The function of (Na(+)-K+)ATPase in the beta 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 Km of 0.4 mM. It was also dependent on Mg2+ (Km range 70-80 microM). 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 beta cell.
Hoenig
M
M
Department of Physiology and Pharmacology, College of Veterinary Medicine, University of Georgia, Athens 30602.
D000818:Animals; D005947:Glucose / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D007340:Insulinoma / Q000201:enzymology*; D007515:Islets of Langerhans / Q000201:enzymology*; D008297:Male; D010190:Pancreatic Neoplasms / Q000201:enzymology*; D051381:Rats; D000254:Sodium-Potassium-Exchanging ATPase / Q000502:physiology*
D007328:Insulin; D000254:Sodium-Potassium-Exchanging ATPase; D005947:Glucose