Parameters affecting the slow inward channel repriming process in frog atrium

Developmental changes in time course of recovery from inactivation in L-type calcium currents of rabbit ventricular myocytes

The mechanisms of recovery from inactivation of the L-type calcium current ( ICa) are not well established, and recovery is affected by many experimental conditions. Little is known about developmental changes of recovery from inactivation of ICa. We studied developmental changes of recovery from inactivation in ICa using isolated adult and newborn (1–4 days) rabbit ventricular myocytes. We used broken-patch and perforated-patch techniques with physiological extracellular ionic concentrations of calcium and sodium and interpulse conditioning potentials of −80 or −50 mV. We also maximized ICa with forskolin. We found that recovery from inactivation did not differ between adult and newborn cells when either EGTA or BAPTA was used to buffer intracellular calcium. Maximizing ICa with forskolin slowed recovery from inactivation in newborn but not in adult cells. In contrast, when the intracellular buffering of the cell was left nearly intact (perforated patch), recovery from inactivation (half-time of recovery) in the newborn cells was significantly slower than for the adult cells when either a conditioning potential of −80 mV (140 ± 9 vs. 58 ± 4 ms, newborn vs. adult; P < 0.05) or −50 mV (641 ± 106 vs. 168 ± 15 ms, newborn vs. adult; P < 0.05) was used. Forskolin significantly increased half-time of recovery for both adult and newborn cells. Dialysis with no calcium buffer showed a slower recovery from inactivation in newborn cells. Intracellular dialysis with a calcium buffer masked differences in recovery from inactivation of ICa between newborn and adult rabbit ventricular cells.

Intracellular calcium and electrical restitution in mammalian cardiac cells

The role of calcium current and changes in intracellular calcium concentration ([Ca2+]i) in regulation of action potential duration (APD) during electrical restitution process was studied in mammalian ventricular preparations. Properly timed action potentials were recorded from multicellular preparations and isolated cardiomyocytes using conventional microelectrodes and EGTA-containing patch pipettes. APD increased monotonically in canine and guinea pig ventricular preparations with increasing diastolic interval (DI), while in rabbit papillary muscles the restitution process was biphasic: APD first lengthened, then shortened as the DI increased. When the restitution process was studied in single cardiomyocytes using EGTA-containing patch pipettes, the restitution pattern was similar in the three species studied. Similarly, no difference was observed in the recovery time constant of calcium current (/Ca-L) measured under these conditions in voltage clamped myocytes. Loading the myocytes with the [Ca2+]i-chelator BAPTA-AM had adverse effects in rabbit and canine cells. In rabbit myocytes steady-state APD lengthened and the late shortening component of restitution was abolished in BAPTA-loaded cells. In canine myocytes BAPTA-load shortened steady-state APD markedly, and during restitution, APD decreased with increasing DI. The late shortening component of restitution, observed in untreated rabbit preparations, was greatly reduced after nifedipine treatment, but remained preserved in the presence of 4-aminopyridine or nicorandil. Beat to beat changes in APD, peak/Ca-L and [Ca2+]i, measured using the fluorescent dye, Fura-2, were monitored in rabbit ventricular myocytes after a 1-min period of rest. In these cells, the shortening of APD was accompanied by a gradual reduction of the peak/Ca-L and elevation of diastolic [Ca2+]i during the initial eight post-rest action potentials. It is concluded that elevation of [Ca2+]i shortens, while reduction of [Ca2+]i lengthens APD in rabbit, but not in canine ventricular myocytes. These differences may probably be related to different distributions of [Ca2+]i-dependent ion currents and/or to differences in calcium handling between the two species.

