HOW SHEATFISH, SILURUS-GLANIS, MAINTAINS OXYGEN-SUPPLY IN HYPOXIA - AKEY EXAMPLE OF OXYGEN HOMEOSTASIS

How oxygen is supplied independently of water oxygenation in water bre athers is an age-old problem of great interest to fish farmers, ecolog ists and fish physiologists alike. This review paper (i) summarizes th e strategy used by the sheatfish Silurus glanis to deal with this prob lem and also (ii) considers the case of S. glanis within a general fra mework that explains how O-2 is supplied in many water breathers. One of the basic concepts that we have elaborated during the past 10 years , partly thanks to the sheatfish model, deals with the notion of O-2 h omeostasis in the internal medium. We demonstrated that, in a wide ran ge of physiologically different water-breathers under resting conditio ns, a low arterial Po-2 (1-3 kPa at 13-15 degrees C; 1 kPa = 7.5 mm Hg ) set most of the time just above the arterial Po-2 at the anaerobic t hreshold (0.7-1.2 kPa), provides a sufficient pressure head to ensure an adequate O-2 supply at the cellular level. This is largely independ ent of Po-2 changes in the external medium in the range 3-40 kPa (i.e. 1.5-20 mg.l(-1) at 13 degrees C). The sheatfish Silurus glanis has a resting Oz consumption that is quite comparable to values measured in teleosts like the carp, eel or tench and in crustaceans and is approxi mately half of the consumption reported for salmonids. The sheatfish i s able to maintain this O-2 consumption constant for at least 24 h dow n to a critical inspired Po-2 = 2-3 kPa (1-1.5 mg.l(-1) at 13 degrees C). Ventilatory flow rate plays a fundamental role in this maintenance while the inspired Pot varies in the external medium. Indeed, ventila tion increases in hypoxic water and decreases in hyperoxic water while the blood flow rate remains constant. The main result of this ventila tory adaptation is the constancy of the arterial Po-2 at 2 kPa, that i s maintained independently of the inspired Po-2 in the range 3-40 kPa (ie. 1.5-20 mg.l(-1) at 13 degrees C). In this inspired Po-2 range the blood pH and the blood O-2 content also remain constant. Consequently , in terms of O-2, the homeostasis of the internal medium is fulfilled . Although we have not studied these mechanisms in Silurus at temperat ures other than 13 degrees C, for temperature changes in the range 10- 23 degrees C at least, it is reasonable to assume that the arterial Po -2 and the critical water Po-2 over which O-2 consumption can be maint ained constant probably remains steady. This conclusion is based on a comparison with reported data in the literature which cover a large ra nge of physiologically different water-breathers. First, in the carp C yprinus carpio, the crayfish Astacus leptodactylus and the crab Carcin us maenas, the arterial Po-2 remains in the same low range whatever th e temperature change from 10-13 degrees C to 23-24.5 degrees C. Second , in the carp Cyprinus carpio, the trout Oncorhynchus mykiss and the c rayfish Astacus leptodactylus, the resting O-2 consumption can be main tained constant down to the same critical Po-2 (3 kPa), independently of the temperature from 10-13 to 20-23 degrees C. It is concluded that the sheatfish Silurus glanis at rest has a substantial capacity for w ithstanding prolonged periods of hypoxia down to Po-2 = 2-3 kPa (1-1.5 mg.l(-1) depending on the temperature). However, we stress that curre ntly we have no idea of this potential in animals reared in intensive fish farms where gills could be physiologically altered or other patho logical disorders could have developed.