Effect of adrenergic agonists on Ca2+-channel currents in single vascular smooth muscle cells

Ca²⁺-channel currents have been measured in enzymatically dispersed single smooth muscle cells of the rabbit ear artery using the whole-cell patch clamp technique. Inward currents were elicited by depolarizing test pulses from a holding potential of–50 mV. These currents were activated from–30 mV onward and reached full activation around 0 mV. -Adrenergic agonists did not affect the background current measured at the holding potential, but markedly reduced the peak amplitude of the voltageactivated Ca²⁺-channel currents. This -adrenergic inhibition also occurred in cells which were internally perfused with solutions containing either 10 M cAMP, 10M cGMP or 0.1 mM GTP, but became irreversible when the pipette solution contained a non-hydrolyzable GTP-analog. The action of -agonists on the voltage-activated Ca²⁺-channel currents was variable, and ranged from no effect at all to a 50% reduction of the current. It is concluded that -agonists do not open receptor-operated Ca²⁺-channels in these smooth muscle cells. The inhibition of the voltageactivated Ca²⁺-currents does not seem to be mediated through changes in cyclic nucleotide levels, but might be mediated through G-proteins. Its physiological relevance remains however unclear. The action of -agonists is consistent with their relaxing effect, but the reason for the nonuniform response has not been elucidated.