Effects of membrane lipid peroxidation by tert butyl hydroperoxide on the sodium current in isolated feline ventricular myocytes

Membrane lipid peroxidation is known to play a pivotal role in the genesis of coronary reperfusion arrhythmias in both experimental and clinical settings. To elucidate the electrophysiological mechanisms underlying these arrhythmias, the effects of tert butyl hydroperoxide (TBH) on the Na+ current (INa) in isolated feline ventricular myocytes were studied using whole-cell patch clamp techniques under 100% O2 bubbling. This agent at 20 mM inhibited INa from 2.2 +/- 1.3 to 1.7 +/- 1.0 nA (P < 0.01, n = 7) without changing time courses of INa inactivation. Twenty millimoles TBH shifted the steady-state inactivation curve for INa from -77.4 +/- 1.7 to -81.3 +/- 1.8 mV when measured at INa half inhibition voltage (P < 0.01, n = 7), but did not affect the slope factor. The kinetics of INa recovery from inactivation remained unchanged. These findings suggest that lipid peroxidation in the membrane by TBH reduces INa conductance and voltage-dependent INa availability, most likely as a result of structural damage to the Na+ channels.

Differential effects of elevating [K]o on three transient outward potassium channels. Dependence on channel inactivation mechanisms

We carried out a systematic study on the effects of elevating [K]o on the properties of a transient outward potassium channel encoded by a cardiac cDNA (RHK1) and compared them with those on two Shaker potassium channels (H-4 and H-37). The amino acid sequences of all three channels are known, and their structure-function relations have been partially characterized. All three channels were expressed in Xenopus oocytes and studied under double-microelectrode voltage-clamp conditions. For all three channels, elevating [K]o caused an increase in the channels' chord conductances and a negative shift in the calculated activation curves. However, in other aspects of channel properties that are related to the channels' inactivation processes, there were differences in the changes induced by increasing [K]o: 1) Elevating [K]o caused a positive shift in the steady-state inactivation curves of RHK1 and H-4 but did not cause any shift in H-37. 2) Elevating [K]o slowed the time course of inactivation of H-37 but did not cause any significant changes in the time course of RHK1 or H-4. 3) Elevating [K]o accelerated the rate of recovery from inactivation of RHK1 and H-4 but slowed the recovery time course of H-37. Our experiments show that elevating [K]o can cause a wide range of effects on the transient outward potassium channels. Furthermore, raising [K]o induced similar changes in RHK1 and H-4 (inactivation mediated by an "N-type" mechanism) that were different from the changes in H-37 (inactivation mediated by a "C-type" mechanism). Therefore, our data suggest that part of the effects of elevating [K]o on channel properties may depend on the channel's inactivation mechanism. This hypothesis is supported by results from experiments studying the effects of elevating [K]o on a mutant RHK1 channel (RHK1 delta 3-25), which apparently lacks the N-type and C-type inactivation mechanisms.

Ca channel gating during cardiac action potentials

How do Ca channels conduct Ca ions during the cardiac action potential? We attempt to answer this question by applying a two-microelectrode technique, previously used for Na and K currents, in which we record the patch current and the action potential at the same time (Mazzanti, M., and L. J. DeFelice. 1987. Biophys. J. 12:95-100, and 1988. Biophys. J. 54:1139-1148; Wellis, D., L. J. DeFelice, and M. Mazzanti. 1990. Biophys. J. 57:41-48). In this paper, we also compare the action currents obtained by the technique with the step-protocol currents obtained during standard voltage-clamp experiments. Individual Ca channels were measured in 10 mM Ca/1 Ba and 10 mM Ba. To describe part of our results, we use the nomenclature introduced by Hess, P., J. B. Lansman, and R. W. Tsien (1984. Nature (Lond.). 311:538-544). With Ba as the charge carrier, Ca channel kinetics convert rapidly from long to short open times as the patch voltage changes from 20 to -20 mV. This voltage-dependent conversion occurs during action potentials and in step-protocol experiments. With Ca as the charge carrier, the currents are brief at all voltages, and it is difficult to define either the number of channels in the patch or the conductance of the individual channels. Occasionally, however, Ca-conducting channels spontaneously convert to long-open-time kinetics (in Hess et al., 1984, notation, mode 2). When this happens, which is about once in every 100beats, there usually appears to be only one channel in the patch. In this rare configuration, the channel is open long enough to measure its conductance in 10 Ca/ 1 Ba. The value is 8-10 pS, which is about half the conductance in Ba. Because the long openings occur so infrequently with Ca as the charge carrier, they contribute negligibly to the average Ca current at any particular time during an action potential. However, the total number of Ca ions entering during these long openings may be significant when compared to the number entering by the more usual kinetics.

