Noradrenaline receptor-mediated potentiation of caffeine-induced Ca( 2+)-activated K(+) currents in bovine ciliary muscle cells.

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 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 [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 alpha(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 alpha(1) adrenoceptor activation and that the NA-induced potentiation occurs at Ca(2+)-activated K(+) channels but not CAF-induced Ca(2+) releasing sites.