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 O
2), 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 O
2 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}O
2) 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 (
P
aO
2) decreased slightly at 1 h, prior to returning to control levels, while partial pressure of carbon dioxide in arterial blood (
P
aCO
2) 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
P
aCO
2 remained approximatively unchanged, the gradual decrease in arterial pH (pH
a) 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 O
2 content (
C
aO
2) gradually decreased (by 38% at 24 h) simultaneously with the decrease in pH
a, while the ratio
P
aO
2/
C
aO
2 increased. In parallel, seawater exposure induced a marked decrease in affinity of haemoglobin for O
2, so that at 24 h, P
50 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
P
aO
2 and later to a stimulation of respiratory neurons resulting from the lowered medullary interstitial pH. The decrease in
C
aO
2 could be interpreted mainly as a consequence of a decreased affinity of haemoglobin for O
2, 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
C
aO
2 in tissue O
2 supply, was interpreted as a passive consequence of the decrease in total vascular resistance occurring during seawater exposure.Abbreviations a.u.
arbitrary units
-
C
aO
2
arterial oxygen content
- pH
50
arterial pH at P
50
-
C
vO
2
venous oxygen content
- Hb
haemoglobin
- HR
heart rate
- Hct
hematocrit
- n
Hill
Hill coefficient
- O
2
standard oxygen consumption
-
P
aCO
2
arterial partial pressure of carbon dioxide
-
P
aO
2
arterial partial pressure of oxygen
-
P
vO
2
oxygen partial pressure in mixed venous blood
- P
50
oxygen tension at half saturation of haemoglobin
-
P
VA,
P
DA
blood pressure in ventral and dorsal aorta
- pH
a
arterial pH
-
PIO
2,
PEO
2
oxygen partial pressure of inspired and expired water
-
PO
2
oxygen partial pressure
-
cardiac output
- SEM
standard error of mean
- S.I.D.
strong ion difference
- SV
cardiac stroke volume
- TO
2
gill oxygen transfer factor
- U
oxygen extraction coefficient
- VA
ventilatory amplitude
- VF
ventilatory frequency
- VR
G, VR
S
branchial and systemic vascular resistances
-
ventilatory flow
-
ventilatory oxygen convection requirement
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