Sodium extrusion induced by ammonium ions in toad skeletal muscle

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Abstract
A study was made of the ability of ammonium ions to stimulate active sodium extrusion in toad skeletal muscle. In sodium‐rich muscles, 15 mM ammonium induced a net loss of sodium against a concentration gradient slightly larger than 5 mM potassium. Similar results were obtained when a sodium efflux was considered. The activation curve of ammonium‐stimulated sodium efflux was strongly S‐shaped, reaching a plateau at 10–15 mM ammonium; this could be due to more than one activation site on the outside membrane, or to some cooperative effect of ions. The increment in the rate constant for sodium efflux produced by both ammonium and potassium was completely abolished by 10−4 M ouabain. The extent of the inhibition by a submaximal dose of this glycoside was independent of the external ammonium concentration up to 30 mM. This behavior corresponds to a non‐competitive type of inhibition.