INTERACTION BETWEEN CA2-OOCYTE( RELEASE FROM INOSITOL TRISPHOSPHATE SENSITIVE STORES AND CA2+ ENTRY THROUGH NEURONAL CA2+ CHANNELS EXPRESSED IN XENOPUS)

Rat cerebellar RNA injected into Xenopus oocytes leads to the expressi on of putative P-type voltage-dependent Ca2+ channels (VDCCs). The mon itoring of intracellular Ca2+ variations by recording the Ca2+ depende nt chloride current in voltage clamped oocytes indicates that activati on 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+ i nflux through VDCCs suggest that the Ca2+ release from intracellular I nsP(3) sensitive stores can by modulated in a differential manner. Nam ely, discrete elevations of cytoslic Ca2+ switch on the Ca2+ release w hereas higher Ca2+ concentrations dampen the release. These results su ggest a functional coupling between P-type VDCCs and InsP(3) receptors .