Slow and fast oscillations of cytoplasmic Ca2+ in pancreatic islets correspond to pulsatile insulin release.

Cytoplasmic Ca2+ concentration ([Ca2+]i) and insulin secretion were monitored in single ob/ob mouse pancreatic islets stimulated by glucose. After culture in 5.5 mM of the sugar, islets responded to 11 mM glucose with pulsatile insulin secretion synchronized with oscillations of [Ca2+]i (0.3-0.5/min). Most islets also showed superimposed regular rapid [Ca2+]i oscillations and insulin transients of similar frequency. Whereas the amplitude of the slow insulin pulses increased in 20 mM glucose, the [Ca2+]i oscillations were replaced by a sustained increase. After culture in the absence of sugar, there was little rise of [Ca2+]i during exposure to 11 mM glucose and only a slight secretory response, which, however, was pulsatile. The slow secretory pulses in the presence of 11 mM glucose were augmented after culture in 11 or 20 mM glucose despite a sustained elevation of [Ca2+]i. Although pulsatile insulin release was not always associated with [Ca2+]i oscillations, the data indicate that the slow and fast [Ca2+]i oscillations do correspond to pulsatile insulin secretion.