Capacitative calcium entry is colocalised with calcium release in Xenopus oocytes: evidence against a highly diffusible calcium influx factor.

Depletion of intracellular calcium stores activates the plasma membrane capacitative calcium entry pathway in many cell types. The nature of the signal that couples the depletion of the intracellular calcium stores to the activation of the plasma membrane calcium influx pathway is as yet unknown. It has recently been suggested that a highly diffusible calcium influx factor is involved in the activation of capacitative calcium entry, and that its action is potentiated by the protein phosphatase inhibitor okadaic acid. Depletion of intracellular calcium stores in a localised region of a Xenopus oocyte was found to evoke capacitative calcium entry exclusively colocalised across the stimulated area of the plasma membrane, arguing against the involvement of a highly diffusible calcium influx factor. Equally, no evidence could be found for the presence of a soluble calcium influx factor in the bulk cytosol 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 necessarily be related to the action of a putative calcium influx factor.