EFFECTS OF EXTERNAL RB-ARTERY ENDOTHELIAL-CELLS( ON INWARD RECTIFIER K+ CHANNELS OF BOVINE PULMONARY)

Inward rectifier (IR) K+ channels of bovine pulmonary artery endotheli al cells were studied using the whole-cell, cell-attached, and outside -out patch-clamp configurations. The effects of Rb+ on the voltage dep endence and kinetics of IR gating were explored, with [Rb+](o) + [K+]( o) = 160 mM. Partial substitution of Rb+ for K+ resulted in voltage-de pendent reduction of inward currents, consistent with Rb+ being a weak ly permeant blocker of the IR. In cells studied with a K+-free pipette solution, external Rb+ reduced inward IR currents to a similar extent at large negative potentials but block at more positive potentials wa s enhanced. In outside-out patches, the single-channel i-V relationshi p was approximately linear in symmetrical K+, but rectified strongly o utwardly in high [Rb+](o) due to a reduced conductance for inward curr ent. The permeability of Rb+ based on reversal potential, V-rev, was 0 .45 that of K+, whereas the Rb+ conductance was much lower, 0.034 that of K+, measured at V-rev-80 mV. The steady state voltage-dependence o f IR gating was determined in Rb+-containing solutions by applying var iable prepulses, followed by a test pulse to a potential at which outw ard current deactivation was observed. As [Rb+](o) was increased, the half-activation potential, V-1/2, changed less than V-rev. In high [K](o) solutions V-1/2 was V-rev-6 mV, while in high [Rb+](o) V-1/2 was V-rev + 7 mV. This behavior contrasts with the classical parallel shif t of V-1/2 with V-rev in K+ solutions. Steady state IR gating was less steeply voltage-dependent in high [Rb+](o) than in K+ solutions, with Boltzmann slope factors of 6.4 and 4.4 mV, respectively. Rb+ decrease d (slowed) both activation and deactivation rate constants defined at V-1/2, and decreased the steepness of the voltage dependence of the ac tivation rate constant by 42%. Deactivation of IR channels in outside- out patches was also slowed by Rb+. In summary, Rb+ can replace K+ in setting the voltage-dependence of IR gating, but in doing so alters th e kinetics.