Adrenergic responses and the Root effect in erythrocytes of freshwater fish

The effects of adrenergic‐stimulation upon the oxygen‐binding capacity of fish erythrocytes have been investigated. The oxygen capacity of rainbow trout, Oncorhynchus mykiss (Walbaum), erythrocytes was lowered by 44% on extracellular acidification (the so‐called ‘Root effect’). Addition of isoproterenol at 20° Ccaused an acid shift of the curve relating oxygen capacity to pH0 by approximately 0.2 pH units, a value which was similar to the change in intracellular pH caused by adrenergic stimulation (Cossins & Kilbey Journal of Experimental Biology, 148, 303–312, 1990). Moreover, when plotted as a function of pHi, the curves for control and adrenalinstimulated erythrocytes were superimposable suggesting that the adrenergic shift in the Root curve was a result of the change in pHi caused by activation of the adrenergic Na+/H+ exchanger. A similarly large adrenergic shift in the Root curve was observed for pike, Esox lucius L., erythrocytes, though not for erythrocytes of carp, Cyprinus carpio L., and tench, Tinea tinea (L.). The pH for the mid‐point of the Root effect in pike erythrocytes was distinctly more acid than for trout, but in both cases corresponded closely with the optimal pH for the adrenergic Na+/H+ exchange mechanism. This suggests a link between the functional characteristics of the exchanger and the oxygen‐binding properties of haemoglobin.