CYTOPLASMIC CA2-DEPENDENT MEMBRANE CURRENTS IN DISPERSED BOVINE CILIARY MUSCLE-CELLS( MOBILIZATION AND CA2+)

Purpose. The dependence of plasmalemma Ca2+ influx and Ca2+ release fr om intracellular stores on Ca2+ activated K+ channels of bovine ciliar y muscle (CM) cells were examined. Methods. The nystatin-perforated pa tch clamp technique for the measurement of membrane currents and a mic roscope based fura-2 fluorescence imaging of [Ca2+](i) were applied to CM cells freshly dissociated with collagenase and identified with smo oth muscle-specific alpha-isoactin. Results. At holding voltages (V-H) of >-60 mV, CM cells showed spontaneous transient outward currents (S TOCs) and caffeine (> 10(-4) M) induced large transient outward curren ts (I-CAF). Both STOCs and I-CAF were abolished by tetraethylammonium chloride (10(-3) M) and charybdotoxin (10(-7) M), but not by apamin (1 0(-6) M), suggesting that both currents are mediated by Ca2+-activated K+ channels similar to those with medium (MK) or large (BK-type) cond uctance. Both STOCs and I-CAF were gradually abolished in the nominall y Ca2+-free and Co2+-containing solutions but were resistant to L-type Ca2+ channel blockers, including nicardipine, verapamil and diltiazem and a N-type channel blocker, omega-conotoxin. The [Ca2+](i)-elevatio n during high K+ (100 mM)-depolarization was prevented by Ca2+-free an d Co2+-containing solutions but not by nicardipine. Conclusions. These results suggest that CM cells possess MK or BK type-like Ca2+-activat ed K+ channels and that L-type Ca2+ channels play minor roles for the maintenance of Ca2+-dependent responses in contrast to other types of smooth muscle cells.