title: The alpha-2 isomer of the sodium pump is inhibited by calcium at physiological levels.
abstract: The inhibition of the (Na,K)ATPase by calcium was investigated in plasma membrane preparations of rat axolemma, skeletal muscle and kidney outer medulla. Ouabain titration curves demonstrated that physiological calcium (0.08-5 microM) inhibited mainly the high affinity alpha 2 isomer. In axolemma all the (Na,K)ATPase had high ouabain affinity and calcium inhibited 40-50% of the activity with a Ki of 1.9 +/- 0.9 x 10(-7) M. In skeletal muscle high and low ouabain affinity components were present in equal amounts and calcium inhibited only the high affinity component with a Ki of 1.3 +/- 0.3 x 10(-7) M. Kidney enzyme had a low affinity for ouabain and showed very little sensitivity to calcium in the physiological range. It was demonstrated that high calcium levels inhibit the enzyme in a general sense, irrespective of the isomer, with a Ki of 6.5 +/- 6 x 10(-4) M for the kidney and 5.9 +/- 4 x 10(-4) M for the axolemma enzymes. In axolemma, enzyme activity was studied as a function of sodium concentration. Physiological calcium reduced Vmax while not significantly changing K 0.5 for sodium binding.
authors.affiliation: Department of Biochemistry and Molecular Biology, Harvard University, Cambridge, Massachusetts 02138.
authors.forename: J E
authors.initials: JE
authors.lastname: McGeoch
mesh_terms: D000818:Animals; D001369:Axons / Q000201:enzymology*; D002118:Calcium / Q000494:pharmacology*; D007536:Isomerism; D007679:Kidney Medulla / Q000201:enzymology*; D007700:Kinetics; D009132:Muscles / Q000201:enzymology*; D009928:Organ Specificity; D010042:Ouabain / Q000494:pharmacology; D051381:Rats; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors*
chemical_list: D010042:Ouabain; D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium