Modulation of Endoplasmic Reticulum Ca2+ Store Filling by Cyclic ADP-ribose Promotes Inositol Trisphosphate (IP3)-evoked Ca2+ Signals

In addition to its well established function in activating Ca²⁺ release from the endoplasmic reticulum (ER) through ryanodine receptors (RyR), the second messenger cyclic ADP-ribose (cADPR) also accelerates the activity of SERCA pumps, which sequester Ca²⁺ into the ER. Here, we demonstrate a potential physiological role for cADPR in modulating cellular Ca²⁺ signals via changes in ER Ca²⁺ store content, by imaging Ca²⁺ liberation through inositol trisphosphate receptors (IP₃R) in Xenopus oocytes, which lack RyR. Oocytes were injected with the non-metabolizable analog 3-deaza-cADPR, and cytosolic [Ca²⁺] was transiently elevated by applying voltage-clamp pulses to induce Ca²⁺ influx through expressed plasmalemmal nicotinic channels. We observed a subsequent potentiation of global Ca²⁺ signals evoked by strong photorelease of IP₃, and increased numbers of local Ca²⁺ puffs evoked by weaker photorelease. These effects were not evident with cADPR alone or following cytosolic Ca²⁺ elevation alone, indicating that they did not arise through direct actions of cADPR or Ca²⁺ on the IP₃R, but likely resulted from enhanced ER store filling. Moreover, the appearance of a new population of puffs with longer latencies, prolonged durations, and attenuated amplitudes suggests that luminal ER Ca²⁺ may modulate IP₃R function, in addition to simply determining the size of the available store and the electrochemical driving force for release.