A seasonal variation in adrenergic response of salmonid erythrocytes

erythrocytes has been illustrated in some studies. Nikinmaa and Jensen (1986) observed a complete lack of β-adrenergic responses in erythrocytes of rainbow trout acclimated to winter conditions. Cossins and Kilbey (1989) observed that the adrenergic response of rainbow trout erythrocytes was reduced in winter fish. Similarly, our recent studies have shown that adrenergic activation of the Na+/H+ exchanger in Arctic charr (Salvelinus alpinus) erythrocytes is greatest in spring (Lecklin and Nikinmaa, 1999). Cossins and Kilbey (1989) suggested that the annual variations in adrenergic responsiveness could be caused by the liberation of young erythrocytes into the circulation if the activity of the Na+/H+ exchanger was elevated in young erythrocytes. In many salmonid species, e.g. Baltic salmon Salmo salar (Härdig and Höglund, 1984), brown trout Salmo trutta (Álvarez et al., 1994) and rainbow trout Oncorhynchus mykiss (Houston et al., 1996), the proportion of immature erythrocytes in circulation increases in spring. However, the adrenergic responsiveness of erythrocytes at different maturational state has not been investigated in detail. In the present study, we have manipulated the age of erythrocytes in circulation by making the fish anaemic and following the production of new erythrocytes. When immature erythrocytes emerged into the circulation, noradrenalineinduced erythrocytic swelling was studied in the newly synthetized and old erythrocytes. In addition, we have investigated whether the newly synthetized and old erythrocytes have different membrane fluidities.