Control of IP(3)-mediated Ca2+ puffs in Xenopus laevis oocytes by the Ca2+-binding protein parvalbumin

1. Elementary events of Ca2+ release (Ca2+ 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 microM) to induce Ca2+ release under resting conditions. 2. Here, we investigated Ca2+ puff activity in non-stimulated Xenopus oocytes using confocal microscopy. The fluorescent Ca2+ dye indicators Calcium Green 1 and Oregon Green 488 BAPTA-2 were injected into oocytes to monitor basal Ca2+ activity. 3. In this preparation, injection or overexpression of parvalbumin, an EF-hand Ca(2+)-binding protein (CaBP), induced Ca2+ 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. 4. Ca2+ puff activity was also evoked by injection of low concentrations of the Ca2+ chelator EGTA, but not by calbindin D(28k), another member of the EF-hand CaBP superfamily. 5. BAPTA and the Ca2+ indicator dye Oregon Green 488 BAPTA-1 evoked Ca2+ 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 Ca2+ puff activity. 6. Together, the data indicate that some IP(3)Rs spontaneously release Ca2+ under resting concentrations of IP(3). These elementary Ca2+ events appear to be below the level of detection of current imaging techniques. We suggest that parvalbumin evokes Ca2+ puffs by coordinating the activity of elementary IP(3)R channel openings. 7. We conclude that Ca2+ 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).