Effects of Ouabain on [3H]Ouabain Binding,86Rb Uptake, Cellular Sodium and Potassium, and Parathyroid Hormone Secretion in Dispersed Bovine Parathyroid Cells*

Abstract

The effects of the cardiac glycoside ouabain on [3H]ouabain binding, 86Rb uptake, cellular Na and K, and PTH [parathyroid hormone] secretion in dispersed bovine parathyroid cells. [3H]Ouabain binds reversibly to a single class of binding sites with an affinity of 6.1 .times. 10-8 M and a binding capacity of 5.8 .times. 105 sites/cell. Ouabain also inhibits the uptake of 86Rb, an analog of K, by 90% with half-maximal inhibition at 7.2 .times. 10-8 M. There is a concomitant, ouabain-induced increase in cellular Na and a reduction in cellular K. The half-maximal effect on cellular monovalent cations takes place at 1.4 .times. 10-7 M oaubain. Finally, ouabain causes a dose- and time-dependent inhibition of low Ca-stimulated PTH secretion. This inhibition does not require extracellular Ca, half-maximal inhibition occurs at 1.1 .times. 10-7 M ouabain. Dispersed bovine parathyroid cells contain abundant binding sites for ouabain, a known inhibitor of Na+-K+-ATPase. Moreover, the ouabain-induced reduction in 86Rb uptake and alterations in cellular Na and K support an inhibition of this enzyme in parathyroid cells by the cardiac glycoside. The close correspondence between ouabain binding and effects on Rb uptake, cellular monovalent cations, and PTH release suggest a role for a Na+-ATPase per se or for monovalent cations in PTH secretion. A change in Na-Ca exchange due to the elevation in cellular Na is a potential mechanism by which ouabain might inhibit PTH secretion.