THE ERYTHROCYTE NA+ H+ EXCHANGERS OF EEL (ANGUILLA-ANGUILLA) AND RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) - A COMPARATIVE-STUDY/

Trout and eel red blood cell Na+/H+ exchangers show widely different r egulatory properties. Catecholamines, cyclic AMP and phorbol esters, w hich activate the trout red cell antiporter, do not affect the eel exc hanger. Unlike the trout red cell exchanger, the eel red cell exchange r is strongly activated by cell shrinkage, allowing a remarkable cell volume recovery. These different regulatory properties probably indica te the existence of different isoforms of the exchangers in nucleated erythrocytes, since sensitivity to catecholamines is known to be depen dent upon the presence of protein kinase A consensus sites on the cyto plasmic domain of the antiporter. After shrinkage of eel erythrocytes, the Na+/H+ exchange rate gradually increases to reach a maximum value after about 10 min. The magnitude of activation is a graded function of cell shrinkage, Deactivation, like activation, is induced by a volu me change and occurs after some delay (lag time). The response of the trout antiporter (beta NHE) to cell shrinkage is much reduced compared with that of the eel antiporter. In addition, the antiporter is deact ivated prior to restoration of the normal control volume, leaving cell volume regulation notably defective. The trout red cell antiporter, w hich is desensitized and enters a refractory state following hormonal activation, is only deactivated (it can be reversibly reactivated) aft er shrinkage-induced activation. This dual control may occur by both p hosphorylation-dependent and phosphorylation-independent mechanisms. I n view of the similarities in the regulatory properties of eel and sal amander (Amphiuma sp.) Na+/H+ exchangers, the expression of a putative K+/H+ exchange mediated by the Na+/H+ exchanger was sought in eel ery throcytes, exchange, neither osmotic swelling nor calyculin-A-dependen t phosphorylation revealed such a K+/H+ exchange.