A novel component to the outward currents of rat ventricular myocytes, I_Kx, has recently been suggested (Himmel et al. 1999). We have investigated the contribution of I_Kx to the outward currents recorded from 103 rat ventricular myocytes at physiological temperatures and in the absence of external Cd²⁺ using conventional whole-cell patch-clamp techniques. Left ventricular myocytes were isolated enzymatically from hearts excised under pentobarbitone anaesthesia (150 mg kg^-1 I.P.) from adult male Wistar rats according to UK legislation. Isolated cells were superfused with a Hepes-buffered Tyrode solution containing 1 mM Ca²⁺ (pH 7.35) at 35 °C. Pipettes were filled with a Hepes-buffered K⁺-rich solution containing 10 mM EGTA and 4 mM ATP (pH 7.2, pCa 7.14). Na⁺ and Ca²⁺ currents were removed using 20 ms pre-pulses to -40 mV and 3 mM nifedipine, respectively. Data are given as means ± S.E.M. and P < 0.05 in a Student’s paired t test was considered statistically significant. The voltage-dependent inactivation of the currents was examined using a two-pulse protocol in which 1 s conditioning pulses from -100 to +5 mV were given immediately prior to a 1 s pulse to +40 mV. Modified Boltzmann equations comprising either a single or two inactivating components were fitted by non-linear least squares to the inactivation of the peak current (I_pk) from conditioning pulses positive to -80 mV. In 81 of the myocytes examined, a modified Boltzmann equation with two inactivating components was accepted as a good fit to the data:

I_pk = I_m1 /(1 + exp ((V_p – V_0.5,1)/V_s,1))

+ I_m2 /(1 + exp ((V_p – V_0.5,2)/V_s,2)) + I_ss,

where I_m1 and I_m2 are the maximal values of the inactivating components, I_ss is the non-inactivating component, V_p is the conditioning pulse potential, V_0.5,1, V_0.5,2, V_s,1 and V_s,2 are the voltages of half-maximal inactivation and slope factors of the respective current components. The mean values of V_0.5,1 (-50.5 ± 0.5 mV) and V_0.5,2 (-29.5 ± 0.7 mV, n = 81, P < 0.001) are consistent with the existence of I_to1 and I_Kx, respectively. In paired t tests, the mean of I_Kx (352 ± 24 pA) was significantly less than that of I_to1 (734 ± 59 pA; P < 0.001) or I_ss (644 ± 28 pA; P < 0.001). However, there was considerable heterogeneity between cells, the relative contribution of I_Kx ranging from 2.5 to 51 % of the total peak outward current. Thus I_Kx is likely to play an important role in the heterogeneity of repolarising K⁺ currents in rat ventricular myocytes.

Financial support from The Wellcome Trust and British Heart Foundation is gratefully acknowledged.

All procedures accord with current UK legislation.