Daily spontaneous running did not alter vagal afferent reactivity

Exercise training alters the cardiopulmonary baroreflex regulation of the circulation; however, the mechanisms responsible are unknown. One possibility is an enhanced afferent response to cardiopulmonary stimulation. We therefore tested the hypothesis that daily spontaneous running (DSR) would enhance cardiopulmonary vagal afferent responses to mechanical (increase in left atrial pressure, LAP) and chemical (phenyl biguanide, PBG) stimulation. Reactivity of single-fiber cardiopulmonary vagal afferents was evaluated in 16 control and 12 DSR anesthetized Sprague-Dawley rats. Rats were weaned at 3 wk of age and randomly assigned to a control or DSR group. Eight to twelve weeks of DSR was associated with a 27% increase in heart weight-to-body weight ratio (3.27 +/- 0.08 vs. 2.56 +/- 0.05 g/kg, P < 0.001) and resting bradycardia (394 +/- 10 vs. 421 +/- 8 beats/min, P = 0.036). However, DSR did not alter the stimulus-response curves to increases in LAP (frequency of discharge vs. LAP) for either the high-frequency (maximum response, sedentary 59.6 +/- 3.2, DSR 60.1 +/- 5.0 spikes/s) or low-frequency (maximum response, sedentary 20.0 +/- 2.9 DSR 20.6 +/- 3.9 spikes/s) receptors. Dose-response curves to chemical stimulation (spikes/s vs. PBG dose) were also not altered by DSR. Thus DSR did not change vagal afferent reactivity to mechanical or chemical stimulation.