Effects of depolarizing agents on the sodium content of rat pancreatic islets

Rat pancreatic islets were used for studying the effects of depolarization on their sodium content. The islet sodium was markedly affected by small variations of extracellular K+. As with increased K+, the presence of low concentrations of glucose (5 mM) and arginine (2 mM) decreased the sodium content. The latter substances did not lower the sodium concentration below the value obtained by depolarization with excessive K+, nor was it possible to obtain a further decrease when 10 mM arginine was combined with 5 mM glucose. The sodium content was also reduced in the presence of 10 mM L-leucine, 10 mM 2-ketoisocaproate and 0.1 mM Ba2+. Tolbutamide differed from the other depolarizing agents in that it increased the sodium concentration, an effect manifested also in the presence of excessive K+. The observation that depolarizing agents other than sulfonylureas do not increase but actually reduce sodium implies that islet cells are exceptional among electrically excitable cells. The observed reduction of sodium may reflect activation of a voltage-sensitive carrier mechanism for outward transport of Na+.