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

Abstract

AbstractThe aerobic metabolism of Bufo americanus is 0.56 ± S.E. 0.12 ml O2 g−1 h−1; lactate accumulates to 0.63 ± S.E. 0.06 mg lactate g−1, during a 10‐min 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 14C activity per gram. Considerable fractions of the tissue 14C content were isolated as glycogen (22–39%), lower fractions as protein (1–24%), 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 (<10% 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%).