Epithelial calcium action potentials in Obelia geniculata trigger brief light flashes from specialized cells by direct activation of cytoplasmic calcium-activated photoprotein obelin. During a series of action potentials, sequential flashes undergo characteristic facilitation and decrement with no change in associated spike waveform. Analysis of the subcellular light distribution shows that facilitation results from two processes: recruitment of calcium entry sites and increased light from previously responding localized sites. We propose a model that accounts for the localized flash facilitation and decrement observed in vivo and is based upon the kinetics of calcium binding and emission of obelin. In this model, obelin emits light only when three calcium ions are bound. Changes in flash intensity during successive action potentials result from calcium bound persistently to to unexpended obelin, effectively lowering the number of calcium ions required for subsequent activation. Accordingly, facilitation or decrement results from the time-dependent availability of singly and doubly bound obelin.
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