ATP-sensitive K⁺ channels in rat ventricular myocytes are blocked and inactivated by internal divalent cations

Abstract

K⁺ currents were recorded from ATP-sensitive channels in inside-out patches from isolated rat ventricular myocytes. In the absence of internal divalent cations the current voltage relationship could be described by constant-field assumptions with a permeability of 1.25×10⁻¹³ cm²/s; outward currents saturated under a high driving force for K⁺ movement. Internal 0.1–5.0 mM Mg²⁺, 0.1 μM Ca²⁺ and 10 mM Na⁺ each depressed the flux of K⁺ ions moving outwards through open channels. Internal 0.1–5.0 mM Mg²⁺, 0.1–1.0 μM Ca²⁺ and 1–10 μM Ba²⁺ and Sr²⁺ blocked K⁺ channel activity in a dose-and voltage-dependent manner. Run-down channels could be reactivated by Mg-ATP, but not by AMP-PNP, ATPγS or Mg-free ATP which suggested that phosphorylation of the channels was involved in their activity. Ca²⁺ (>=1 μM) and Sr²⁺ (1 mM) markedly inactivated K⁺ ATP channels, millimolar Ba²⁺ or Mg²⁺ were less effective. This suggested that the run down of the channels was a Ca²⁺-dependent dephosphorylation of the K⁺ channel protein.