Aldosterone actions on basolateral Na+/H+ exchange in Madin-Darby canine kidney cells

In recent studies, there has been a re-evaluation of the polarity of Na⁺/H⁺ exchange in Madin-Darby canine kidney (MDCK) cells. This study was designed to examine aldosterone actions on basolaterally located Na⁺/H⁺ exchange of MDCK cell monolayers grown on permeant filter supports; pH_i was analysed in the absence of bicarbonate by using the pH-sensitive fluorescent probe 2,7-bis(carboxyethyl)-5,6-carboxyfluorescein. Pre-exposure of MDCK cells to aldosterone led within 10–20 min to an alkalization of pH_i ( 0.3 pH unit); this effect is prevented by an addition of dimethylamiloride to the basolateral superfusate. Addition of aldosterone led to stimulation of the basolaterally located Na⁺/H⁺ exchange activity (Na⁺-dependent recovery from an acid load); this effect required preincubation (more then 3 min) and was observed at 0.1 nM aldosterone. Preexposure (15 min) of MDCK monolayers to phorbol 12-myristate 13-acetate also led to an activation of Na⁺/H⁺ exchange; pre-exposure to 8-bromo-cAMP led to inhibition of Na⁺/H⁺ exchange activity. An inhibitory effect of aldosterone was observed if Na⁺/H⁺ exchange activity was analysed in the presence of aldosterone; the highest inhibitory effects (20%–30%) occurred at concentrations of 5 nM and higher. Aldosterone-dependent inhibition does not require preincubation and is fully reversible; it was only observed at low (20 mM) but not at high Na⁺ concentrations (130 mM). The data suggest that aldosterone has an instantaneous inhibitory effect on basolaterally located Na⁺/H⁺ exchange activity under conditions of low Na⁺, but stimulates the rate of transport activity upon preincubation under conditions of physiological Na⁺ concentrations.