THE ENTRY AND CLEARANCE OF CA2-CELLS FROM THE BULLFROGS SACCULUS( AT INDIVIDUAL PRESYNAPTIC ACTIVE ZONES OF HAIR)

Neurotransmitter is released when Ca2+ triggers the fusion of synaptic vesicles with the plasmalemma. To study factors that regulate Ca2+ co ncentration at the presynaptic active zones of hair cells, we used las er-scanning confocal microscopy with the fluorescent Ca2+ indicator fl uo-3. The experimental results were compared with the predictions of a model of presynaptic Ca2+ concentration in which Ca2+ enters a cell t hrough a point source, diffuses from the entry site, and binds to fixe d or mobile Ca2+ buffers. The observed time course and magnitude of fl uorescence changes under a variety of conditions were well fit when th e model included mobile molecules as the only Ca2+ buffer. The results confirm the localized entry of Ca2+ underlying neurotransmitter relea se and suggest that Ca2+ is cleared from an active zone almost exclusi vely by mobile buffer.