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Aims: Present study was performed to compare the dynamics of human L-type calcium current (ICa,L) flowing during rectangular voltage pulses, voltage ramps, and action potentials (APs) recorded from epicardiac and endocardiac canine ventricular cells., Methods: ICa,L was recorded in single myocytes isolated from undiseased human hearts using the whole cell voltage clamp technique., Results: The decay of ICa,L was monotonic when using rectangular pulses or endocardial APs as voltage commands, whereas the current became double-peaked (displaying a second rise and fall) during epicardial (EPI) APs or voltage ramps used to mimic EPI APs. These ICa,L profiles were associated with single-hooked and double-hooked phase-plane trajectories, respectively. No sustained current was observed during the AP commands. Kinetics of deactivation and recovery from inactivation of human ICa,L were determined using twin-pulse voltage protocols and voltage ramps, and the results were similar to those obtained previously in canine cells under identical experimental conditions., Conclusions: ICa,L can inactivate partially before and deactivate during the phase-1 repolarization of the epicardiac AP, and reopening of these channels seems to be associated with formation of the dome.

Aim: The aim of this study was to compare the action potential configuration, contractility, intracellular Ca2+ and H+ concentrations in mammalian cardiac tissues bathed with Krebs and Tyrode solutions at 37 degrees C., Results: In Langendorff-perfused guinea-pig hearts, loaded with the fluorescent Ca2+-indicator Fura-2, or H+-sensitive dye carboxy-SNARF, shifts from Krebs to Tyrode solution caused intra-cellular acidification, increased diastolic pressure and [Ca2+]i, decreased systolic pressure and [Ca2+]i, leading to a reduction in the amplitude of [Ca2+]i transients and pulse pressure. Contractility was also depressed in canine ventricular trabeculae when transferred from Krebs to Tyrode solution. Shifts from Krebs to Tyrode solution increased the duration of action potentials in multicellular cardiac preparations excised from canine and rabbit hearts but not in isolated cardiomyocytes. All these changes in action potential morphology, contractility, [Ca2+]i and [H+]i were readily reversible by addition of 26 mmol L(-1) bicarbonate to Tyrode solution. Effects of dofetilide and CsCl, both blockers of the delayed rectifier K current, on action potential duration were compared in Krebs and Tyrode solutions. Dofetilide lengthened rabbit ventricular action potentials in a significantly greater extent in Tyrode than in Krebs solution. Exposure of canine Purkinje fibres to CsCl evoked early after depolarizations within 40 min in all preparations incubated with Tyrode solution, but not in those bathed with Krebs solution., Conclusion: It is concluded that the marked differences in action potential morphology, [Ca2+]i, [H+]i and contractility observed between preparations bathed with Krebs and Tyrode solutions are more likely attributable to differences in the intracellular buffering capacities of the two media.

Action potential configuration in ventricular and atrial myocardium, as well as rate-dependent changes in ventricular action potential duration (APD) were studied and compared in healthy and diabetic rats. Diabetes was induced by a single injection of streptozotocin (STZ, 65 mg kg(-1) i.v.). Conventional microelectrode techniques were applied to record action potentials after the establishment of diabetes (2, 6, 10 and 18 weeks after STZ-treatment). Untreated age-matched animals were used as controls. Both depolarization and repolarization were significantly retarded following STZ-treatment. However, the time course of development of diabetic changes in atrial and ventricular myocardium was different. APD was significantly lengthened from week 2 of diabetes in ventricular, but only from week 6 in atrial preparations. In atrial myocardium, lengthening of APD was more pronounced at early rather than late phases of repolarization. The maximum rate of depolarization (Vmax) was significantly reduced from the 6th week of diabetes in both preparations. No differences were observed in action potential amplitude (except at week 18) and in the resting membrane potential in diabetic rats. Diabetic ventricular preparations showed a positive APD-frequency relationship at any level of repolarization, in contrast to control muscles, where APD25 and APD50 values lengthened. But APD75 and APD90 values were not changed significantly with increase in the pacing frequency. The results indicate that development of diabetic alterations are not fully identical in atrial and ventricular myocardium of the rat, probably owing to differences in density and kinetics of ionic currents responsible for atrial and ventricular action potentials.

Female sexual steroids are known to modify the expression of various K+ channels and thus they can alter cardiac repolarization. In the present work, using conventional microelectrode techniques, action potential characteristics were studied in atrial myocardium isolated from virgin, late pregnant, early (1-3 days) post-partum and late (2-3 weeks) post-partum rabbits. No changes in action potential configuration were observed during pregnancy. However, the duration, overshoot and amplitude of action potentials were significantly increased in the early (1-3 days) post-partum period. Resting potential and maximum rate of depolarization remained unchanged. The observed changes were transient, normal action potential characteristics were obtained at weeks 2-3 post-partum. 4-aminopyridine (1 mmol L(-1)). caused a marked lengthening of action potential duration in all preparations obtained from non-pregnant and pregnant rabbits, whereas this 4-aminopyridine-induced prolongation was moderate in those preparations excised from the hearts of early post-partum animals. Action potential configuration was not affected by pinacidil (10 micromol L(-1)) or glibenclamide (5 micromol L(-1)) in non-pregnant or pregnant animals. In preparations obtained from early post-partum rabbits, pinacidil significantly shortened action potential duration, which was reverted by glibenclamide. The lengthening of action potential duration together with the decreased sensitivity to 4-aminopyridine observed in early post-partum animals may probably be caused by reduction of the transient outward K+ current at this stage. The results also suggest that electrophysiological alterations in the early post-partum period may probably be more pronounced than those associated with pregnancy itself.

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.

The whole-cell configuration of the patch-clamp technique was applied to study and compare ion currents in single ventricular and atrial cardiocytes isolated from human myocardium. In ventricular cardiocytes the K+ inward rectifier current (IK1) was three times larger than in atrial cardiocytes, while its inactivation kinetics were twice as slow when measured at -140 mV. The magnitude of these variables depended on the test potential but was independent of changes in holding potential. A transient outward current (I(to)) was observed in both ventricular and atrial cardiocytes. The amplitude of the inactivating component of Ito was not significantly different in atrial and ventricular cells, but the time course of inactivation was significantly longer in atrial than in ventricular cardiocytes. Steady-state inactivation of Ito in atrial cells was well described by a two-state Boltzmann function having a midpoint potential of -41.4 mV and a slope factor of 6.9 mV-1. No discernible K+ delayed rectifier current (IK) was observed in either cell type. In four of the 12 atrial cells studied, a time dependent inward current was observed at negative test potentials having a 240 +/- 21 ms time constant for activation and an amplitude of 101 +/- 28 pA. This current, which resembled the pacemaker current (I(f)), was not observed in any of the ventricular cells examined.