Metabolism of Isotopic Lactate by the Isolated Perfused Dog Gastrocnemius

The metabolism of C 13 -lactate was studied in the isolated perfused dog gastrocnemius. Under the conditions employed, 12–13% of the respiratory CO 2 was derived from the administered lactate (carboxyl-labeled) in unstimulated muscles, whereas 25–29% was so derived in muscle contracting at a rate of 2/sec. (from either carboxyl or α,ß-labeled lactate). Thus, the increase in the rate of conversion of lactate to CO 2 associated with contraction was greater than the increase in the rate of CO 2 production. Evidence for the conversion of administered lactate to glycogen was found only from the α,ß-labeled compound in unstimulated preparations given insulin and in the presence of moderate blood glucose levels. Such conversion was not demonstrated in any of the repetitively contracting muscles given carboxyl- or α,ß-labeled lactate or in unstimulated preparations administered carboxyl-labeled lactate.