Sodium-proton exchange in crayfish

Recently, three proton pump inhibitors were shown to have no effect on proton excretion and little on Na uptake in tapwater-adapted (TW) crayfish, while all three reduced Na-H exchange in salt-depleted (SD) animals. It appeared that the exchange was mediated by the Na channel-H pump in SD crayfish but not in TW animals. An alternative, a 2Na-1H exchanger, might function in the latter. To test this, the effects of amiloride (AM) and ethylisopropyl AM (EIPA) on Na fluxes were observed. AM inhibits the Na channel but is a much weaker blocker of Na-H exchangers. In contrast, EIPA inhibits exchangers but acts weakly on the Na channel. If an exchanger functions in TW crayfish, we should expect EIPA to block Na influx in them with AM having a weaker action. The reverse should be true in SD animals. Experimental data showed that EIPA was a potent inhibitor of Na influx in TW crayfish with half-maximal inhibition at about 0.2 microM. However, AM proved to be equipotent. In SD crayfish, EIPA was as effective as in TW animals, and again AM was equally potent. The data fail to show the expected differential action. Therefore, AM and its analogues cannot be used to distinguish between the two models of Na-H exchange in crayfish.