Identifying the role of group III/IV muscle afferents in the carotid baroreflex control of mean arterial pressure and heart rate during exercise

This study sought to comprehensively investigate the role of metabolically and mechanically sensitive group III/IV muscle afferents in carotid baroreflex responsiveness and resetting during both electrically evoked (EVO, no central command) and voluntary (VOL, requiring central command) isometric single-leg knee-extension (15% of maximal voluntary contraction; MVC) exercise. Participants (n=8) were studied under control conditions (CTRL) and following lumbar intrathecal fentanyl injection (FENT) to inhibit -opioid receptor-sensitive lower limb muscle afferents. Spontaneous carotid baroreflex control of mean arterial pressure (MAP) and heart rate (HR) were assessed following rapid 5s pulses of neck pressure (NP, +40mmHg) or suction (NS, -60mmHg). Resting MAP (8710mmHg) and HR (708bpm) were similar between CTRL and FENT conditions (P>0.4). In terms of spontaneous carotid baroreflex responsiveness, FENT did not alter the change in MAP or HR responses to NP (+135mmHg, P=0.85; +93bpm; P=0.99) or NS (-135mmHg, P=0.99; -2411bpm; P=0.49) at rest or during either exercise protocol, which were of a remarkably similar magnitude to rest. In contrast, FENT administrationreduced the exercise-induced resetting of the operating point for MAP and HR during both EVO (116 +/- 10mmHg to 100 +/- 15mmHg and 93 +/- 14bpm to 82 +/- 10bpm) and VOL (107 +/- 13mmHg to 100 +/- 17mmHg and 89 +/- 10bpm to 72 +/- 10bpm) exercise bouts. Together, these findings document that group III/IV muscle afferent feedback is critical for the resetting of the carotid baroreflex MAP and HR operating points, independent of exercise-induced changes in central command, but not for spontaneous carotid baroreflex responsiveness.