Calcium antagonists and myocardial protection

Painful and asymptomatic ischemia has been associated with left ventricular dysfunction, an important variable related to survival in patients with coronary artery disease. The treatment of patients with coronary artery disease with agents such as calcium channel blockers has been directed at reducing ischemia by restoring the balance between myocardial oxygen supply and demand, which ultimately serves to protect against myocardial dysfunction. Once ischemia has occurred, calcium channel blockers may protect myocardial cellular integrity and function. By reducing intracellular calcium overload during ischemia, mitochondrial function is preserved and adenosine triphosphate stores are maintained. Numerous in vitro and isolated heart preparations have shown that ischemia in the presence of calcium blockade is associated with less cellular dysfunction than in the situation of ischemia in the absence of calcium channel blockade.

The spasmogenic effect of caffeine in trachealis isolated from control and actively sensitized guinea-pigs

The spasmogenic activity of caffeine (10 mM) was evaluated in tracheal strips obtained from control and sensitized guinea-pigs then pretreated with indomethacin (2.8 microM) and cooled to 20 degrees C. The contraction elicited by caffeine was inhibited by verapamil (100 microM), trifluoperazine (100 and 500 microM) and dantrolene (50 and 500 microM) in the control and the sensitized tissues but was unaffected by disodium cromoglycate (39 microM). However, the same concentration of verapamil produced significantly less inhibition of the caffeine-induced contraction in sensitized compared to control tissues while the reverse was found for trifluoperazine and dantrolene. Exposure to a Ca2+-free, EGTA-containing medium resulted in 33% inhibition of the response to caffeine in control tissues but no inhibition in sensitized tissues. These results suggest the existence of differences in calcium movements in response to caffeine between control and sensitized tissues that may reflect abnormalities in calcium handling by the sensitized tissue.

Inactivation of calcium current in bull-frog atrial myocytes

1. A single-microelectrode technique has been used to study the voltage dependence and the kinetics of inactivation and reactivation of a tetrodotoxin-resistant inward current (ICa) in single cells from bull-frog atrium. 2. In most cases the kinetics of both inactivation and reactivation can be well described as a single-exponential process. 3. Several different observations indicate that inactivation of ICa in these cells is controlled by both voltage-dependent and current-dependent processes, as has been demonstrated previously in heart (Kass & Sanguinetti, 1984; Lee, Marban & Tsien, 1985) and in other tissues (Hagiwara & Byerly, 1981; Tsien, 1983; Eckert & Chad, 1984). 4. Evidence in favour of a voltage-dependent inactivation mechanism included: (a) In paired-pulse measurements of steady-state inactivation ('f infinity') a 'conventional' steady-state f infinity vs. membrane potential (Vm) relationship was obtained in the range of membrane potentials from -60 to 0 mV. (b) Increasing [Ca2+]o from 2.5 to 7.5 mM, which resulted in a 2-3-fold increase in ICa, did not produce any significant increase in the amount of inactivation. (c) Using a 'gapped' double-pulse protocol non-monotonic U-shaped inactivation relationships were obtained, i.e. positive to approximately +20 mV some removal of inactivation occurred. However, f never approached a value near 1.00 at very depolarized potentials; it reached a maximum between 0.5 and 0.6. (d) In constant [Ca2+]o and at fixed Vm, the kinetics of ICa inactivation were independent of peak size of ICa. This was demonstrated by: (i) varying the holding potential (-90 to -30 mV), (ii) using paired-pulse 'recovery' protocols, and (iii) partial block by La3+ (1-10 microM) and Cd2+ (0.1 mM). (e) Influx of Ca2+ ions was not an obligatory prerequisite for development of inactivation. In all ionic conditions (Ca2+, Sr2+, Ba2+, Na+-free and Ca2+-free Ringer solutions) currents displayed inactivation phenomena, although the extent and kinetics of inactivation were dependent upon ionic conditions. Outward currents recorded above the reversal potential for ICa exhibited time- and voltage-dependent inactivation, and could be inactivated by brief depolarizing pre-pulses that produced no net inward current flow. Evidence against a possible role of the electrogenic Na+-Ca2+ exchanger in producing inactivation of these outward currents was obtained.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium current restitution in mammalian ventricular myocytes is modulated by intracellular calcium

