CAPACITATIVE CALCIUM-ENTRY IS COLOCALIZED WITH CALCIUM-RELEASE IN XENOPUS OOCYTES - EVIDENCE AGAINST A HIGHLY DIFFUSIBLE CALCIUM INFLUX FACTOR

Depletion of intracellular calcium stores activates the plasma membran e capacitative calcium entry pathway in many cell types. The nature of the signal that couples the depletion of the intracellular calcium st ores to the activation of the plasma membrane calcium influx pathway i s as yet unknown. It has recently been suggested that a highly diffusi ble calcium influx factor is involved in the activation of capacitativ e calcium entry, and that its action is potentiated by the protein pho sphatase inhibitor okadaic acid. Depletion of intracellular calcium st ores in a localised region of a Xenopus oocyte was found to evoke capa citative calcium entry exclusively colocalised across the stimulated a rea of the plasma membrane, arguing against the involvement of a highl y diffusible calcium influx factor. Equally, no evidence could be foun d for the presence of a soluble calcium influx factor in the bulk cyto sol of Xenopus oocytes. The potentiation of capacitative calcium entry by okadaic acid resembled that mediated by the activation of protein kinase C, thus suggesting that okadaic acid activity may not necessari ly be related to the action of a putative calcium influx factor.