Lactation-induced changes in calcium handling by rat pancreatic islets.

Glucose-stimulated insulin release occurred at a lower rate in pancreatic islets removed from lactating than non-lactating rats. This defect was corrected in the presence of either gliclazide or a calcium-agonist. With both agents present, insulin release from islets of lactating rats was greater. When islets were prelabelled with 45calcium, gliclazide stimulated to the same extent 45Ca outflow in islets from lactating and non-lactating rats, respectively. However, when the islets were prelabelled with 45Ca in the presence of gliclazide, the administration of Ba2+ increased effluent radioactivity more markedly in islets from non-lactating than lactating rats. This suggests that lactation favours, in gliclazide-stimulated islets, the sequestration of 45Ca in non-labile subcellular pools. When D-glucose was used instead of Ba2+, the greater lability of 45Ca in islets from non-lactating animals was apparently masked by a lesser efficiency in the metabolism and cationic effects of D-glucose in the non-lactating rats. The calcium-ionophoretic effect of islet extracts was higher in lactating than non-lactating rats. These results support the view that a depletion of endogenous calcium stores accounts, in part at least, for the decreased insulin secretory responsiveness to D-glucose in lactation, since the latter apparently favours the function of those systems involved in either the entry of calcium into or its sequestration within the islet cells.