Controlof 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 discrete clusters of inositol 1,4,5 trisphosphate receptors (IP3Rs) at low concentrations of IP3. Ca2+ puffs have rarely been observed unless elicited by either hormone treatment or introduction of IP3 into the cell. However, cells appear to have sufficient concentrations of IP3 (0.1‐3.0 μM) 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 Ca2+‐binding protein (CaBP), induced Ca2+ puffs in resting Xenopus oocytes. This activity was inhibited by heparin, an IP3R channel blocker, and by 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 D28k, 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 Ca2+ buffer must be mobile in order to increase Ca2+ puff activity. 6 Together, the data indicate that some IP3Rs spontaneously release Ca2+ under resting concentrations of IP3. 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 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 resting concentrations of IP3.