Mechanisms of ammonia and acid extrusion by the little skate, Raja erinacea

The little skate (Raja erinacea) excretes ammonia and titratable acid at a rate of 6.37 and 40–95 μM. 100g‐1. hr‐1, respectively, at an external pH of 7.8 to 8.0. Transfer into Na‐ and K‐free artifical sea water does not alter the rate of ammonia efflux but reduces titratable acid efflux to zero. Addition of amiloride (10‐4M) to the external sea water does not alter the ammonia efflux but reduces the titratable acid efflux by 49%. Thus, while ammonia efflux is apparently not via Na (or K)/NH4 exchange, excretion of titratable acid is apparently via Na (or K)/H exchange.