Alanine has long been recognized as an important substrate for hepatic gluconeogenesis through the glucose-alanine (Cahill) cycle which plays an important role in the maintenance of euglycemia during times of caloric deficiency. The Cahill cycle involves the transamination of pyruvate by the amino group of glutamate, producing alpha-ketoglutarate and alanine. Alanine formed in skeletal muscle during exercise can be sent to the liver where it is used to produce glucose and safely remove the NH3+ as urea. This process is catalyzed by the glutamic-pyruvic transaminase (GPT) enzyme, of which two distinct isoforms exist: cytosolic GPT1 and mitochondrial GPT2. However, the precise role of these different enzymes in alanine metabolism remains to be fully elucidated and is an ongoing subject of debate. Likewise, the potential efficacy of exogenous alanine administration as a strategy to improve skeletal muscle glycogen recovery following exercise has not been examined. The following studies were conducted to: 1) evaluate the metabolic effects of L-alanine administration following a bout of exhaustive exercise and 2) determine the role hepatic GPT2 plays in gluconeogenesis from alanine during exercise.

Administration of L-alanine to C57BL/6 mice kept fasted after an exhaustive bout of exercise did not significantly alter glycogen content in the gastrocnemius during 1 hour of recovery; despite the observation that blood glucose concentrations were elevated at this time compared to mice treated with sterile saline. In addition, treatment with L-alanine resulted in significantly increased blood lactate concentrations at 30 and 60 minutes of recovery.

Liver specific GPT2--/-- mice are overtly normal and survive to adulthood with normal exercise tolerance. Gene expression analysis by qPCR reveals LS-GPT2--/-- mice have higher levels of GPT1 mRNA, which may act to compensate for the loss of GPT2. Indeed, liver specific deletion of GPT2 and the mitochondrial pyruvate carrier 2 (MPC2) resulted in reduced exercise time to exhaustion. Impaired gluconeogenesis was also observed in double knockout mice following 1 hour of recovery from exercise in the fasted state.

These studies demonstrate that immediately following exercise alanine is not a limiting substrate for skeletal muscle glycogen replenishment or hepatic gluconeogenesis. In addition, we show that loss of GPT2 alone is not sufficient to reduce exercise performance or gluconeogenesis due to compensatory changes in gene expression.