have been measured and they bear no relationship to the changes in the initial rate of repolarization. It is concluded that the major component of the transient outward current in sheep Purkinje fibers is unlikely to be a Ca-activated current

Charge movements during the Na+-Ca2+ exchange in heart sarcolemmal vesicles

The Na+-Ca2+ exchange was studied in a highly purified vesicular preparation derived from heart sarcolemma. The initial velocity of the Na+-driven Ca2+ influx, which was monitored continuously with a specific electrode, was 15 nmol/mg of protein per sec; the total Ca2+-accumulation capacity was 80 nmol/mg of protein. The Na+-Ca2+ exchange generated a current that was compensated for by the uptake of tetraphenylphosphonium in (Ph4P+) (when the latter was present in the medium), the influx of K+, and the efflux of Cl-. The movements of Ph4P were followed with a specific electrode. Ca2+ in the concentration range 3-50 microM induced an increase in the permeability of the sarcolemmal membrane to K+. Under conditions of optimal charge neutralization by K+ (i.e., in the presence of valinomycin), the Km (Ca2+) of the exchanger was 1.5 microM. The Na+-Ca2+ exchange was inhibited by chlorpromazine and was not inhibited by vanadate.