THE REGULATION OF CAPACITATIVE CALCIUM-ENTRY BY CALCIUM AND PROTEIN-KINASE-C IN XENOPUS OOCYTES

Agonists linked to the phosphoinositide signaling pathway evoke capaci tative calcium entry in Xenopus oocytes. This entry pathway can also b e activated by injection of inositol 2,4,5-trisphosphate, application of thapsigargin, or incubation in calcium-free medium. A variety of pr otocols revealed highly nonlinear behavior of the calcium entry in tha psigargin-treated oocytes suggestive of positive and negative feedback by calcium at the level of its own entry. These feedback mechanisms m ay account for the highly damped calcium oscillations we observed in t hapsigargin-treated oocytes. Low level activation of protein kinase C potentiated calcium influx in thapsigargin-treated oocytes, apparently by blocking the inactivation of the calcium influx. Higher levels of protein kinase C activity inhibited capacitative calcium entry.