Control of IP3-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 disc rete clusters of inositol 1,4,5 trisphosphate receptors (IP(3)Rs) at low co ncentrations of IP3. Ca2+ puffs have rarely been observed unless elicited b y either hormone treatment or introduction of IP3 into the cell. However, c ells appear to have sufficient concentrations of IP3 (0.1-3.0 muM) to induc e Ca2+ release under resting conditions. 2. Here, we investigated Ca2+ puff activity in non-stimulated Xenopus oocyt es using confocal microscopy. The fluorescent Ca2+ dye indicators Calcium G reen 1 and Oregon Green 488 BAPTA-2 were injected into oocytes to monitor b asal Ca2+ activity. 3. In this preparation, injection or overexpression of parvalbumin, an EF-h and Ca2+-binding protein (CaBP), induced Ca2+ puffs in resting Xenopus oocy tes. This activity was inhibited by heparin, an IP3R channel blocker, and b y mutation of the Ca2+-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+ pu ff activity, while the dextran conjugate of Oregon Green 488 BAPTA-1 did no t. These data indicate that a Ca2+ buffer must be mobile in order to increa se Ca2+ puff activity. 6. Together, the data indicate that some IP(3)Rs spontaneously release Ca2 under resting concentrations of IP3. These elementary Ca2+ events appear t o be below the level of detection of current imaging techniques. We suggest that parvalbumin evokes Ca2+ puffs by coordinating the activity of element ary IP3R channel openings. 7. We conclude that Ca2+ release can be evoked not only by hormone-induced increases in IP3, but also by expression of mobile cytosolic CaBPs under re sting concentrations of IP3.