Phasic characteristic of elementary Ca(2+) release sites underlies quantal responses to IP(3).

Ca(2+) liberation by inositol 1,4,5-trisphosphate (IP(3)) is 'quantal', in that low [IP(3)] causes only partial Ca(2+) release, but further increasing [IP(3)] evokes more release. This characteristic allows cells to generate graded Ca(2+) signals, but is unexpected, given the regenerative nature of Ca(2+)-induced Ca(2+) release through IP(3) receptors. Two models have been proposed to resolve this paradox: (i) all-or-none Ca(2+) release from heterogeneous stores that empty at varying [IP(3)]; and (ii) phasic liberation from homogeneously sensitive stores. To discriminate between these hypotheses, we imaged subcellular Ca(2+) puffs evoked by IP(3) in Xenopus oocytes where release sites were functionally uncoupled using EGTA. Puffs were little changed by 300 microM intracellular EGTA, but sites operated autonomously and did not propagate waves. Photoreleased IP(3) generated flurries of puffs-different to the prolonged Ca(2+) elevation following waves in control cells-and individual sites responded repeatedly to successive increments of [IP(3)]. These data support the second hypothesis while refuting the first, and suggest that local Ca(2+) signals exhibit rapid adaptation, different to the slower inhibition following global Ca(2+) waves.