Opposing effects of glucose and tolbutamide on the sodium content of rat pancreatic islets

Abstract

Abstract. Integrating flame photometry was employed for measuring sodium in rat pancreatic islets incubated in media buffered with HEPES or bicarbonate. The sodium content decreased by nearly 40% when the islets were exposed to 5 mmol/l glucose, no further reduction being seen with additional rise of the concentration to 20 mmol/l. Whereas the depressing effect of glucose was mimicked by 100 μmol/l quinine, increased sodium contents were noted after inhibition of the Na/K pump (removal of extracellular K+ or addition of 1 mmol/l ouabain) or exposure of the islets to 1 mmol/l tolbutamide. Although promoting sodium accumulation in the islet cells, tolbutamide counteracted the increase in sodium obtained on withdrawal of K+ from the incubation medium. It is suggested that tolbutamide in addition to its major effect in promoting the entry of Ca2+ also facilitates insulin release by suppressing the outward transport of this ion.