Local Ca 2+ Entry Through L-Type Ca 2+ Channels Activates Ca 2+ -Dependent K + Channels in Rabbit Coronary Myocytes

Abstract

Abstract —Large-conductance Ca 2+ -dependent K + channels (K Ca ), which are abundant on the sarcolemma of vascular myocytes, provide negative feedback via membrane hyperpolarization that limits Ca 2+ entry through L-type Ca 2+ channels ( I CaL ). We hypothesize that local accumulation of subsarcolemmal Ca 2+ during I CaL openings amplifies this feedback. Our goal was to demonstrate that Ca 2+ entry through voltage-gated I CaL channels can stimulate adjacent K Ca channels by a localized interaction in enzymatically isolated rabbit coronary arterial myocytes voltage clamped in whole-cell or in cell-attached patch clamp mode. During slow-voltage-ramp protocols, we identified an outward K Ca current that is activated by a subsarcolemmal Ca 2+ pool dissociated from bulk cytosolic Ca 2+ pool (measured with indo 1) and is dependent on L-type Ca 2+ channel activity. Transient activation of unitary K Ca channels in cell-attached patches could be detected during long step depolarizations to +40 mV (holding potential, −40 mV; 219 pS in near-symmetrical K + ). This local interaction between the channels required the presence of Ca 2+ in the pipette solution, was enhanced by the I CaL agonist Bay K 8644, and persisted after impairment of the sarcoplasmic reticulum by incubation with 10 μmol/L ryanodine and 30 μmol/L cyclopiazonic acid for at least 60 minutes. Furthermore, we provide the first direct evidence of simultaneous openings of single K Ca (67 pS) and I CaL (3.9 pS) channels in near-physiological conditions, near resting membrane potential. Our data imply a novel sensitive mechanism for regulating resting membrane potential and tone in vascular smooth muscle.