Presence of a Na⁺/H⁺ exchanger in brush border membranes isolated from the kidney of the spiny dogfish,Squalus acanthias

Summary

A membrane fraction, rich in brush border membranes, was prepared from renal proximal tubules of the spiny dogfish,Squalus acanthias, and the sodium-proton exchange mechanism in these membrane vesicles was investigated by both a rapid filtration technique and the fluorescence quenching of acridine organe.²²Na⁺ uptake was stimulated by an outwardly directed H⁺ gradient, and was inhibited by amiloride at a single inhibitory site with an apparentK _i of approximately 1.7×10⁻⁵ M. In the presence of an H ⁺_i >H ⁺_o gradient, the$K_{{\text{m}}_{{\text{Na}}} + } {\text{and}} V_{\max _{{\text{Na}}} + }$ of the Na⁺/H⁺ exchanger were 9.7±0.8 mM and 48.0±12.0 nmol·mg protein⁻¹·min⁻¹, respectively. The uptake of Na⁺ was electroneutral in the presence of a H⁺ gradient, indicating a stoichiometry of 1. In the fluorescence studies, quenching of acridine orange occurred in the presence of an outwardly directed Na⁺ gradient which was inhibited by amiloride. Thus, an electroneutral Na⁺/H⁺ exchanger with properties similar to those found in the mammalian kidney is also present in the spiny dogfish and may contribute to the urinary acidification of this marine animal.