The function of (Na⁺K⁺)ATPase in the β cell: Characterization of the enzyme in a glucose-responsive insulinoma

(Na⁺K⁺)ATPase is necessary for the maintenance of the membrane potential. The activity of this enzyme was studied in purified plasma membranes from a glucose-responsive rat insulinoma. Ouabain-sensitive (Na⁺K⁺)ATPase activity showed expected ATP dependency with a K_m of 0.4 mm. It was also dependent on Mg²⁺ (K_m range 70-80 μM). In the presence of Mg and ATP, half-maximal activity was obtained at a Na concentration of 30 mm and the enzyme activity increased sigmoidally with a Hill coefficient of 1.5. No direct effect on enzyme activity was observed with the insulin secretagogues glucose, fructose, glyceraldehyde, and ketoisocaproate, or with dibuturyl-cAMP and the phosphodiesterase-inhibitor isobutyl methyl xanthine. It is concluded that (Na⁺K⁺)ATPase is not directly influenced by known secretagogues associated with insulin release by the β cell.