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PHYSIOLOGICAL RELEVANCE OF IMMEDIATELY RELEASABLE POOL RECOVERY, AND ITS Ca2+ MODULATION. Yanina D.Alvarez and Fernando D.Marengo. LFBM. IFIByNE-CONICET.FCEyN.U.B.A. Inside ready releasable pool, there is in chromaffin cells a small group of vesicles named immediately releasable pool (IRP), which can be release by short pulses due to the proximity between vesicles and P/Q Ca2+ channels. This work analyzes the effect of cytosolic Ca2+ on IRP refilling, and how stimulation frequency affects the maintenance of IRP exocytosis. IRP size was estimated by patch clamp cell capacitance measurements at various free Ca2+ concentrations in the patch pipette. Surprisingly, pipette Ca2+ concentrations between 0 to 300nM show no significant change in IRP size (32±1fF, n=83). We next studied the effect of Ca2+ on IRP recovery after the application of a 50 ms depolarization to deplete completely the pool. The addition of 200 nM free Ca2+ to the patch pipette increased the initial recovery velocity in 37% respect to 0 calcium added (at 2 s, 0.35±0.05 % vs 0.57±0.06%, p<0.02). These results suggest that while IRP is saturated in basal conditions, the refilling after depletion depends on [Ca2+]i. By IRP definition, a single action potential (AP) should release just some vesicles from IRP. An AP like depolarization (APld) induced a capacitance jump of 8.9±0.7fF (25 % of the IRP), which was significantly decreased to 3±1fF (p<0.02) by a P/Q calcium channel blocker, as expected. Because the physiological AP firing rate varies from 0.5 to 10 Hz, we wonder if the refilling velocity of IRP was enough to sustain secretion at this frequency range. By application of paired APld separated by variable time intervals, we found that for intervals between 200ms (5Hz) and 10s (0.1Hz) exocytosis recovers with a exponential time constant of 0.8±0.1s, but a facilitation process was found for time intervals < 200 ms.