Distinguishable substrate pools for muscle glyconeogenesis in lactate-supplemented recovery from exercise

The formation of muscle glycogen from substrates other than glucose (glyconeogenesis) has now been demonstrated 1) from circulating lactate when this lactate is elevated and 2) from intramuscular substrate, which equilibrates with the products of local glucose metabolism but not with circulating lactate [Am. J. Physiol. 267 (Endocrinol. Metab. 30): E210-E218, 1994]. The purpose of the present studies was to examine the interaction of recovery from low-intensity exercise (4-h swim) and supplementation with exogenous lactate in determining the distribution of carbon flux between these two pathways for the glyconeogenic process in the gastrocnemius muscles. Ten protocols were defined using [14C]bicarbonate (no local incorporation into glycogen), [U-14C]lactate (tracks circulating lactate), and recycled [1-14C]glucose (tracks local substrate formation and glyconeogenesis). During recovery, lactate was infused to increase circulating concentrations 15- to 20-fold. Glucose and saline infusions during recovery were used as controls. The results indicate that prior exercise primarily promotes the local incorporation of recycled glucose label produced within the muscle into glycogen. Exogenous lactate stimulates the incorporation of circulating lactate into muscle glycogen. The contribution of the two substrate pools to glycogen synthesis appears to be additive, indicating the independence of muscle glycogenesis from these two sources.