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

Large-conductance Ca-dependent K channels (KCa), which are abundant on the sarcolemma of vascular myocytes, provide negative feedback via membrane hyperpolarization that limits Ca entry through L-type Ca channels (ICaL). We hypothesize that local accumulation of subsarcolemmal Ca 21 during ICaL openings amplifies this feedback. Our goal was to demonstrate that Ca entry through voltage-gated ICaL channels can stimulate adjacent KCa 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 KCa current that is activated by a subsarcolemmal Ca pool dissociated from bulk cytosolic Ca pool (measured with indo 1) and is dependent on L-type Ca channel activity. Transient activation of unitary KCa channels in cell-attached patches could be detected during long step depolarizations to 140 mV (holding potential, 240 mV; 219 pS in near-symmetrical K). This local interaction between the channels required the presence of Ca in the pipette solution, was enhanced by the ICaL agonist Bay K 8644, and persisted after impairment of the sarcoplasmic reticulum by incubation with 10 mmol/L ryanodine and 30 mmol/L cyclopiazonic acid for at least 60 minutes. Furthermore, we provide the first direct evidence of simultaneous openings of single KCa (67 pS) and ICaL (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. (Circ Res. 1999;84:1032-1042.)