Restitution of the conventional L-type calcium current (ICa) was studied in dog or guinea pig ventricular myocytes to understand its time course and regulation. Whole-cell ICa free of other overlapping currents was recorded with a suction pipette. The intracellular environment was varied by intracellular dialysis. The properties of ICa were similar in dog and guinea pig ventricular myocytes, except that the amplitude of ICa was larger in the latter (2.2 +/- 0.5 nA in guinea pig cells and 0.9 +/- 0.2 nA in dog cells, n = 8 for both). In both types of cells during restitution a holding voltage (Vh) negative to -50 mV induced a transient increase in ICa above the control level (ICa overshoot). This overshoot was inhibited by substituting barium for calcium, lowering [Ca]0, increasing intracellular calcium buffering capacity, ryanodine (1-2 microM), or caffeine (10 mM). The overshoot peaked 30-100 msec after repolarization from the conditioning depolarization and gradually declined over the following 2-3 seconds. During the overshoot, although the amplitude of ICa was larger its half-time of decay was longer than the control. The maximum overshoot occurred following a conditioning step to plateau voltages and it was decreased by prolonging the conditioning step from 50 to 100 or 500 msec. It is concluded that intracellular calcium regulates restitution of the L-type calcium channels in mammalian ventricular myocytes and that the sarcoplasmic reticulum is involved in this process.

Inactivation, reactivation and pacing dependence of calcium current in frog cardiocytes: correlation with current density

1. Ca2+ currents were measured in single cells isolated from frog ventricle using the whole-cell patch clamp technique and a perfused pipette. K+ currents were blocked with intracellular (120 mM) and extracellular (20 mM) Cs+. 2. A single type of Ca2+ current (ICa) was found in these cells. The current activated at voltages positive to -30 mV, exhibited a symmetrical current-voltage relationship with a peak at 0 mV, and was slowly inactivating with Ba2+ as charge carrier. 3. Large variations in ICa amplitude were observed from cell to cell (ICa at 0 mV = 293.1 +/- 283.3 pA; N = 152). These variations were not due simply to differences in cell membrane area, which was estimated by cell membrane capacitance (Cm), because the density of Ca2+ current (dICa = ICa/Cm) also varied significantly from cell to cell (1.3-28 pA/pF at 0 mV; mean +/- S.D. = 4.49 +/- 3.96; N = 152). 4. The inactivation curve of ICa was a complex function of membrane potential. 200 ms pre-pulses to voltages between -60 and +20 mV progressively inactivated ICa elicited by a subsequent test pulse with half-maximal inactivation occurring for pre-pulses to approximately -40 mV. With pre-pulses positive to +20 mV, ICa elicited by the test pulse became progressively larger. The degree of inactivation induced by a 200 ms depolarization to potentials more positive than +20 mV varied significantly from cell to cell, while no such variations were observed in the negative range of membrane potentials. 5. The time course of reactivation (i.e. removal from inactivation) of ICa at -80 mV often exhibited an overshoot. The amplitude of the overshoot varied between 100% (i.e. no overshoot) and approximately 180% in eighty-one cells. 6. The degree of inactivation at positive potentials (+100 mV) and the amplitude of the overshoot were strongly correlated with the Ca2+ current density. The overshoot was more pronounced, the reactivation was faster, and the inactivation at positive potentials was less in cells with lower ICa density. 7. Increasing the stimulation frequency from 0.125 to 2 Hz induced a positive staircase of ICa in cells with ICa density less than 2 pA/pF and a negative staircase in cells with ICa density greater than 3 pA/pF. 8. Perfusing the patch pipette with 5 mM-BAPTA instead of EGTA reduced the amplitude of the overshoot and slightly slowed the inactivation kinetics. Replacing extracellular Ca2+ ions by Ba2+ ions completely suppressed the overshoot.(ABSTRACT TRUNCATED AT 400 WORDS)

