Noradrenaline receptor-mediated potentiation of caffeine-induced Ca2+-activated K+ currents in bovine ciliary muscle cells

Purpose. Adrenoceptor-mediated modulation of a caffeine (CAF)-induced [Ca2+](i) elevation and resulting Ca2+-activated K+ current (I-CAF) in bovine ciliary muscle (CM) cells were investigated. Methods. The nystatin-perforated patch clamp technique for the measurement of membrane currents and a microscope based fura-2 fluorescence imaging of [Ca2+](i) were applied to CM cells freshly dissociated with collagenase and identified with smooth muscle-specific alpha-isoactin. Results. Under voltage-clamped conditions, noradrenaline (NA) potentiated I-CAF in a NA concentration-dependent manner without producing current responses to NA when NA was applied alone. NA-induced potentiation of I-CAF occurred within 20 sec after the application of NA, while the NA-potentiated I-CAF gradually recovered to the control level within 30 min after removal of NA. Despite the little current response to NA applied alone, NA elicited a [Ca2+](i) elevation in a manner similar to that induced by CAF although the NA-induced [Ca2+](i) elevation was smaller than the CAF-induced [Ca2+](i) elevation. In contrast to the significant potentiation of I-CAF with NA, NA produced little potentiation of the CAF-induced [Ca2+](i) elevation. The NA-induced potentiation of I-CAF was antagonized by an a 1 adrenoceptor antagonist, prazosin. Neither clonidine nor isoproterenol had an effect on I-CAF, suggesting that alpha(2) and beta adrenoceptor are not involved in the response to NA. Conclusions. These results suggest that NA potentiates I-CAF via a 1 adrenoceptor activation and that the NA-induced potentiation occurs at Ca2+-activated K+ channels but not CAF-induced Ca2+ releasing sites.