Metabolic fates of lactate during recovery from activity in an anuran amphibian, Bufo americanus

S.E. 0.12 ml 02 g-' h-'; lactate accumulates to 0.63 f S.E. 0.06 mg lactate gpl, during a 10-niin bout of enforced activity. About 20% of the energy expended during activity is derived from anaerobic metabolism and 80% from aerobic metabolism. Aerobic metabolic rate decreases rapidly after the cessation of activity and reaches resting levels by 1 hr post-activity. Whole body lactate content declines more slowly and in a linear fashion, reaching resting levels about 1 hr post-activity. About 7% or less of the 14C-lactate administered after activity was estimated to have been eliminated by aerobic metabolism of the lactate; the remaining 14C activity was recovered in body tissues and urine or was excreted across the skin. Most tissues examined (liver, skin, ventricle, lungs, stomach, intestine) had similar 14C activities per gram of tissue, but skeletal muscle had a higher I4C activity per gram. Considerable fractions of the tissue 14C content were isolated as glycogen (22-39%)' lower fractions as protein (124%), and little as lipid (2-4%). There are some differences in the distribution of 14C label, depending on whether 14C-lactate or 14C-glucose were administered at rest, or after activity, but little of the 14C label was aerobically metabolised for either substrate. The observed fraction of lactate oxidised by aerobic metabolism (<lo% of total 14C lactate) is less than the theoretical value for the most energetically economical metabolism of lactate (17%: a Meyerhof quotient of 5) and considerably less than the fraction of lactate oxidised by mammals (20-90%). The aerobic metabolism of Bufo americanus is 0.56 The aerobic and anaerobic metabolic capacities (Cushman et al., '76; Jackson and Silverblatt, '74; of amphibians have received considerable atten- Quinn and Burggren, '83). In vitro studies of the tion during the last decade. Aerobic metabolic rate fate of lactate accumulated during electrical stimincreases five- to tenfold during activity and rap- ulation of isolated amphibian muscle indicate that idly declines to resting values after the cessation a considerable fraction of the lactate is reconverted of activity (Seymour, '73; Hillman, '76; Hillman to glycogen (Meyerhof, '20; Bendall and Taylor, and Withers, '79; Hillman et al., '79; Withers, '80; '70; Connett, '791, but it is not known if this occurs Withers and Hillman, '81; Quinn and Burggren, invivo. '83). Anaerobic metabolism elevates whole body We undertook the present study to determine the lactate levels by 5-20-fold during activity; lactate metabolic fates of lactate in anuran amphibians levels decline slowly after the cessation of activity after the cessation of strenuous activity, using I4C(Bennett and Licht, '74; Hutchison and Turney, radioisotope tracer studies. We investigated the '75; Cushman et al., '76; Putnam, '79a,b; F'reslar following potential in vivo fates for lactate in the and Hutchison, '78; Hutchison and Miller, '79a,b; American toad (Bufo arnericanus) after 10 min of Quinn and Burggren, '83). activity: 1) oxidation to CO,; 2) renal and cutaThe in vivo fate of the lactic acid that is accu- neous excretion; 3) conversion to glycogen; 4) conmulated during activity is unclear. The time course version to lipid; and 5) conversion to protein. and magnitude of changes in V02 and whole body lactate Concentration after activity Ceases are COncop and conversion of the remainder to glucose or