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 Ca 2+ entry through voltage-gated I-CaL channels can stimulate adjacent K-Ca c hannels by a localized interaction in enzymatically isolated rabbit coronar y 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 fro m bulk cytosolic Ca2+ pool (measured with indo 1) and is dependent on L-typ e Ca2+ channel activity. Transient activation of unitary K-Ca channels in c ell-attached patches could be detected during long step depolarizations to +40 mV (holding potential, -40 mV; 219 pS in near-symmetrical K+). This loc al interaction between the channels required the presence of Ca2+ in the pi pette solution, was enhanced by the I-CaL agonist Bay K 8644, and persisted after impairment of the sarcoplasmic reticulum by incubation with 10 mu mo l/L ryanodine and 30 mu mol/L cyclopiazonic acid for at least 60 minutes. F urthermore, we provide the first direct evidence of simultaneous openings o f single K-Ca (67 pS) and I-CaL (3.9 pS) channels in near-physiological con ditions, near resting membrane potential. Our data-imply a novel sensitive mechanism for: regulating resting membrane potential and tone in vascular s mooth muscle.