Evidence for a slowly exchangeable pool of calcium in the pancreatic beta cell plasma membrane.

1. Exposure to media deprived of Ca2+ resulted in prompt and transient stimulation of 45Ca efflux from beta cell-rich pancreatic islets microdissected from ob/ob-mice and to some extent also from the isolated neurohypophysis. 2. Particular high efflux rates were reached when the Ca2+-deficient medium contained EGTA, but there was no effect of the chelator on the total amount of radioactivity mobilized from the islets. 3. The removal of extracellular Ca2+ was less effective in promoting the 45Ca efflux in the absence of Na+ and no stimulatory response was seen in the presence of 1 mM-La3+. 4. The 45Ca washout was stimulated whether or not the media used for the loading or subsequent perifusion of the islets were supplemented with 20 mM-D-glucose. However, there was no response to a second exposure to a Ca2+-deficient medium even subsequent to redistribution of intracellular calcium induced by temporary lowering of the temperature. 5. It is suggested that the islet 45Ca released by the removal of extracellular Ca2+ originates from a distinct plasma membrane pool which is exchanged slowly compared to most of the calcium at the beta cell periphery.