Local Ca2+ entry through L-type Ca2+ channels activates Ca2+-dependent K+ channels in rabbit coronary myocytes

Large-conductance Ca2+-dependent K+ channels (K-Ca), which are abundant on the sarcolemma of vascular myocytes, provide negative feedback via membrane hyperpolarization that limits Ca2+ entry through L-type Ca2+' channels (I-CaL). We hypothesize that local accumulation of subsarcolemmal Ca2+ during I-CaL openings amplifies this feedback. Our goal was to demonstrate that Ca2+ 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 Ca2+ pool dissociated from bulk cytosolic Ca2+ pool (measured with indo 1) and is dependent on L-type Ca2+ 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 Ca2+ 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 mu mol/L ryanodine and 30 mu mol/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.