Bioluminescence in the hydrozoan coelenterate Obelia results from calcium activation of a photoprotein contained in light-emitting cells (photocytes) scattered in the animal's endoderm. The influx of calcium into non-luminescent endodermal cells through conventional voltage-dependent calcium channels is required for the excitation-luminescence coupling. Our results suggest that the subsequent diffusion of this calcium, via gap junctions, into the neighboring photocytes triggers a localized luminescence response. Following intense stimulation, the local rise in calcium elicits a secondary wave of luminescence that is supported by a voltage-independent calcium permeability mechanism in the photocyte plasma membrane. These two mechanisms for elevating internal calcium in light-emitting cells can account for the spatial and temporal features of intra-cellular luminescence in Obelia.
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