FOREARM TRAINING ATTENUATES SYMPATHETIC RESPONSES TO PROLONGED RHYTHMIC FOREARM EXERCISE

We previously demonstrated that nonfatiguing rhythmic forearm exercise at 25% maximal voluntary contraction (12 2-s contractions/min) evokes sympathoexcitation without significant engagement of metabolite-sensi tive muscle afferents (B. A. Batman, J. C. Hardy, U. A. Leuenberger, M . B. Smith, Q. X. Yang, and L. I. Sinoway. J. Appl. Physiol. 76: 1077- 1081, 1994). This is in contrast to the sympathetic nervous system res ponses observed during fatiguing static forearm exercise where metabol ite-sensitive afferents are the key determinants of sympathetic activa tion. In this report we examined whether forearm exercise training wou ld attenuate sympathetic nervous system responses to rhythmic forearm exercise. We measured heart rate, mean arterial blood pressure (MAP), muscle sympathetic nerve activity (microneurography), plasma norepinep hrine (NE), and NE spillover and clearance (tritiated NE kinetics) dur ing nonfatiguing rhythmic forearm exercise before and after a 4-wk uni lateral forearm training paradigm. Training had no effect on forearm m ass, maximal voluntary contraction, or heart rate but did attenuate th e increase in MAP (increase in MAP: from 15.2 +/- 1.8 before training to 11.4 +/- 1.4 mmHg after training; P < 0.017), muscle sympathetic ne rve activity (increase in bursts: from 10.8 +/- 1.4 before training to 6.2 +/- 1.1 bursts/min after training; P < 0.030), and the NE spillov er (increase in arterial spillover: from 1.3 +/- 0.2 before training t o 0.6 +/- 0.2 nmol . min(-1). m(-2) after training, P < 0.014; increas e in venous spillover: from 2.0 +/- 0.6 before training to 1.0 +/- 0.5 nmol . min(-1). m(-2) after training, P < 0.037) seen in response to exercise performed by the trained forearm. Thus forearm training reduc es sympathetic responses during a nonfatiguing rhythmic handgrip parad igm that does not-engage muscle metaboreceptors. We speculate that thi s effect is due to a conditioning-induced reduction in mechanically se nsitive muscle afferent discharge.