Enhanced ER Ca2+ store filling by overexpression of SERCA2b promotes IP3-evoked puffs

Liberation of Ca2+ from the endoplasmic reticulum (ER) through inositol trisphosphate receptors (IP3R) is modulated by the ER Ca2+ content, and overexpression of SERCA2b to accelerate Ca2+ sequestration into the ER has been shown to potentiate the frequency and amplitude of IP3-evoked Ca2+ waves in Xenopus oocytes. Here, we examined the effects of SERCA overexpression on the elementary IP3-evoked puffs to elucidate whether ER [Ca2+] may modulate IP3R function via luminal regulatory sites in addition to simply determining the size of the available store and electrochemical driving force for Ca2+ release. SERCA2b and Ca2+ permeable nicotinic plasmalemmal channels were expressed in oocytes, and hyperpolarizing pulses were delivered to induce Ca2+ influx and thereby load ER stores. Puffs evoked by photoreleased IP3 were significantly potentiated in terms of numbers of responding sites, frequency and amplitude following transient Ca2+ influx in SERCA-overexpressing cells, whereas little change was evident with SERCA overexpression alone or following Ca2+ influx in control cells not overexpressing SERCA. Intriguingly, we observed the appearance of a new population of puffs that arose after long latencies and had prolonged durations supporting the notion of luminal regulation of IP3R gating kinetics.