Ammonium replaces potassium in supporting sodium transport by the Na-K-ATPase of renal proximal straight tubules

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Abstract
Ammonium is capable of replacing potassium to support the hydrolysis of ATP by the Na-K-ATPase in many tissues. Whether ammonium supports the transport function of the Na-K-ATPase in the kidney (where such a substitution could be physiologically important) has not been studied, however. To address this issue, we determined the rates of fluid absorption and bicarbonate absorption (measured as total carbon dioxide) in the proximal straight tubule of the rabbit where both processes are dependent on sodium transport by the Na-K-ATPase. Both fluid absorption and total carbon dioxide absorption were significantly inhibited by potassium removal from the bath and perfusate. When ammonium was included in the perfusate and bath (replacing potassium completely), both fluid absorption and bicarbonate absorption occurred at rates indistinguishable from the rates observed with potassium present (and ammonium absent). We conclude that ammonium can replace potassium in supporting sodium transport by the Na-K-ATPase of the proximal straight tubule. Interaction between ammonium and potassium on the Na-K-ATPase could be important in this and other nephron segments.