Ca2+-channel-dependent and -independent inhibition of exocytosis by extracellular ATP in voltage-clamped rat adrenal chromaffin cells

Membrane currents and capacitance were measured to examine the effects of extracellular ATP on exocytosis in voltage-clamped rat adrenal chromaffin cells. ATP reversibly inhibited Ca2+ current (I-Ca) and exocytosis. The dependency of exocytosis on I-Ca evoked by 1-s depolarizations was determined. However, inhibition of exocytosis was 2.6 times larger than that estimated from the reduction of I-Ca, implying the existence of a Ca2+-channel-independent pathway. This inhibition did not rely on a further reduction of the intracellular Ca2+ concentration spike. ATP reduced the rate of exocytosis induced by clamping the intracellular Ca2+ concentration. Pertussis toxin blocked the inhibitory effects of ATP on I-Ca and exocytosis. Although RB-2, a P-2Y antagonist. blocked the inhibitory effect of ATP on I-Ca, RB-2 itself produced large increase or decrease in membrane capacitance. Adenosine inhibited I-Ca via a pertussis-toxin-sensitive pathway but did not significantly inhibit exocytosis. Our data show that extracellular ATP inhibits exocytosis via inhibition of I-Ca by activation of a pertussis-toxin-sensitive G-protein linked to P-2Y receptors. Furthermore, our data strongly suggest that ATP activates another pathway, which is also G-protein dependent and accounts for the majority of the inhibitory effect of ATP on exocytosis.