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Laboratoire Jean Maetz, Département de Biologie du Commissariat à l'Energie Atomique, Villefranche-sur-Mer, France.

1. In the presence of catecholamine the nucleated red blood cells of trout show a large increase in cell volume as a result of an accumulation of sodium and chloride due to activation of an amiloride-sensitive, cyclic AMP-dependent Na+-H+ exchanger allowing Na+ to enter in exchange for internal H+. 2. The activation of this cyclic AMP-dependent Na+-H+ exchange is considered to be involved in an adaptive response to hypoxia by increasing the oxygen-carrying capacity of erythrocytes. But cell swelling could increase resistance to blood flow and thus impair the expected physiological advantages for oxygen transport. The effect of catecholamine on the deformability properties of the red blood cells has been studied by measuring the rate at which blood flows through a Nucleopore filter (5 microM). 3. The results show that stimulation by catecholamine in fact increases the erythrocyte deformability, a response which must favour the supply of oxygen at the tissue level. 4. Hormonal stimulation increases the cellular cyclic AMP content (and cyclic AMP-dependent phosphorylation of cytoskeleton proteins could influence cell deformability) and the cell volume. It has been shown that when cellular cyclic AMP content is increased under conditions where the cell cannot swell, the erythrocyte becomes more rigid and not more deformable. Conversely the results show a systematic coincidence between cell swelling and deformability increase. The precise way in which volume change and deformability are interrelated needs more study.