Stimulus-secretion coupling of glucose-induced insulin release. LI. Divalent cations and ATPase activity in pancreatic islets.
Pancreatic islets can be viewed as a fuel-sensor organ. The amount of ATP used by the islet cells for the maintenance of adequate Ca2+ gradients across membranes is not known. An indirect approach to this issue consists in the measurement of Ca-ATPase activity. The kinetics of Ca-ATPase in islet homogenates yielded a Km for ATP close to 0.1 mM and two Km values for Ca2+ close to 0.13 and 4-6 microM, respectively. Within limits, the Ca-ATPase appeared as a distinct entity from Mg-ATPase. Several divalent cations, including Mg2+, inhibited the Ca-ATPase activity. Calmodulin also inhibited, significantly albeit modestly Ca-ATPase. The activity of the enzyme was increased at high pH or in the presence of bicarbonate. The reaction velocity at close-to-physiological concentrations of ATP, Ca2+ and H+ suggests that the consumption of ATP by the Ca-ATPase may account for a major fraction of the overall rate of ATP breakdown in intact islets.
D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002413:Cations, Divalent; D004533:Egtazic Acid / Q000494:pharmacology; D005947:Glucose / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000201:enzymology* / Q000378:metabolism; D007700:Kinetics; D051381:Rats
D002413:Cations, Divalent; D007328:Insulin; D004533:Egtazic Acid; D000251:Adenosine Triphosphatases; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D005947:Glucose