Effects of Ca2+ on the sodium pump observed in cardiac myocytes isolated from guinea pigs.
It is presently unknown whether Ca2+ plays a role in the physiological control of Na+/K+-ATPase or sodium pump activity. Because the enzyme is exposed to markedly different intra- and extracellular Ca2+ concentrations, tissue homogenates or purified enzyme preparations may not provide pertinent information regarding this question. Therefore, the effects of Ca2+ on the sodium pump were examined with studies of [3H]ouabain binding and 86Rb+ uptake using viable myocytes isolated from guinea-pig heart and apparently maintaining ion gradients. In the presence of K+, a reduction of the extracellular Ca2+ increased specific [3H]ouabain binding observed at apparent binding equilibria: a half-maximal stimulation was observed when extracellular Ca2+ was lowered to about 50 microM. The change in [3H]ouabain binding was caused by a change in the number of binding sites accessible by ouabain instead of a change in their affinity for the glycoside. Ouabain-sensitive 86Rb+ uptake was increased by a reduction of extracellular Ca2+ concentration. Benzocaine in concentrations reported to reduce the rate of Na+ influx failed to influence the inhibitory effect of Ca2+ on glycoside binding. When [3H]ouabain binding was at equilibrium, the addition of Ca2+ decreased and that of EGTA increased the glycoside binding. Mn2+, which does not penetrate the cell membrane, had effects similar to Ca2+. In the absence of K+, cells lose their tolerance to Ca2+. Reducing Ca2+ concentration prevented the loss of rod-shaped cells but failed to affect specific [3H]ouabain binding observed in the absence of K+. These results indicate that a large change in extracellular Ca2+ directly affects the sodium pump in cardiac myocytes isolated from guinea pigs.
Hagane
K
K
Department of Pharmacology and Toxicology, Michigan State University, East Lansing.
Akera
T
T
Stemmer
P
P
D000818:Animals; D001566:Benzocaine / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005260:Female; D006168:Guinea Pigs; D066298:In Vitro Techniques; D008297:Male; D008345:Manganese / Q000494:pharmacology; D009206:Myocardium / Q000378:metabolism*; D010042:Ouabain / Q000378:metabolism; D011188:Potassium / Q000494:pharmacology; D012413:Rubidium / Q000493:pharmacokinetics; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000032:analysis
D008345:Manganese; D010042:Ouabain; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D012413:Rubidium; D011188:Potassium; D002118:Calcium; D001566:Benzocaine