ABSTRACT
Glycolysis in muscle during the anaerobic component of severe exercise rapidly produces lactate (La−) and metabolic protons 
in equal amounts, some of which may enter the blood (Krebs, Wood & Alberti, 1975). In vivo experimentation using strenuously exercised fish has yielded two patterns: La− accumulating in blood either in greater or lesser amounts than 
. It was rare to find both species at the same concentration, as is the usual case in man (Keul, Keppler & Doll, 1967). In the dogfish (Piiper, Meyer & Drees, 1972) and rainbow trout (Turner, Wood & Clark, 1983a) after exercise, ΔLa− in blood greatly exceeded 
. The opposite discrepancy occurred in the starry flounder (Wood, McMahon & McDonald, 1977), muskellunge (Beggs, Holeton & Crossman, 1980), and flathead sole (Turner, Wood and Hōbe, 1983b), where ΔLa− was consistently lower than 
These discrepan cies could be caused either by differential removal rates of the two ionic species from the blood space or by differential release rates from the myotome. We have recently argued that the latter is the dominant process (Turner et al. 1983a,b). However, this is extremely difficult to prove with in vivo methodology, as measurements in a single body compartment with periodic sampling give little information about the dynamic equilibrium between production, metabolism and fluxes between compartments. In vitro muscle preparations can be used to overcome many of these difficulties (e.g., Mainwood & Worsley-Brown, 1975; Benadé & Heisler, 1978). Isolated-perfused rainbow trout trunks have been employed to examine the physiology of systemic vascular resistance control (Wood & Shelton, 1975; Wood, 1977) and substrate utilization by muscle (Moen & Klungsoyr, 1981). In the present study, by strenuously exercising intact trout, then rapidly preparing them as isolated-perfused trunks, it was possible to determine net 
and La− movement from muscle to perfusate during recovery while simultaneously monitoring muscle [La−] by repetitive biopsy.
