tions. Oxygen consumption, oxygen utilisation co- efficient and ventilatory flow rate were calculated from these data. Hypercapnia (Pwco =5 Tort) markedly in-.
Journal of Comparative Physiology. B
J Comp Physiol (1983) 151:185-190
9 Springer-Verlag 1983
Comparison of the Effects of Exogenous and Endogenous Hypercapnia on Ventilation and Oxygen Uptake in the Rainbow Trout (SMmogMrdneri R.) S. Thomas 1, B. Fievet 1, L. Barthelemy 2, and C. Peyraud 1 1 Laboratoire de Physiologic Animale*, Facult6 des Sciences et Techniques, F-29283 Brest C6dex, France 2 Laboratoire de Physiologic*, Facult6 de M~decine, F-29200 Brest, France Accepted December 5, 1982
Summary. Ventilatory frequency, oxygen tensions of inspired and expired water and of water entering and leaving the experimental chamber were measured for trout in hypercapnic or hyperoxic conditions. Oxygen consumption, oxygen utilisation coefficient and ventilatory flow rate were calculated from these data. Hypercapnia (Pwco =5 Tort) markedly increased ventilation even after 72 h of hyperoxia while 21;/o2 remained unchanged. Thus external CO 2 stimulates ventilation even when the fish is breathing hyperoxic water and blood Pco~ is elevated. Hyperoxia (PWo=400~450Torr) decreased ventilation but increased ~;/o2The changes in A~/o~are discussed in comparison with the changes in ventilatory activity.
Introduction It is generally accepted that in fish an increase in exogenous CO 2 leads to hyperventilation which is more or less developed for different species and depends upon the magnitude of imposed Pwco ~ variations (Cameron and Randall 1972; Janssen and Randall 1975; Eddy etal. 1977; Saunders 1962; Peyraud and Serfaty 1964; Dejours 1975). In contrast, hyperoxia provokes a marked hypoventilation although inducing a secondary rise in Paco2 in carp (Dejours 1973) or trout (Wood and Jackson 1980). However, a revival of ventilatory * Equipe de recherche associ~e du Centre National de la Recherche Scientifique No. 07622
activity because of this endogenous hypercapnia has never been noticed (Dejours 1975; Dejours et al. 1977). Most authors infer from these observations that CO 2 is not a strong stimulus for ventilation in fish. One question is whether CO2 has different effects according to its endogenous or exogenous origin, or if the difference observed in these two cases is attributable to the presence of hyperoxia. In a recent study, Wood and Jackson (1980) have presented evidence for an internal diffusive or perfusive limitation for CO 2 excretion associated with branchial constriction during hyperoxia. Thus it seems that in order to dissociate the effects of endogenous hypercapnia from those of hyperoxia, one experimental possibility would be to place the fish in hyperoxic conditions and to test the effects of exogenous CO2 on ventilation. The purpose of the present work is to compare the effects of three types ofhypercapnia on ventilation and oxygen uptake in trout (Salmo gairdneri R.): 1. Exogenous hypercapnia obtained by increasing Pwco 2. 2. Endogenous hypercapnia resulting from hyperoxia. 3. Exogenous hypercapnia associated with endogenous hypercapnia induced by hyperoxia.
Materials and Methods Experimental Animals Rainbow trout (Salmo gairdneri R.) (400 600 g) were obtained from a hatchery (Lesneven, Brittany) and acclimated to laboratory conditions in tanks supplied with city tap water at 15 ~ (ionic composition (main ions): Na +, 1.0mEq.l-X; K +, 0 m E q - l - 1 ; Ca ++ , 0.8mEq.1 1; Mg++, 0 . 5 m E q . l - 1 ; CI-, 1.0 mEq- 1-1 ; SO4 -, 0.6 mEq. 1 1; NO3, 0.3 mEq-1- 1).
