Extracellular ATP regulates exocytosis by inhibiting multiple Ca2+ channeltypes in bovine chromaffin cells

Feedback modulation of voltage-dependent Ca2+ channels by ATP is a well doc umented phenomenon in bovine chromaffin cells. However, its influence in th e control of hormone release is at present poorly understood. By using comb ined patch-clamp and fura-2 fluorescence measurements we provide evidence t hat the three Ca2+ channel types (L, N and P/Q) expressed in bovine chromaf fin cells are inhibited by ATP (30 mu M), and that their involvement in the secretory response, as assayed by capacitance measurements, is roughly pro portional to their contribution to the whole-cell Ca2+ current (I-Ca) both in the absence and presence of ATP. ATP did not modify the capacitance incr ease observed in cells dialyzed with Ca2+-EGTA buffers (1.5 mu M free Ca2+) , thus excluding a direct effect of ATP on the secretory machinery. Voltage predepolarizations or long chemical (2 s, 70 mM KCI) depolarizations atten uate the effect of ATP on exocytosis by partially relieving the inhibition of I-Ca. Likewise, a strong stimulation that depletes the readily releasabl e pool of vesicles prevents an inhibitory effect of ATP on the secretory re sponse. While these results lend support to the hypothesis of autocrine mod ulation of exocytosis by endogenously released ATP acting on P2y-purinocept ors to inhibit I-Ca, feedback regulation of the rate of release will be a c omplex function of the occupancy of those receptors and of the electrical a nd secretory activity of the cell.