PRITCHARD. HUNTER, and LASKER: EXERCISE AND BIOCHEMICAL CHANGES 



than the controls; at this subthreshold speed 

 lactate levels of red and white muscle were 

 higher than the controls and differed statistically 

 (P = 0.02) . We have no explanation for these 

 differences except to suggest that the high 

 muscle lactate concentration in the control an- 

 imals may have obscured changes resulting from 

 moderate exercise. A larger sample size may 

 be required to obtain reliable measurements of 

 differences in lactic acid concentration caused 

 by moderate exercise. 



Muscle lactate level did not appear to be re- 

 lated to fatigue at any swimming speed. Lac- 

 tate levels in fish that fatigued at the threshold 

 speed were not different from the controls. Fish 

 that failed at superthreshold s])eeds had a higher 

 muscle lactate level than did the controls but 

 the level did not differ from that of fish that 

 swam at the same speed but were removed be- 

 fore they became exhausted. These results sug- 

 gest that high lactic acid concentration in muscle 

 was not the principal cause of exhaustion. 



Red muscle contained considerably more fat 

 per unit weight than white muscle. Indeed, 

 white muscle fat levels were almost undetectable 

 in many cases (Table 3). White muscle fat 

 levels did not differ from the control at any 

 speed level. Red muscle fat did not differ from 

 the controls at threshold and superthreshold 

 speeds but at the subthreshold speed the mean 

 level of fat in the red muscle was lower than 

 the controls and differed statistically from them 

 (P = 0.02). Thus only when the fish swam 

 for at least 6 hr at subthreshold speed was there 

 evidence of fat utilization in the red muscle.* 

 The reduction in fat in the red muscle suggests 

 that the red muscle system may have been used 

 at the subthreshold velocity. On the other hand, 

 presence of high muscle lactate in both red and 

 white muscle and the drop in red muscle and 

 liver glycogen at subthreshold speeds implies 

 that the white muscle was also active. 



Table 3. — Fat analyses in red and white muscle of jack 

 mackerel following various forced swimming conditions. 

 Where 0.0% is given for white muscle, only traces of fat 

 were found with the chloroform-methanol extraction. 

 For convenience zeros were used for averaging. Values 

 given as percent dry weight of tissue. 



' In an earlier and preliminary experiment, five 

 smaller jack mackerel, mean length 9.2 cm, swam at 

 the subthreshold speed of 12.7 L" Vsec (48 cm/sec) for 

 48 hr without failure and we recorded a decrease in the 

 mean fat content of red muscle from 23.7% (range, 20.4- 

 28 4%; K = 5) to 18.0% (range, 16.2-20.8%; n — 5) 

 (P<0.05). 



1 Differed from the controls, P = 0.02, Mann Whitney U lest (Siegel, 

 1956). 



DISCUSSION 



Control levels of jack mackerel white muscle 

 glycogen were similar to those recorded by Can- 

 adian workers for mixed red and white muscle 

 in salmonids (Black, Robertson, and Parker, 

 1961; Black et al, 1962; Connor et al, 1964) 

 and to those from a variety of marine teleosts 

 (Beamish, 1968; Eraser et al, 1966; Witten- 

 berger, 1968; Wittenberger et al., 1969). Red 

 muscle glycogen has not often been separately 

 determined. Our mean control value of 750 mg 

 percent was somewhat higher than the mean of 

 420 mg percent reported by Wittenberger 

 (1968) for Tnichuncs mediterrayieiis ponticus, 

 a related species from the Black Sea. Fraser 

 et al. (1966) gave a range of 215 to 279 mg per- 

 cent for red muscle glycogen of cod, based on 

 analysis of three fish in a relaxed (anesthesized) 

 state. Wittenberger et al. (1969) reported 320 



383 



