Glucose inhibits the high-affinity (Ca2+ + Mg2+)-ATPase in the plasma membrane of a glucose-responsive insulinoma.
(Ca2+ + Mg2+)-ATPase enzyme activity of a purified plasma membrane preparation from a glucose responsive rat insulinoma, was characterized as Ca2(+)-dependent dephosphorylation of [gamma-32P]ATP. A high-affinity enzyme with a Km(ATP) ranging from 20 to 30 microM and a Km(Ca2+) of 1 microM was identified. Glucose inhibited this high-affinity enzyme in a dose-dependent manner, with no significant inhibition at a concentration between 0 and 5 mM, 50% inhibition at 13.3 mM and 94.5% inhibition at 30 mM. The inhibitory effect of glucose was immediate and rapidly reversible. The effect was stereospecific for the alpha-anomer. These findings support the concept that glucose acts directly at the beta-cell plasma membrane and is involved in the maintenance of elevated intracellular free calcium concentrations associated with insulin release by directly or indirectly inhibiting energy-dependent calcium efflux. Glyceraldehyde (20 mM) increased enzyme activity 3-fold, while other metabolic fuels had no effect. This suggests that inhibition of the enzyme is not an obligatory requirement for insulin release. Calmodulin stimulated the enzyme activity in calmodulin-depleted but not in undepleted membranes. Trifluoperazine (30-100 microM) inhibited (Ca2+ + Mg2+)-ATPase in a dose-dependent manner (14-61% activity) and the activity was also inhibited by vanadate (0.1-1.0 mM) and NaCl (150 mM).
Hoenig
M
M
Department of Physiology and Pharmacology, College of Veterinary Medicine, University of Georgia, Athens 30602.
Lee
R J
RJ
Ferguson
D C
DC
D007516:Adenoma, Islet Cell / Q000201:enzymology*; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000037:antagonists & inhibitors*; D002118:Calcium / Q000494:pharmacology; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors*; D002147:Calmodulin / Q000494:pharmacology; D002462:Cell Membrane / Q000201:enzymology; D005947:Glucose / Q000494:pharmacology*; D007340:Insulinoma / Q000201:enzymology*; D008297:Male; D010190:Pancreatic Neoplasms / Q000201:enzymology*; D010766:Phosphorylation; D051381:Rats; D013237:Stereoisomerism; D014268:Trifluoperazine / Q000494:pharmacology; D014638:Vanadates / Q000494:pharmacology
D002147:Calmodulin; D014268:Trifluoperazine; D014638:Vanadates; D000255:Adenosine Triphosphate; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D005947:Glucose; D002118:Calcium