Parathyroid hormone regulation of Na+/H+ exchange in opossum kidney cells: polarity and mechanisms

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Abstract
In previous work we have shown that parathyroid hormone (PTH) inhibits Na+/H+ exchange in cellular suspensions of OK (oppossum kidney) cells (an established renal epithelial cell line) in a dose-dependent manner. PTH effects could be mimicked by pharmacological activation of both protein kinase A and protein kinase C (Helmle-Kolb et al. 1990). In the present paper we extend these observations and analyze the PTH-dependent control of Na+/H+ exchange in OK cells kept in epithelial configuration (monolayer). Na+/H+ exchange activity is examined by microfluorometry using the intracellularly trapped pH-sensitive dye 2′7′-bis-(2-carboxyethyl)-5,6-carboxyfluorescein. Cells recovered from an acid load (NH4Cl prepulse) after addition of apical Na+. Ethylisopropylamiloride inhibits Na+-dependent pHi recovery at micromolar concentrations. PTH leads to an inhibition of apical Na+/H+ exchange activity; inhibition is observed even at a concentration of 5 pM PTH. PTH given at maximally effective concentrations (24 nM) reduces the total Na+/H+ exchange capacity by 60%–70%. Apical as well as basolateral hormone additions elicit an inhibitory response at low (5 pM) or high (24 nM) concentrations. Forskolin (activation of protein kinase A) and phorbol esters (activation of protein kinase C) lead to an inhibition of Na+/H+ exchange activity (60%–70% inhibition). These observations suggest that Na+/H+ exchange activity is preferentially located in the apical membranes of OK cells kept in monolayer configuration. Na+/H+ exchange activity is partially inhibited by PTH (even at physiological hormone concentrations) and PTH-dependent inhibition can be mimicked by pharmacological activation of either protein kinase A or protein kinase C. Interestingly, PTH-related regulatory mechanisms are induced from both the apical and basolateral cell surface suggesting a location of receptors at both cell surfaces.