Interaction between Ca2+ release from inositol trisphosphate sensitive stores and Ca2+ entry through neuronal Ca2+ channels expressed in Xenopus oocyte.

Rat cerebellar RNA injected into Xenopus oocytes leads to the expression of putative P-type voltage-dependent Ca2+ channels (VDCCs). The monitoring of intracellular Ca2+ variations by recording the Ca2+ dependent chloride current in voltage clamped oocytes indicates that activation of these Ca2+ channels by depolarization gives rise to two distinct components of cytosolic Ca2+ elevation. If the early component (T-1) can be directly attributed to the Ca2+ entry through VDCCs, the second delayed one (T-2) is related to a Ca2+ release from InsP(3) sensitive stores activated following Ca2+ entry. Modifications of cytosolic Ca2+ by direct injection of Ca2+ into oocytes or by increasing the Ca2+ influx through VDCCs suggest that the Ca2+ release from intracellular InsP(3) sensitive stores can by modulated in a differential manner. Namely, discrete elevations of cytoslic Ca2+ switch on the Ca2+ release whereas higher Ca2+ concentrations dampen the release. These results suggest a functional coupling between P-type VDCCs and InsP(3) receptors.