The Consequences of Short Term Exercise, Various Levels of Stress and Training on Ion Regulation in Different Species of Fish

Abstract

Initially, this study examined the mechanisms by which Na+ and Cl-are regulated in freshwater rainbow trout during exercise and stress. Aerobic exercise (~2 body lengths sec⁻¹) caused a brief increase in diffusive Na⁺ efflux (Jₒᵤₜᴺᵃ⁺) and a brief decline in plasma Na+ and Cl⁻. This disturbance was rapidly compensated by a 3 fold increase in Jᵢₙᴺᵃ⁺ and Jᵢₙᶜˡ⁻(over the first 10-12 h exercise), and by a reduction in Jₒᵤₜᴺᵃ⁺ to 40% of routine by 7 h of exercise. The compensation produced a significant increase in whole body Na⁺ while whole body Cl⁻ remained unchanged. In contrast, confinement stress (for 4 or 8 h) caused an 8 fold increase in Jₒᵤₜᴺᵃ⁺and Jₒᵤₜᶜˡ⁻ which was sustained for at least the first 5 h of stress and resulted in large decreases in whole body Na+ and Cl-. Compensation of the losses was not complete until 24 h post-stress and was achieved by increases in Jᵢₙᴺᵃ⁺ and Jᵢₙᶜˡ⁻ (of similar magnitude and timing to that of exercise) as well as reductions in Jₒᵤₜᴺᵃ⁺ and Jₒᵤₜᶜˡ⁻ to nearly zero. We conclude that Jᵢₙ increased because of an activation of inactive transport sites in the gills while Jₒᵤₜ was reduced by a reduction in branchial ionic permeability, both responses mediated hormonally. Although the hormonal control mechanisms are as yet poorly defined, we argue that growth hormone and prolactin are responsible for the Jᵢₙ and Jₒᵤₜ regulation, respectively, and rule out either cortisol or epinephrine as having any role, at least with respect to the rapid NaCI regulation evident during exercise. The ability of rainbow trout to rapidly regulate ion balance was then investigated to determine whether it is unique to rainbow trout, exists in stream-dwelling animals or whether it is wide spread in fish regardless of preferred habitat. Common shiners, considered to be an active species and smallmouth bass, considered to be less active, were the two species of comparison. Common shiners demonstrated rapid increases in Jᵢₙᴺᵃ⁺ during exercise and confinement, a lack of change in whole body Na⁺ and Cl⁻ during exercise and a large Na⁺ and Cl⁻ loss during stress. In contrast, smallmouth bass experienced minimal increases in Jᵢₙᴺᵃ⁺ during exercise and no change during stress with ion loss occurring during both exercise and stress. It was concluded that the relative ability to regulate ion balance in response to stress and exercise may reflect the frequency with which the animal experiences that challenge in its natural habitat. Consequently, common shiners probably possess a similar uptake mechanism to that of rainbow trout while the mechanisms in smallmouth bass may exist, but in attenuated form. The final analysis investigated whether or not exercise training affected the magnitude of the disturbance to Na⁺ balance produced by both acute and chronic stress. This was important in that it could be applied to the improvement of fish stocking techniques. Trained fish demonstrated the ability to reduce ion loss produced by stress despite significantly high levels of cortisol, glucose and oxygen consumption. Similar results were produced by both acute and chronic stress and it was established that the rainbow trout's ability to regulate ions during stress, without altering the release of cortisol and catecholamines was improved by training.