CYTOSOLIC CA2-COMPETENT VESICLES IN CHROMAFFIN CELLS( ACTS BY 2 SEPARATE PATHWAYS TO MODULATE THE SUPPLY OF RELEASE)

Recovery from depletion of the readily releasable pool of vesicles (RR P) in adrenal chromaffin cells was studied at differing basal [Ca2+](i ) or following protein kinase C (PKC) activation by phorbol esters. Fo llowing depletion, the pool size was estimated at varied times from ce ll capacitance jumps in response to paired depolarizations. The experi mentally observed RRP recovery time course and steady-state size could be predicted from the measured [Ca2+](i) signal assuming a Michaelis- Menten-type regulation of the vesicle supply by Ca2+. An elevated recr uitment activity was observed at increased [Ca2+](i) even when protein kinase C was blocked, but maximum effects could be obtained only afte r stimulation of PKC by phorbol esters or by prolonged elevations in [ Ca2+](i). We suggest that, in chromaffin cells, elevated cytosolic Ca2 + modulates exocytotic plasticity via PKC-dependent and -independent p athways.