Different ability of trifluoperazine to inhibit agonist-induced contraction of lung parenchyma strips from control and sensitized guinea-pigs

There is increasing interest in the therapeutic potential of calcium antagonists in asthma. Among them the use of calmodulin antagonists deserves consideration. In the present work the effect of trifluoperazine on contractions generated by different mechanisms (CaCl2, KCl, acetylcholine, histamine and 5-hydroxytryptamine) in lung parenchyma strip isolated from control and actively sensitized guinea-pigs has been studied. Trifluoperazine produced both in unsensitized and sensitized lung strips, a concentration-dependent, right, downward displacement of the concentration-response curves to the agonists used, although the sensitization procedure resulted in a potentiation in the ability of trifluoperazine to inhibit agonist-induced contractions. The basis for this greater potency of trifluoperazine in sensitized tissues remains to be elucidated but raises attention to the future use of selective calmodulin antagonists in the management of asthma.

An intrinsic potential-dependent inactivation mechanism associated with calcium channels in guinea-pig myocytes

1. Currents through Ca2+ channels of single guinea-pig ventricular myocytes were studied using patch electrodes for whole-cell recording. Currents through Na+ and K+ channels were suppressed by the application of drugs or the substitution of impermeant ions. 2. Inactivation of the Ca2+ current (ICa) was investigated using a two-pulse protocol. The amount of inactivation left behind by a pre-pulse appeared to be related to current magnitude, as others have reported. The dependence of inactivation on the pre-pulse potential was partially U-shaped, as the amount of inactivation peaked at 0 mV and then declined with more positive pre-pulses. 3. Non-specific current carried by monovalent ions through Ca2+ channels (Ins) was induced by lowering the extracellular Ca2+ concentration with EGTA. Ins peaked in an inward direction at -20 mV, reversed direction at +22 mV, and became a large outward current at more positive potentials. 4. Ins inactivated with a slow time course. The inactivation was not due to accumulation or depletion phenomena. Studies using two-pulse protocols showed that the amount of inactivation left by a pre-pulse was directly related to the pre-pulse potential. 5. The addition of micromolar amounts of free Ca2+ to the external solution induced outward rectification of Ins. Inward currents were small or absent, while larger outward currents could still be seen at very positive potentials. Ca2+-channel inactivation still occurred under these conditions, even in the absence of any significant ionic movement. 6. The time courses of Ins inactivation and recovery were studied. The half-time of Ins inactivation decreased with larger depolarizations. Recovery of Ins was very slow, but could be accounted for by changes in the surface charge of the membrane. 7. It is concluded that Ins inactivation is due solely to a voltage-dependent inactivation process which is intrinsic to myocardial Ca2+ channels. Voltage-dependent inactivation appears to account for a significant proportion of total Ca2+-channel inactivation at negative potentials, and appears to account for almost all of the inactivation at very positive potentials, even in the presence of millimolar concentration of external Ca2+.

