KIRSCHNER, L. B: The Mechanism of Sodium Uptake in Crayfish.

Recently, we reported that 3 proton pump inhibitors, known to block Na-H exchange in frog skin had no effect on proton movement and little on Na uptake in tapwater-adapted (TW)crayfish. In contrast, all 3 reduced Na-H exchange in salt-depleted (SD) crayfish. It appeared that the exchange was mediated by the Na channel-proton pump mechanism in SD animals but not in those adapted to TW. We suggested that a 2Na-1H exchanger might function in the latter (Zetino et al, Comp. Biochem. Physiol. 128A 863-872 2001). It is known that amiloride (AM) is a powerfful inhibitor of Na channels in several epithelial systems but is a much weaker blocker of Na-H exchangers. The converse is true for the analogues ethylisopropyl-AM (EIPA) and hexamethyl-AM (HMA). If our inferences are correct we should expect EIPA and HMA to inhibit Na influx in TW crayfish with AM having a weaker action. In SD animals AM should be a stronger inhibitor than either of the analogues. The experimentral data showed the following: EIPA and HMA were potent inhibitors of Na influx in TW animals. The concentration for half-maximal inhibition was about 0.2 microM for both. Surprisingly, AM was equipotent in these animals. In addition, all 3 compounds were equally powerful inhibitors in SD animals and acted at about the same concentrations as in TW animals. The data show that there is no differential action of AM, EIPA and HMA in either TW or SD crayfish. AM and its analogues cannot be used to distinguish between the 2 models of Na-H exchange in crayfish.