To distinguish between the significance of membrane thiols and intracellular thiols of rat pancreatic islets in calcium uptake and insulin secretion, lanthanum-nondisplaceable uptake of ⁴⁵Ca and secretion of insulin were studied in the presence of the membrane-penetrating thiol oxidant diazene dicarboxylic acid bis-(N′-methylpiperazide) (DIP), on the one hand, and the corresponding nonpenetrating thiol oxidant salt of DIP (bis-N′-methyl iodide, i.e. DIP + 2), on the other hand. In contrast to DIP + 2, DIP inhibited glucose-induced (20 mM) uptake of ⁴⁵Ca and insulin secretion. Furthermore, DIP + 2 stimulated ⁴⁵Ca uptake and insulin secretion in the presence of low glucose (3 mM). DIP inhibited these effects of DIP + 2.⁴⁵Ca uptake and insulin release evoked by the ionophore A-23187 were not inhibited by DIP, however.

The present data collected from isolated pancreatic islets have several implications. Firstly, the reduced state of the intracellular glutathione to GSSG ratio, which causes the superficial thiol to remain in the reduced state, constitutes a prerequisite for glucose-stimulated ⁴⁵Ca uptake and insulin release. Secondly, direct oxidation of superficial thiols leads to ⁴⁵Ca uptake and subsequent secretion of insulin. Finally, the redox state of the intracellular glutathione system does not appear to be of importance in the stimulus-secretion coupling steps following the uptake of calcium.