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

Abstract

Purpose. Adrenoceptor-mediated modulation of a caffeine (CAF)-induced [Ca 2+ ] i elevation and resulting Ca 2+ -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 [Ca 2+ ] i were applied to CM cells freshly dissociated with collagenase and identified with smooth muscle-specific a-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 [Ca 2+ ] i elevation in a manner similar to that induced by CAF although the NA-induced [Ca 2+ ] i elevation was smaller than the CAF-induced [Ca 2+ ] i elevation. In contrast to the significant potentiation of I CAF with NA, NA produced little potentiation of the CAF-induced [Ca 2+ ] 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 a 2 and ß 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 Ca 2+ -activated K + channels but not CAF-induced Ca 2+ releasing sites.