The purpose of this study was to investigate the specific role of

leucine in the stimulation of the mammalian target of rapamycin

signalling pathway. Six male subjects performed four heavy

resistance exercise sessions, each separated by approximately one

week. Subjects were randomly supplemented with one of four

drinks: placebo (flavored water), leucine or essential amino acids

(EAA) with and without leucine. Immediately following each

exercise session, four subjects were infused with a flooding dose of

L-[2H5] phenylalanine (Inf) while two subjects served as controls

(Ctrl). Muscle biopsies were taken before and one hour after

exercise. In the Ctrl group, resistance exercise resulted in a

substantial increase (45-fold) in p70 kinase phosphorylation

when all EAA were ingested, whereas ingestion of leucine alone

had no greater effect than that of placebo. In the Inf group,

however, ingestion of leucine alone and EAA increased p70

phosphorylation to a similar extent (35-fold). The divergent

signalling response in the two groups suggests that leucine alone is

insufficient to increase p70

phosphorylation. Indeed, in the Inf

group, there was a strong correlation (r=0.91) between

p70 phosphorylation and the product of muscle leucine and

phenylalanine levels. These results suggest that the stimulatory

effect of leucine on p70 phosphorylation is dependent on

elevated muscle phenylalanine levels. Supported by the Swedish

National Centre for Research in Sports