VOLTAGE-DEPENDENT AND TIME-DEPENDENT BLOCK OF DELAYED RECTIFIER K+ CURRENT IN RABBIT SINOATRIAL NODE CELLS BY EXTERNAL CA2+ AND MG2+

1. The properties of the delayed rectifier K+ current (I-K) of rabbit isolated sino-atrial node cells were investigated in high (140 mM) [K](o) using the whole-cell-clamp technique. 2. Hyperpolarizing clamp pu lses from O mV induced an instantaneous current jump (I-V relation lin ear) followed by a time-dependent increase in inward current to a peak , whereas depolarizing clamp pulses induced little outward current. Th e peak I-V relation showed a strong in-ward rectification. The inwardl y rectifying current was blocked by E-4031. 3. The inward K+ current i nduced by hyperpolarizing clamp pulses from O mV relaxed after reachin g its peak. The rate of the relaxation increased as the membrane poten tial became more negative and concentrations of external Ca2+ or Mg2were increased. The steady-state current was smaller as the relaxation of the current accelerated on increasing [Ca2+](o) or [Mg2+](o). 4. D epolarizing clamp pulses from -80 mV induced an increase in inward cur rent, reaching a steady state. The amplitude of the steady-state curre nt became smaller and the rate of current increase became slower as [C a2+](o) or [Mg2+](o) was increased. 5. The effects of Ca2+ and Mg2+ ar e well explained by a time- and voltage-dependent blockade of the K+ c hannel by these ions. The fractional electrical distance of the bindin g site calculated from the voltage dependence of the blocking rate con stant is 0.69 for Ca2+ and 0.88 for Mg2+. The blocking rate constant a t O mV for Ca2+ is about 15 times faster than that for Mg2+, indicatin g stronger effects of Ca2+. 6. A re-interpretation of I-K in sino-atri al node cells is proposed: there are two independent gates (an activat ion gate which opens on hyperpolarization and an inactivation gate whi ch closes on hyperpolarization) and a binding site for Ca2+ and Mg2+ i nside the channel. Binding of these ions, which is facilitated by hype rpolarization, causes channel blockade, resulting in the observed volt age dependence of I-K in physiological concentrations of Ca2+ and Mg2.