Regulation of the Na⁺/H⁺ exchanger under conditions of abolished proton gradient: Isosmotic and hyperosmotic stimulation

Activation of the Na⁺/H⁺ exchanger following isosmotic and hyperosmotic stimuli was investigated in an osteoblast cell line (RCJ 1.20). The pH dependence of the transporter activity was studied under conditions of abolished proton gradient (pH_i = PH*) across the membrane. The isotonic response is Na⁺* dependent, increases towards higher pH. values, displaying a sigmoidal dependence on pH₁** (Hill coefficient ≈ 1.8) and is controlled by pH*. The greater than first order dependence of pH suggest that H⁺* inhibits the exchange beyond the rate expected from competition with the Na⁺* alone. This may be due to the existence of an external H⁺ regulatory site with a negative cooperative effect on the intra- or extracellular transport site. The hyperosmotic activation is Na⁺* independent, parallels the sigmoidal pH dependence of the isosmotic stimulus (Hill coefficient ≈ 2.0) and is mediated through an increase of the V_max without a change in the intracellular proton sensitivity.