An in vivo examination of the effects of leucine on skeletal muscle protein synthesis in the fasting rat

Abstract

Leucine has been shown to stimulate protein synthesis in in vitro muscle preparations. However, in vivo studies have been unable to reproduce these results consistently. The purpose of the current study was to examine the effects of leucine on in vivo protein synthesis using a bolus injection of 14C-tyrosine as a tracer. Intraperitoneal injections of leucine resulted in a linear increase in muscle free leucine up to 6 times control values. Leucine stimulated protein synthesis in skeletal muscles from fasted rats. The extensor digitorum longus and plantaris synthesis rates in 1-day fasted rats were increased with leucine in the absence of added insulin, while synthesis rates in the soleus remained unchanged. In fed rats, there was no stimulation of protein synthesis with leucine administration. These results indicate that leucine has the potential to stimulate muscle protein synthesis in vivo, however, the response is dependent on the nutritional status and the muscles selected.