Effect of blood flow and muscle contraction on noradrenaline spillover in the canine gracilis muscle

Abstract

Many authors have reported that, during exercise, noradrenaline spillover increases and fractional extraction decreases. It has been suggested that the increase in blood flow to active muscles may contribute to these effects. Muscle contraction also causes changes in many factors that may affect noradrenaline spillover and fractional extraction. In this experiment, we studied the effect of muscle contraction and blood flow on noradrenaline and adrenaline spillover and fractional extraction in the in situ canine gracilis muscle. The low intensity stimulation protocol enabled us to have muscle contractions without any effect on the local concentration of noradrenaline, as measured by microdialysis, and noradrenaline spillover. Fractional extraction of both noradrenaline and adrenaline was unaffected by increasing blood flow three and four times its resting value. In addition, noradrenaline spillover was increased by the higher blood flow, from 188 to 452 pg·min-1 at rest and from 246 to 880 pg·min-1 during stimulation. Stimulation of muscle contraction caused a significant increase in fractional extraction of noradrenaline and a nonsignificant increase in adrenaline extraction. In addition, an adrenaline spillover was observed in certain conditions. In light of our results, it seems that blood flow may not be the main factor decreasing fractional extraction of noradrenaline during exercise. However, blood flow could contribute to the increase in noradrenaline spillover observed in the active muscles during exercise.Key words: skeletal muscle, spillover, fractional extraction, stimulation, adrenaline.