Effect of amiloride on sodium transport across body surfaces of freshwater animals.

KIRSCHNER, LEONARD B., LEWIS GREEN~ALD, AND THEODORE H. KERSTETTER. Effect of amiloride on sodium transport across body surfaces of freshwater animals. Am. J. Physiol. 224(4): 832-837. 1973.-Amiloride, added to an external bathing solution at a concentration of 1 X 10m4 M, inhibited sodium influx in intact trout (79 %), frogs (460/,), and crayfish (90 yO). It had no effect on efflux across the body surface. Chloride fluxes, measured simultaneously, were also unaffected. Ammonia efflux was inhibited by amiloride to a lesser extent in trout (300/,) and crayfish (54’7,). It was not excreted at measurable rates by the frog in the presence or absence of the inhibitor. Hydrogen ion excretion, calculated on the assumption that only NH3 (not NHJ+) was excreted, was inhibited by amiloride in all three animals: trout 56 %, frogs 407,, and crayfish >90%. The data suggest that the initial step in active sodium transport across the body surface is similar in freshwater animals from at least two phyla. Under natural conditions (a dilute external medium), this step appears to require a coupled Na+/H+ exchange to maintain electrical neutrality. The amiloride data also admit the possibility of NHJ+ excretion balancing a fraction of the Na+ influx in some animals.