Effects of ouabain on [3H]ouabain binding, 86Rb uptake, cellular sodium and potassium, and parathyroid hormone secretion in dispersed bovine parathyroid cells

Abstract

We have compared the effects of the cardiac glycoside ouabain on [3H]ouabain binding, 86Rb uptake, cellular sodium and potassium, and PTH secretion in dispersed bovine parathyroid cells. [3H]ouabain binds reversibly to a single class of binding sites with an affinity of 6.1 X 10(-8) M and a binding capacity of 5.8 X 10(5) sites/cell. Ouabain also inhibits the uptake of 86Rb, an analog of K, by 90% with half-maximal inhibition at 7.2 X 10(-8) M. There is a concomitant, ouabain-induced increase in cellular sodium and a reduction in cellular potassium. The half-maximal effect on cellular monovalent cations takes place at 1.4 X 10(-7 M ouabain. Finally, ouabain causes a dose- and time-dependent inhibition of low calcium-stimulated PTH secretion. This inhibition does not require extracellular calcium; half-maximal inhibition occurs at 1.1 X 10(-7) M ouabain. These results show that 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 sodium and potassium support an inhibition of this enzyme in parathyroid cells by the cardiac glycoside. Finally, the close correspondence between ouabain binding and effects on Rb uptake, cellular monovalent cations, and PTH release suggest a role for Na+-K+-ATPase per se or for monovalent cations in PTH secretion. A change in sodium-calcium exchange due to the elevation in cellular sodium is a potential mechanism by which ouabain might inhibit PTH secretion.