SUMMARY1. The nature, magnitude and kinetics of the 4-aminopyridine-sensitive early outward current (Ito) were analysed in isolated ventricular myocytes from the septum, the apex and the left ventricular free wall of rat ventricles using the wholecell voltage clamp method. The modulatory effect of pressure overload-induced cardiac hypertrophy on the regional variations of Ito was assessed in each topographical class of cells.2. Voltage clamp experiments were performed at room temperature (20-25°C) in the absence of Na+ on both sides of the membrane and in the presence of 3 mM CoCl2.Ito was studied from a holding potential of -80 mV and determined by subtraction of total outward currents elicited by the same protocols in the presence of 3 mm 4-aminopyridine (4-AP) from those obtained in its absence.3. In normal hearts, membrane passive properties were very similar in each topographical class of cells. Our results confirmed that the predominant early outward current in rat ventricular cells was 4-AP-sensitive, time and voltage dependent, and demonstrated that the magnitude of the current varied on a regional basis: current density of Ito in left ventricular free wall cells (30-1 +9-2 pA/pF at + 60 mV) was larger than in apex cells (20-2 + 1-7 pA/pF) or in septum cells (1 1-9 + 3-3 pA/pF). We noticed a larger variability in data from left ventricular free wall compared with other regions.4 magnitude appeared not to be modified, the current density-voltage curves were slightly shifted to more positive potentials and significantly decreased as compared to normal cells (in pA/pF, at + 60 mV): 8-4 + 5-0 in the left free wall group, 1 1-6 + 2-0 in the apex group, and 3-8+ 1-5 in the septum group. Steady-state activation and inactivation parameters were not clearly modified, but kinetics were slowed down.7. We conclude, therefore, that Ito is differentially distributed among different regions of the normal rat ventricle and we propose that this regional heterogeneity may be related to different distributions of functional channel densities, rather than alterations in whole-cell kinetics or single-channel properties. Pressure overloadinduced hypertrophy reduces Ito current availability by decreasing current densities without any significant change of whole-cell kinetics, while a homogenizing tendency of the ionic profile is observed among the studied regions. One possible explanation for the hypertrophy-induced variations may be an absence of Ito channel neosynthesis, leading to a decrease of channel density per surface area unit.

Long-term left coronary occlusion induces significant alterations in both Ca2+ and K+ currents that occur with similar amplitude in both ventricles. They include a marked reduction in Ito amplitude as well as a slowing of ICaL inactivation. Both factors could contribute to the disturbances in cellular electrical behaviour and the occurrence of arrhythmias in the post-myocardial infarcted heart.

Modulation of the regional distribution of the action potential by left ventricular hypertrophy and the role of the L-type Ca2+ current (I(Ca)) and transient outward current (I(to)) in the action potential duration (APD) were investigated in normal and hypertrophied rat ventricular myocytes from the apex (A), septum (S) and left ventricular free wall (FW) by using whole cell current- and voltage-clamp techniques. Hypertrophy was induced by abdominal aortic constriction. In control cells, the APD measured at 20% repolarization (APD20) assumed the shortest values in the A and the longest in the S, whereas FW cells showed intermediate values. Hypertrophy significantly prolonged the APD20 and increased APD variability within the A and FW regions but did not modify the APD in S cells. Analysis of the APD, I(Ca), and I(to) at the instant of 20% repolarization in the same cell showed that in control cells the shortest APD20 was associated with a prominent I(to) in the A and FW, whereas the long APD20 was identified with a lower I(to) in S myocytes. Hypertrophy-induced prolongation ofAPD20 was associated with a reduction in the I(to) in the A and FW. Significant correlations could be established between the APD20 and the "net current," defined as the algebraic addition of I(to) and I(Ca) in the A and FW control groups but not in the control S or hypertrophied cells whatever their origin. Our results indicate that interregional APD heterogeneity is lost while intraregional APD variability is increased in the A and FW during the hypertrophic process. These effects are largely due to a change in the balance between the I(Ca) and I(to), which is a major contributing factor to the heterogeneity of the initial phase of repolarization in the normal rat ventricle.

Characteristics of the slow inward current (Isi) in human ventricular myocytes isolated from septal specimens obtained in patients undergoing corrective cardiac surgery were studied using the whole-cell clamp method. A first series of experiments was performed under normal standard superfusion. Clamping from -60 mV evoked an inward current with a threshold at about -35 mV, a maximum around +10 mV and an apparent reversal potential at about +55 mV. No overlapping transient or background outward currents were detected in the -60 to +30 mV potential range, but time-dependent and steady-state outward currents were elicited at potentials above +30 mV. An overlap of steady-state activation and inactivation curves was present between -30 and +10 mV and a slight relief from inactivation was observed for voltages positive to +10 mV. The time course of inactivation consisted of fast and slow phases with time constants differing by a factor of eight. Slow time constants of inactivation were shorter at potentials that elicited larger Isi, and longer at potentials inducing smaller Isi. Recovery from inactivation evolved slowly with 100% reactivation occurring in about 4000 ms. Switching the holding potential from -60 to -40 mV led to a reversible decline of Isi without any change of the decay time constants. Isi was significantly increased by 0.1 microM isoproterenol. Total or partial inhibition by inorganic (2 mM Mn2+, 3 mM Co2+, 1 mM Cd2+) and organic (1 microM methoxyverapamil, 5 microM diltiazem) calcium antagonists did not unmask any transient outward current. However, a consistent increase of Isi was reversibly observed with 3 mM 4-aminopyridine while using standard solutions. A second series of experiments carried out with K(+)- and Na(+)-free solutions did not demonstrate any significant change from data observed with standard solutions except a reduction of outward currents at steps above +30 mV and alteration of inactivation kinetics. In this experimental setting, 4-aminopyridine also increased Isi but to a lesser degree. We conclude that Isi, as compared to the outward currents, is dominant in the diseased human ventricular cells we have studied.