The effects of catecholamines on tension reactivation in cardiac muscle

The effects of adrenaline and the beta-agonist isoprenaline on the time course of tension reactivation were studied in several cardiac tissues. The aim of the study was to assess whether experimental evidence can be found for a role of the sarcoplasmic reticulum in the reactivation of tension. It was assumed that calcium recycles between different parts of the reticulum, and that this recycling may affect tension repriming. Isoprenaline was assumed to enhance such recycling by increasing the uptake of calcium, following its release during a preceding contraction. Isoprenaline (in the range of 40 nM to 4 microM) was found to enhance tension repriming in adult guinea pig atria. However, in adult rat atria, isoprenaline often gave a complex effect, with a smaller degree of repriming at short intervals, and enhanced repriming at longer intervals. This was thought to reflect the balance between the enhancing effect of the drug on calcium recycling and an augmented release from the sarcoplasmic reticulum (SR). In striking contrast, there was no effect of isoprenaline on tension repriming in neonatal guinea pig atria and a retardation in neonatal rat atria. This was interpreted as reflecting the lack of a sarcoplasmic network in the neonatal tissue. The effects of isoprenaline on tension repriming in the frog atrium (which also has a sparse sarcoplasmic reticulum network) were also found to be complex; low concentrations (40 nM) enhanced the process, and high concentrations (0.4 microM) retarded it. Intermediate levels often produced a 'crossover' effect: more reactivation at short intervals, and less at long intervals. The interpretation of these results was that there are two processes which interact to determine the amount of tension produced at short intervals after each contraction: the basal reactivation process and some augmenting mechanism superimposed on it. This mechanism is probably related to other behavioural features of cardiac muscle, such as rate-dependent increases in membrane calcium currents. It is relevant mainly in those cases where tension repriming depends on membrane calcium currents. Further experiments (in the frog atrium) with elevated calcium and with the alpha-adrenergic agonist phenylephrine (both of which slowed down the reactivation process) also support this idea. These agents elevate internal calcium levels, and presumably saturate the augmenting mechanism (by producing maximal tension responses).(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of a change in stimulation rate on action potentials, currents and contractions in rat ventricular cells

The effects of a change in stimulation rate on electrical activity and accompanying contraction were investigated in ventricular cells isolated from rat heart; the cells were stimulated to contract either by brief depolarization pulses which evoked action potentials, or, under voltage-clamp conditions, by step depolarizations. An increase in stimulation rate from 0.3 to 3 Hz resulted in a gradual reduction in the amplitude of contraction and attenuation of the late phase of the action potential. These changes were less marked at more depolarized potentials. The ventricular cells were voltage clamped at -40 mV and initially stimulated at 0.3 Hz by step depolarizations to 0 mV for 10 or 100 ms, which activated the second inward current (Isi) and an accompanying contraction. The amplitude and time course of contraction were similar with the two pulse durations. When the duration of the depolarization was 100 ms, an increase in stimulation rate to 3 Hz caused a gradual decline in the amplitude of Isi and of the evoked contraction; at the same time extra contractions and small, transient inward currents appeared in addition to the evoked contractions and Isis. There was a reduction in the early component of decay of Isi at 3 Hz. With a depolarizing pulse duration of 10 ms, an increase in stimulation rate to 3 or to 4.2 Hz did not change the amplitude of the evoked Isi or contraction and no extra contractions or currents appeared. Intracellular EGTA abolished all contractions in the cells and an increase in the rate of stimulation with 100 ms pulses did not then induce transient inward currents. There was some decrease in the Isi amplitude but this was not as marked as in the absence of EGTA and the time course of current decay was similar at the two rates. Ryanodine prevented the appearance of extra contractions and currents when the stimulation rate was increased to 3 Hz and, as in the presence of intracellular EGTA, there was a small decrease in Isi amplitude while the time course of decay was similar at the two stimulation rates. The time course of recovery of Isi from inactivation, as shown by a double-pulse procedure, was altered when the duration of the first pulse was reduced from 100 to 10 ms, an extra inactivation of Isi being seen at pulse intervals of 20-100 ms. This extra component of inactivation was not seen with intracellular EGTA or in the presence of ryanodine.(ABSTRACT TRUNCATED AT 400 WORDS)

Determinants of postrepolarization refractoriness in depressed mammalian ventricular muscle

Functional determinants of postrepolarization refractoriness were studied with microelectrodes in isolated cat and dog ventricular muscle preparations mounted in a three-chambered bath. Frequency-dependent conduction delay and block were readily manifested when the central segment (1 mm) was superfused with high potassium (20-30 mM) Tyrode's solution. Conduction disorders were attributed to postrepolarization refractoriness involving slow recovery in the amplitude of elicited subthreshold depolarizations in depressed fibers distal to the central blocked zone. Investigations of subthreshold phenomena in homogeneously depressed tissues indicated that a relatively large local response participated in the voltage displacement induced by subthreshold depolarizing currents. The local response was blocked by tetrodotoxin (10 micrograms/ml) or verapamil (2 micrograms/ml) when resting membrane potential was near -70 or -50 mV, respectively. At either level of reduced membrane potential, gradual recovery in diastolic excitability correlated closely with time-dependent recovery of the local response, the rate of which was also proportional to the current intensity applied. Thus, postrepolarization refractoriness in depressed ventricular muscle fibers is a function of the time for recovery of active subthreshold properties (the local response), as well as intensity of excitatory current input. These factors may play a role in the development of delayed conduction and reentry that occur at faster heart rates under ischemic conditions.

