Properties of the delayed rectifier potassium current in porcine sino‐atrial node cells

Abstract

1 Whole-cell currents were recorded in single, spontaneously active cells dissociated from porcine sino-atrial node, and the conductance and gating properties of the delayed rectifier K⁺ current (I_K) were investigated. 2 The isolated cells exhibited spontaneous action potentials at a rate of 80.5 ± 5.4 min⁻¹ (mean ± s.e.m., n= 11). Under Ca²⁺ current block, depolarization from -40 mV to various potentials activated a time-dependent outward current (I_K). The activation curve of I_K showed a half-activation potential (V_½) of 20.5 ± 2.1 mV and a slope factor (S) of 16.4 ± 1.2 mV (n= 8). 3 As the duration of the depolarizing pulse to either +10 or +60 mV was prolonged, the amplitude of the tail current increased in proportion to that of the activated outward current during depolarization. 4 E4031 (2-5 μM), a selective blocker for the rapidly activating component of I_K (I_K,r), hardly affected I_K, but chromanol 293B, a selective blocker for the slowly activating component (I_K,s), inhibited I_K with an IC₅₀ of 8.79 μM. 5 The reversal potential of I_K was -75.2 ± 2.3 mV with 5.4 mM external and 150 mM internal K⁺. The time courses of activation and deactivation of I_K were fitted by the sum of two exponential functions at various potentials. The relationship between the time constants and membrane potential showed a bell-shaped curve with a peak at around -10 mV for both fast and slow components. 6 The results indicate that in porcine sino-atrial node cells I_K is largely derived from I_K,s and that I_K,s plays a functional role in the slow diastolic depolarization. I_K,s may, in part, account for the relatively slower heart rate of pigs than that of rabbit in which I_K,r is a functionally dominant component of I_K.