Rainbow trout, fitted with arterial catheters, were exercised to exhaustion by manual chasing and then injected with either saline (controls), the β-agonist isoproterenol or the β-antagonist propranolol. Blood acid-base status, branchial unidirectional and net fluxes of Na+ and Cl, and net fluxes of ammonia and acidic equivalents (JHnet) were monitored over the subsequent 4 h of recovery. These same parameters were also monitored in normoxic, resting fish following isoproterenol injection and in exercised fish following acute post-exercise elevation of external NaCl concentration. In addition to confirming an important role for β-adrenoreceptors in the regulation of branchial gas exchange and red cell oxygenation and acid-base status, we find a significant β-adrenergic involvement in the flux of lactic acid from muscle and in JHnet across the gills. Both isoproterenol infusion (into nonexercised fish) and exhaustive exercise were found to cause net acid excretion. The post-exercise JHnet was further augmented by elevating [NaCl] but was not affected, in this instance, either by β-stimulation or blockade, indicating that JHnet was not entirely regulated by a β-adrenergic mechanism. On the basis of a detailed analysis of unidirectional Na+ and Cl fluxes, we conclude that the increase in JHnet following exercise arose mainly from increased Na+/H+(NH4+) exchange and that the upper limit on JHnet was set by the supply of external counterions and by the increase in branchial ionic permeability that invariably accompanies exhaustive exercise.

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