Abstract

Ca²⁺ liberation by inositol 1,4,5-trisphosphate (IP₃) is 'quantal', in that low [IP₃] causes only partial Ca²⁺ release, but further increasing [IP₃] evokes more release. This characteristic allows cells to generate graded Ca²⁺ signals, but is unexpected, given the regenerative nature of Ca²⁺-induced Ca²⁺ release through IP₃ receptors. Two models have been proposed to resolve this paradox: (i) all-or-none Ca²⁺ release from heterogeneous stores that empty at varying [IP₃]; and (ii) phasic liberation from homogeneously sensitive stores. To discriminate between these hypotheses, we imaged subcellular Ca²⁺ puffs evoked by IP₃ in Xenopus oocytes where release sites were functionally uncoupled using EGTA. Puffs were little changed by 300 M intracellular EGTA, but sites operated autonomously and did not propagate waves. Photoreleased IP₃ generated flurries of puffs—different to the prolonged Ca²⁺ elevation following waves in control cells—and individual sites responded repeatedly to successive increments of [IP₃]. These data support the second hypothesis while refuting the first, and suggest that local Ca²⁺ signals exhibit rapid adaptation, different to the slower inhibition following global Ca²⁺ waves.