The addition of ammonium ions to the external medium results in an inhibition of the sodium influx and net uptake in Carassius auratus, while intraperitoneal injection of ammonium produces the opposite effect. The simultaneous chloride balance is not significantly affected by these treatments. The addition of bicarbonate ions to the external medium results in a reduction of the influx and net flux of chloride, while injection of bicarbonate produces the opposite effect. The simultaneous sodium balance is not significantly altered. The effects of the external additions are reversible after elimination of the excess ammonium or bicarbonate ions by rinsing. Inhibition of carbonic anhydrase in the gill by injection of acetazoleamide produces a simultaneous inhibition of both sodium and chloride exchanges. These results confirm the hypothesis of an exchange of sodium for ammonium, and of bicarbonate for chloride across the gill. A tentative schematic representation of the ionic absorption mechanisms in the branchial cell of the fresh-water teleosts is given. Similarities with other biological membranes and especially with the renal tubule are pointed out.

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