Control of IP3-mediated Ca2+ puffs in Xenopus laevis oocytes by the Ca2+-binding protein parvalbumin

Elementary events of Ca 2+ release (Ca 2+ puffs) can be elicited from discrete clusters of inositol 1,4,5 trisphosphate receptors (IP 3 Rs) at low concentrations of IP 3 . Ca 2+ puffs have rarely been observed unless elicited by either hormone treatment or introduction of IP 3 into the cell. However, cells appear to have sufficient concentrations of IP 3 (0.1-3.0 μM) to induce Ca 2+ release under resting conditions. Here, we investigated Ca 2+ puff activity in non-stimulated Xenopus oocytes using confocal microscopy. The fluorescent Ca 2+ dye indicators Calcium Green 1 and Oregon Green 488 BAPTA-2 were injected into oocytes to monitor basal Ca 2+ activity. In this preparation, injection or overexpression of parvalbumin, an EF-hand Ca 2+ -binding protein (CaBP), induced Ca 2+ puffs in resting Xenopus oocytes. This activity was inhibited by heparin, an IP 3 R channel blocker, and by mutation of the Ca 2+ -binding sites in parvalbumin. Ca 2+ puff activity was also evoked by injection of low concentrations of the Ca 2+ chelator EGTA, but not by calbindin D 28k , another member of the EF-hand CaBP superfamily. BAPTA and the Ca 2+ indicator dye Oregon Green 488 BAPTA-1 evoked Ca 2+ puff activity, while the dextran conjugate of Oregon Green 488 BAPTA-1 did not. These data indicate that a Ca 2+ buffer must be mobile in order to increase Ca 2+ puff activity. Together, the data indicate that some IP 3 Rs spontaneously release Ca 2+ under resting concentrations of IP 3 . These elementary Ca 2+ events appear to be below the level of detection of current imaging techniques. We suggest that parvalbumin evokes Ca 2+ puffs by coordinating the activity of elementary IP 3 R channel openings. We conclude that Ca 2+ release can be evoked not only by hormone-induced increases in IP 3 , but also by expression of mobile cytosolic CaBPs under resting concentrations of IP 3 .