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We studied the effects of beta-adrenergic stimulation on the formation of adenosine 3',5'-cyclic monophosphate (cAMP) and on the cell volume in carp red blood cells in normoxia (PO2 = 150 mmHg) and hypoxia (PO2 = 8 mmHg). Accumulation of cAMP was the prerequisite for adrenergic cell swelling. Cell swelling was induced by beta-agonists, forskolin, and 8-bromo-cAMP. The amount of cAMP required for adrenergic swelling was minimal; swelling was observed at cAMP concentrations greater than 100 nM. Maximal increase in cell volume was observed at 200 nM cAMP. These values were independent of both the oxygen tension and the beta-agonist used. Norepinephrine caused the largest accumulation of cAMP, followed by isoproterenol and epinephrine. At hypoxic conditions, the cAMP concentrations obtained after stimulation with the natural catecholamines norepinephrine and epinephrine were greater than at normoxic conditions. The catecholamines caused appreciable cell swelling at lower concentrations in hypoxia than in normoxia. Thus the number of beta-adrenergic receptors, and their inherent ligand affinities, may be higher in hypoxic than in normoxic carp red blood cells. Oxygen tension had a pronounced effect on the magnitude of the adrenergic swelling. The maximal increase in cell volume was 5-7% in hypoxia, whereas in normoxia it was only approximately 2%. This was not due to differences in cAMP formation, but possibly to a greater activity of the Na(+)-H+ exchanger in hypoxic than in normoxic conditions.

The occurrence and pH dependence (pHe 7-8) of the adrenergic red cell responses of two salmonids, trout and whitefish, and a percinid, pikeperch were studied. These are all species that live in well-oxygenated waters. The responses were compared to those of carp, which tolerates oxygen-deficient waters.The adrenergic responses of trout and whitefish red cells were pronounced. In these species red cell swelling, the accumulation of sodium and chloride in the cell, and the increase in red cell oxygen content at atmospheric oxygen tension were maximal at pH 7.3. In contrast, pikeperch red cells responded to β-adrenergic stimulation only at extracellular pH 7.1. In carp, the adrenergic response, occurring below extracellular pH 7.5, was small as compared to the two salmonids. In each case the onset of the adrenergic response coincided with the onset of the Root effect.The differences in the adrenergic responses between the two salmonids and pikeperch suggest that the occurrence of the adrenergic response is not directly related to the environmental oxygen requirements of the species, but may be linked to the activity pattern.

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.

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