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

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Abstract
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. Particular high efflux rates were reached when the Ca2+-deficient medium contained ethylene glycol-bis(.beta.-aminoethyl ether)N,N,N'',N''-tetraacetic acid, but there was no effect of the chelator on the total amount of radioactivity mobilized from the islets. 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+. 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 on response to a 2nd exposure to a Ca2+-deficient medium even subsequent to redistribution of intracellular Ca induced by temporary lowering of the temperature. The islet 45Ca released by the removal of extracellular Ca2+ apparently originates from a distinct plasma membrane pool which is exchanged slowly compared to most of the Ca at the .beta. cell periphery.