Effects of direct transfer from freshwater to seawater on respiratory and circulatory variables and acid-base status in rainbow trout |
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Authors: | Valérie Maxime Jean-Pierre Pennec Claude Peyraud |
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Affiliation: | (1) Laboratoire de Physiologie Animale, URA-CNRS 648, Faculté des Sciences et Techniques, 6 Avenue Victor Le Gorgeu, 29287 Brest Cedex, France |
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Abstract: | Summary The effects of increased ambient salinity (35 mg · ml-1) were studied at 1, 6, and 24 h after direct transfer of rainbow trout from freshwater to seawater. Two series of experiments were carried out successively. The first series was designed to simultaneously study all the respiratory (except Hb affinity for O2), circulatory, and acid-base variables in each fish. In this series, fish were fitted with catheters chronically inserted into the cardiac bulbus, the dorsal aorta, and the opercular and buccal cavities. In the second series, designed to study haemoglobin O2 affinity, fish were fitted with only a dorsal aorta catheter. The ventilatory flow () was markedly increased just after transfer (by 55% at 1 h), then more moderately (by 20% at 6 h and 32% at 24 h). The initial hyperventilation peak was associated with frequent couphing motions. These ventilatory changes resulted essentially from increase in ventilatory amplitude. Initially, standard oxygen consumption (MM}O2) decreased slightly, the moderately increased (by 12% at 24 h), so that the oxygen convection requirement () increased substantially. In spite of an increased ventilation, the partial pressure of oxygen in arterial blood (PaO2) decreased slightly at 1 h, prior to returning to control levels, while partial pressure of carbon dioxide in arterial blood (PaCO2) was not significantly decreased. Gill oxygen transfer factor decreased substantially at 1 h (by 35%) then more moderately (by 7% at 1 h and 12% at 24 h). These results suggest a decrease in gas diffusing capacity of the gills. As PaCO2 remained approximatively unchanged, the gradual decrease in arterial pH (pHa) from 7.94 to 7.67 at 24 h must therefore be regarded as a metabolic acidosis. The strong ion difference decreased markedly because the concentration of plasma chloride increased more than that of sodium. Arterial O2 content (CaO2) gradually decreased (by 38% at 24 h) simultaneously with the decrease in pHa, while the ratio PaO2/CaO2 increased. In parallel, seawater exposure induced a marked decrease in affinity of haemoglobin for O2, so that at 24 h, P50 was increased by 26% above the value obtained in freshwater-adapted trout. The increase in could be ascribed initially (at 1 h) to the decrease of PaO2 and later to a stimulation of respiratory neurons resulting from the lowered medullary interstitial pH. The decrease in CaO2 could be interpreted mainly as a consequence of a decreased affinity of haemoglobin for O2, likely to be due to the blood acidosis and a predictable increase in chloride concentration within erythrocytes. Cardiac output () slightly decreased at 1 h, then progressively increased by 30% at 24 h. Branchial vascular resistance increased at 1 h by 28%, then decreased by 18% of the control value at 24 h. Systemic vascular resistance decreased markedly by 40% at 24 h. As heart rate (HR) remained significantly unchanged, the cardiac stroke volume initially decreased then increased in relation to the changes in . The increase of , allowing compensation for the effect of decreased CaO2 in tissue O2 supply, was interpreted as a passive consequence of the decrease in total vascular resistance occurring during seawater exposure.Abbreviations a.u. arbitrary units - CaO2 arterial oxygen content - pH50 arterial pH at P50 - CvO2 venous oxygen content - Hb haemoglobin - HR heart rate - Hct hematocrit - nHill Hill coefficient - O2 standard oxygen consumption - PaCO2 arterial partial pressure of carbon dioxide - PaO2 arterial partial pressure of oxygen - PvO2 oxygen partial pressure in mixed venous blood - P50 oxygen tension at half saturation of haemoglobin - PVA, PDA blood pressure in ventral and dorsal aorta - pHa arterial pH - PIO2, PEO2 oxygen partial pressure of inspired and expired water - PO2 oxygen partial pressure - cardiac output - SEM standard error of mean - S.I.D. strong ion difference - SV cardiac stroke volume - TO2 gill oxygen transfer factor - U oxygen extraction coefficient - VA ventilatory amplitude - VF ventilatory frequency - VRG, VRS branchial and systemic vascular resistances - ventilatory flow - ventilatory oxygen convection requirement |
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Keywords: | Fish Salinity Respiration Circulation Acid-base status |
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