J Comp Physiol B (1985) 156:192203 Journal of Comparative Biochemica[, Systemic, and Environ- Physiology B monta, Physiology 9 Springer-Verlag 1985 Kinetics of the acclimational responses of tench to combined hypoxia and hypercapnia I. Respiratory responses Frank B. Jensen and Roy E. Weber Institute of Biology,Odense University, DK-5230 Odense M, Denmark Accepted July 7, 1985 Summary. Exposing tench to environmental hy- poxia-hypercapnia reduces routine O2 consump- tion, sharply decreases arterial 02 tension and the Po2 difference between the water and the blood, and results in marked swelling of the erythrocytes. These changes are rapidly reversed upon return to normoxia. Hypoxic-hypercapnic conditions lower the blood NTP/Hb ratio to a new steady state level within 24 h, by reducing GTP/Hb but not ATP/ Hb. A similar selective reduction of eryhtrocytic GTP content forms the initial response of blood incubated in vitro to anoxic conditions. The swelling as well as the reduced GTP/Hb ratio in the erythrocytes appear to improve 02 loading in the gills during environmental hypoxia- hypercapnia. Introduction The frequent occurrence in aquatic environments of hypoxia, which may drastically hamper meta- bolic performance, has resulted in many studies dealing with the acute effects of hypoxia in fish, and adaptational responses observed in the various aspects of its respiratory physiology (e.g. Holeton and Randall 1967; Wood and Johansen 1972; Weber and Lykkeboe 1978; Lomholt and Johan- sen 1979; Booth 1979; Soivio and Nikinmaa 1981). Due to the inherent reciprocality of environmental O2 and CO2 changes, originating in microbial, as well as animal, respiratory processes, however, en- vironmental hypoxia will almost invariably be as- Symbols and abbreviations: a arterial; GTP guanosine triphos- phate; Hct hematocrit; I inspired; NTP nucleoside triphos- phate; w water sociated with a degree of hypercapnia. This partic- ularly applies to stagnant waters, or waters partly covered by floating aquatic plants, as those often inhabited by the tench. Whereas normoxic hyper- capnia has frequently been used to perturb acid- base balance and to induce regulatory acid-base responses (reviewed by Heisler 1984), the ecophy- siologically relevant condition of combined hy- poxia and hypercapnia has only received minor attention (Jensen and Weber 1982; Thomas 1983). Tench acclimated to hypoxia-hypercapnia show pronounced capacity for adaptive responses in the respiratory properties and acid-base status of the blood (Jensen and Weber 1982). In order to understand these adaptational processes, and the underlying mechanisms, we have studied the time courses of the compensatory changes induced by hypoxia-hypercapnia, in the metabolic rate and blood respiratory properties (present paper) and in the extra- and intracellular acid-base status of the blood (Jensen and Weber 1985 b). Materials and methods In vivo experiments Animals and experimental set-up. These experiments were car- ried out in the summerof 1984 on tench (Tinca tinca), 541 • 53 g (x• N=6) in weight, which were obtained from ponds near Gr~sten, Southern Jutland in Denmark. The fish were kept at least 14 days before experimentation in holding tanks with normoxicwater (Po2 > 130 mmHg) at 16 ~ and were sub- jected to a 12 h light - 12 h dark rhythm. Followinganaesthetization in MS 222 the fish were placed on an operating table and the dorsal aorta cannulated accord- ing to Soivio et al. (1975). The fish were then placed in an experimental chamber with recirculating normoxic water, and allowed to recover for 48 h. The recirculating system consisted of an equilibration aquarium that was thermostated at 16 ~ and connected, via gastight tubing, to the experimental chamber, which was sub- merged in another aquarium, that also was kept at 16 ~