An increase in the functional surface area (FSA) of the gills of freshwater fish facilitates increased oxygen consumption (MOO2) but also increases ion loss across the gills - a phenomenon that has been termed the osmorespiratory compromise. This study on the rainbow trout confirms that Na+ loss (JoutNa) accelerates with increasing MO2 but also shows that the former always exceeds the latter, i.e. the ion/gas ratio (IGR, JoutNa/MO2) increases. Since an increase in FSA should affect JNaout and MO2 equally, the increase in the IGR is attributed to an increase in ion permeability and is thought to arise through opening of paracellular diffusion channels via disruption of tight junctions — a conclusion supported by the effects of brief osmotic shock, low external Ca2+ concentration and catecholamine infusion on ion losses across the gills. By analogy with extensive studies on ‘leaky’ epithelia, these treatments disrupt tight junctions by cell shrinkage, by displacement of Ca2+ from tight junction surfaces or by increasing intralamellar pressure, respectively. While enforced exercise or handling stress substantially increased the IGR of the trout above routine levels, animals were also capable of reducing IGR to about one-tenth of routine levels. This regulation may, in part, be due to decreasing intralamellar pressure, achieved in part by down-regulation of adrenergic receptors, but there appears to be a significant additional level of control that is probably exerted directly at the tight junctions. Such a degree of control is only possible, however, if animals do not continue to exercise and if Ca2+ levels in the water are not reduced. With continued exercise, the IGR returns to routine levels. Consequently, ion losses remain substantially elevated, suggesting that they may ultimately limit maximum sustainable activity. Measurement of the IGR under routine conditions is proposed as a simple means of predicting ion losses during activity and of analyzing the nature of the osmorespiratory compromise.

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