The role of cAMP in regulating the β-adrenergic response of rainbow trout (Oncorhynchus mykiss) red blood cells.

The β-adrenergic response of teleost red blood cells (RBCs) enables the fish to maintain or even enhance the oxygen affinity of haemoglobin during various stress situations. The role of CAMP in the pronounced β-adrenergic response of hypoxic rainbow trout RBCs was studied. Rainbow trout RBCs were incubated with three different β-agonists (noradrenaline, adrenaline and isoproterenol, 10(-9) - 10(-4) M) at two oxygen tensions (PO 2, 155 and 8 mmHg), and thereafter cAMP accumulation and cellular water content were measured.The cAMP concentration of non-stimulated trout RBCs was ca. 1200 nmol/kg dw. Of the three β-agonists used, isoproterenol was the most effective in formation of cAMP, followed by noradrenaline and adrenaline. Oxygen tension affected the accumulation of cAMP in two ways. At physiological catecholamine levels (1-100 nM) there was either no difference between normoxic and hypoxic cells or a slight increase in the normoxic ones. At high catecholamine concentrations the accumulation of cAMP was greater in the hypoxic than in the normoxic cells. Oxygen tension also affected the magnitude of cell swelling but had no effect on the catecholamine concentrations causing half-maximal swelling (EC50-values). The results indicate that, at physiological catecholamine levels, the β-adrenergic response of rainbow trout RBCs is mainly regulated on the level of the Na(+)/H(+) exchange.