Environmental versus functional hypoxia/anoxia in sole Solea solea: The lactate paradox revisited

The effects of hypoxia (12 and 6% air saturation), anoxia and forced exercise on intermediary metabolism were compared and the first 4 h of recovery after exhaustive exercise examined in common sole Solea solea. Intermediates of energy metabolism and lactate production were analyzed in white muscle, liver and blood. Rates of ATP production by anaerobic metabolism for the 4 different conditions were 52, 150, 1777, and 6834 pmol 100g(-1) h(-1), respectively. Total anaerobic ATP yield was 625, 1799, 3074, and 3132 mu mol ATP 100g(-1). Under hypoxia the energy consumption of the fish appeared to be depressed (i.e. below standard metabolic rate), while under anoxia the rate was at routine metabolic rate, and under forced exercise was twice the active metabolic rate. The muscle/blood lactate ratio after forced exercise was comparable to those found previously for other flatfish species; however, hypoxia exposure resulted in a completely inverse situation with blood lactate levels up to 20 mM, whereas anoxia resulted in an intermediate situation. The results challenge the present concept of lactate 'releasers' and 'non-releasers' describing the transfer of lactate from muscle tissue into the blood. It is concluded that the distinction between lacate 'releasers' and 'non-releasers' can no longer be applied on a species basis but must be related to type and intensity of the induced stress situation.