Changing surface charge with salicylate differentiates between subgroups of calcium-antagonists

Sodium salicylate (5-10 mM) has been used to distinguish the effects of the three calcium-antagonist subgroups which had been previously differentiated in functional studies. Sodium salicylate (10 mM) reduced the antagonistic effects of verapamil and diltiazem on Ca2+-induced contractions of K+ (40 mM)-depolarized taenia preparations from the guinea-pig caecum. In contrast, salicylate had no effect on the potency of nifedipine and increased the inhibitory effects of cinnarizine and flunarizine. Sodium salicylate (10 mM) had little effect on Ca2+-induced contractions per se. In preparations pretreated with calcium-antagonists and recontracted with high concentrations of Ca2+, salicylate (5 mM) caused an additional contraction when the preparations had been pretreated with verapamil or diltiazem but had no effect in control or nifedipine-treated preparations. In contrast, salicylate relaxed Ca2+-induced contractions in tissues which had been pretreated with cinnarizine, flunarizine, pimozide, bepridil, fendiline, perhexiline and with the calmodulin antagonist W-7. The mechanism of action of salicylate was investigated. Inhibition of prostaglandin biosynthesis or of oxidative phosphorylation by salicylate was not responsible for these effects because indomethacin (28 microM) and 2,4-dinitrophenol (20 microM) did not differentiate between calcium antagonists. The effects of salicylate are ascribed to an increase in negative surface charge on the membrane because other agents changing surface charge (3,5-dichlorosalicylate, 0.3 mM; benzoate, 20 mM) have similar effects and their potency is dependent on their affinity for lipid membranes. Furthermore, salicylate increased the effectiveness of the cationic local anaesthetic, (+)-propranolol (100 microM), but did not change the effects of the neutral local anesthetic, benzocaine (1 mM). It is argued that salicylate increases the effectiveness of cinnarizine by increasing accumulation of this drug in the cell membrane or at intracellular sites whereas the reduced effectiveness of verapamil and diltiazem is secondary to a change in the state of the Ca2+ channel.

The surprising heart: a review of recent progress in cardiac electrophysiology

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Calcium-mediated inactivation of the calcium conductance in cesium-loaded frog heart cells

Ca current inactivation was investigated in frog atrial muscle under voltage-clamp conditions. To inhibit the outward currents, experiments were performed on Cs-loaded fibers and in 20 mM Cs (K-free) Ringer with 4-AP added. Inactivation, produced by a conditioning pulse, was measured by reducing the current during a subsequent test pulse. The extent of inactivation increased initially with prepulse amplitude and then decreased as the prepulse potential became progressively positive. Relative inactivation follows a U-shaped curve. When Sr was substituted for Ca, both the degree and the rate of inactivation decreased. Relative inactivation appeared to be linearly related to the amount of divalent cations (Ca and Sr) carried into the cell during the prepulse. Elevating Ca enhanced peak current and accelerated its decline. Elevating Mg decreased peak current and slowed its decline. An application of Na-free (LiCl) solution resulted in a somewhat smaller but faster inactivating current. Adrenaline increased and D600 decreased the maximal Ca conductance with little alteration in the inactivation rate; Co decreased both peak current and the rate of inactivation. Enhancement of the outward currents, reduced driving force, and intracellular surface charge screening do not adequately account for the above results. Evidence was considered that Ca entry mediates most of Ca current inactivation in frog atrial fibers. Removal from inactivation was also investigated in normal-Ca, Ca-rich, and Sr solutions. Recovery after partial inactivation by high depolarization was biphasic. Recovery was slowed by 10 Ca and accelerated by 1.8 Sr, whereas opposite effects have been shown on activation.