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

The 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%).