Estimation of Skeletal Muscle Interstitial Adenosine During Forearm Dynamic Exercise in Humans

Abstract

Abstract —It has been proposed that adenosine is a metabolic signal that triggers activation of muscle afferents involved in the exercise pressor reflex. Furthermore, exogenous adenosine induces sympathetic activation that mimics the exercise pressor reflex, and blockade of adenosine receptors inhibits sympathetic activation induced by exercise. Thus, we hypothesize that adenosine is released locally by the muscle during exercise. We used microdialysis probes, placed in the flexor digitorium superficialis muscle, to estimate muscle interstitial adenosine levels in humans. We estimated resting in vivo muscle interstitial adenosine concentrations (0.292±0.058 μmol/L, n=4) by perfusing increasing concentrations of adenosine to determine the gradient produced in the dialysate. Muscle interstitial adenosine concentrations increased from 0.23±0.04 to 0.82±0.14 μmol/L (n=14, P <0.001) during intermittent dynamic exercise at 50% of maximal voluntary contraction. Lactate increased from 0.8±0.1 to 2.3±0.3 mmol/L ( P <0.001). Lower intensity (15% maximal voluntary contraction) intermittent dynamic exercise increased adenosine concentrations from 0.104±0.02 to 0.42±0.16 μmol/L (n=7). The addition of ischemia to this low level of exercise produced a greater increase in adenosine (from 0.095±0.02 to 0.48±0.2 μmol/L) compared with nonischemic exercise (0.095±0.02 to 0.25±0.12 μmol/L). These results indicate that microdialysis is useful in estimating adenosine concentrations and in reflecting changes in muscle interstitial adenosine during dynamic exercise in humans.