Ouabain and low extracellular potassium inhibit PTH secretion from bovine parathyroid cells by a mechanism that does not involve increases in the cytosolic calcium concentration

Abstract

We have previously found that high extracellular calcium (Ca++) concentrations inhibit PTH release in association with a threefold to fourfold rise in cytosolic Ca++ concentration. Recent data have also shown that low extracellular potassium (K+) concentration or ouabain also inhibits PTH release to an extent comparable to that seen with high Ca++ and produce a marked rise in the intracellular sodium (Na+) content. These results suggested that low K+ and ouabain might modulate PTH release through increases in cytosolic Ca++ related to alterations in Na+-Ca++-exchange. In the present studies, we have examined further the mechanism(s) by which inhibition of the Na+-K+-ATPase regulates PTH release. Exposure of cells loaded with the Ca++-sensitive dye QUIN-2 to low K+ produced a 10% to 17% increase in cytosolic Ca++ at 0.5 to 1.0 mmol/L extracellular Ca++, which was statistically significant only at 0.75 mmol/L Ca++. In contrast, low K+ caused a statistically significant decrease in cytosolic Ca++ at 1.5 to 2 mmol/L Ca++, while ouabain lowered cytosolic Ca++ significantly by 23% to 46% at all Ca++ concentrations examined (0.5 to 2 mmol/L). Low K+ or ouabain had no effect on cellular levels of ATP or GTP or intracellular pH measured using the pH-sensitive dye BCECF [2', 7'-bis(carboxyethyl)-5,6-carboxyfluorescein]. The inhibition of secretion by low K+ or ouabain, unlike that due to high extracellular Ca++, was not reversed by TPA (12-O-tetradecanoyl phorbol 13-acetate), an activator of protein kinase C. Low K+ did produce a modest (30% to 40%) lowering of agonist-stimulated but not basal cAMP content.(ABSTRACT TRUNCATED AT 250 WORDS)