186
S. Thomas et al. : Exogenous and Endogenous Hypercapnia in Trout
Trout were anaesthetized with neutral MS 222 (0.1 g.1-1) and operations were performed under artificial ventilation with aerated water containing a maintenance dose of neutralised MS 222 (0.01 g" L - ~). PE 90 catheters were placed in the buccal cavity transpalatinally and behind the cleithrum bone for the continuous sampling of inspired and expired water, respectively. In each series, some fishes were implanted with catheters (Clay-Adams PE 90) in the subclavian artery as described by Thomas et al. (1980). Surgical areas were treated with antibiotics (penicillin-streptomycin mixture) in order to prevent postoperative infections. After the operation, the fish were allowed to recover in experimental perspex chambers supplied by tap water at a rate of 1 1. min-~. The dimension and form of the chambers were such that the fish remained unrestrained but were unable to turn. Before any measurements were made, a recovery period of 48 h was allowed. Experimental Protocol
Trout were submitted to three types of conditions after reference measurements in normoxia (Pwo2 = 155 Torr) and normocapnia (Pwco 2= 0.25 Torr) had been performed: Series I: 3 days in hypercapnic water (Pwco ~= 5 Torr). Series H: 3 days in hyperoxia (Pwo2 =400~450 Torr). Series H h trout were submitted to 72 h in hyperoxic water (Pwo2 = 400-450 Torr) while hypercapnic tests (Pwco 2 = 5 Torr) were performed at I h, 24 h, 36 h, 48 h and 72 h. Each test lasted one hour with the intention of determining whether exogenous hypercapnia can stimulate ventilation under hyperoxic conditions. During the measurements water was passed at a constant rate (5 ml/min) by a peristaltic pump via the buccal or opercular catheter through the Po2 measuring cell (Radiometer PHM 71 analyser and E 5036 radiometer electrodes). In each series 5 fish were fitted with an arterial catheter in order to monitor blood acid-base variations within the first 24 h. Blood Pao2 measurements were made using Po2 measuring cells (Radiometer PHM 71 analyser and E 5036 electrodes). Dorsal aortic blood pH was measured using a Radiometer BMS 2MK2 analyser and Paco ~ calculated using the Astrup method (Astrup 1956). The measuring devices were frequently calibrated either by using blood or water samples previously equilibrated in vitro with water saturated gas mixtures obtained from W6sthoff pumps with appropriate Po2 and Pco~, or by using buffer solutions for pH equilibration. The measuring ceils were kept at the same temperature as the fish (15 ~ Measurements of Pwo2 inlet (water entering the experimental chamber), Pwo: inspired, Pwo~ expired and Pwo~ outlet (water leaving the chamber) allowed calculation of the util•177 or extraction coefficient:
Uwo~=
Pinup - Poxp
P/nsp and oxygen consumption:
Severinghaus et al. (1956a, b) and Wood and Johansen (1973) (~CO2=54 pM'1-1 .Torr 1 at 15 ~ and pK] and pK~ values (6.24 and 9.80 at 15 ~ were experimentally determined by using the method of Siggaard-Andersen (1963) as modified by Truchot (1974). Normal haematocrit (Hct) in trout being close to 20%, animals exhibiting Hct below 10% were excluded. The significance (P < 0.05) of the changes occurring was tested using Student's t-test.
Results
Series I
Table I summarises the changes observed in the acid-base status of trout blood after 1 h and 24 h of hypercapnia. A rise in Pwco 2 initially induces a phase of hypercapnic acidosis characterised by a decrease in pHa and a rise in Paco 2. A second phase of pH compensation occurs later correlated Table 1. Mean values_S.E, of water Po2, Pco2 and pH and blood acid-base characteristics at zero time and I and 24 h after transfer from normocapnic water (t= 0) to hypercapnic water (n=5)
t=0
t=l h
t=24h
Pwo2 (Torr)
155 •
155 +5
155 •
5.00 +0.15
•
Pwco 2 (Torr)
0.25
pHw
7.90
5.00
•
•
6.66 _+0.05
110
_+5
135 __5
115 _+6
Paco 2 (Torr)
2.20 • 0.30
6.60 • 0.20
7.10 + 0.20
pHa
7.92 _+0.05
7.65 • 0.04
7.82 -+ 0.04
[HCO3] + [CO3 -] (mEq-1-1)
7.30 • 0.70
10.00 • 0.50
17.80 • 0.50
Pao2 (Tort)
6.66
Table 2. Mean values • S.E. of ventilatory characterisctics and oxygen consumption of fish breathing hypercapnic water for 72 h, Those values that are not significantly different (P
