Parathyroid hormone (PTH) increased intracellular cyclic AMP and reduces Na+/phosphate cotransport activity in OK cells [Malmström & Murer (1986) Am. J. Physiol. 251, C23-C31; Caverzasio, Rizzoli & Bonjour (1986) J. Biol. Chem. 261, 3233-3237]. It was also shown that PTH activates phosphoinositide metabolism in OK cells [Hruska, Moskowitz, Esprit, Civitelli, Westbrook & Huskey (1987) J. Clin. Invest. 79, 230-239]. In the present paper we show that tumour-promoting phorbol esters are effective in reducing Na+/phosphate cotransport. The Ca2+ ionophores A23187 and ionomycin had only a small effect on Na+/phosphate cotransport; added together, A23187 and phorbol esters showed a synergistic action. Phorbol esters and phorbol esters plus ionomycin stimulated prostaglandin synthesis as well as cyclic AMP production; acetylsalicylic acid prevented phorbol-ester-induced prostaglandin synthesis and cyclic AMP production, but had no effect on inhibition of Na+/phosphate cotransport. In suspensions of OK cells, PTH and thrombin produced a rise in intracellular Ca2+. In contrast with PTH, thrombin did not elevate cellular cyclic AMP in suspended OK cells. PTH and thrombin reduced Na+/phosphate cotransport in suspended OK cells. It is suggested that two regulatory cascades are involved in PTH action on Na+/phosphate cotransport: cyclic AMP/kinase A and Ca2+/diacylglycerol/kinase C.

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