Naloxone does not affect the cardiovascular and sympathetic adjustments to static exercise in humans

Previous studies suggested that endogenous opiates may attenuate the cardiovascular and sympathetic adjustments to static exercise. We tested whether this effect originates from exercising skeletal muscle. Eight men performed 2 min of static handgrip (30% maximum) followed by 2 min of posthandgrip muscle ischemia after three interventions: 1) control, 2) intra- arterial injection of naloxone HCl (60 μg) or vehicle (saline) in the exercising arm, and 3) systemic infusion of naloxone (4 mg) or vehicle. Naloxone and vehicle trials were performed double blind on separate days. Preexercise baseline muscle sympathetic nerve activity (burst frequency), heart rate, and blood pressure were similar across interventions on either day. During static handgrip, control, intra-arterial, and systemic administration of vehicle and naloxone elicited similar increases in total muscle sympathetic nerve activity (58 ± 24 vs. 68 ± 26, 146 ± 49 vs. 132 ± 42, 137 ± 54 vs. 164 ± 44%, respectively), heart rate (9 ± 2 vs. 8 ± 3, 16 ± 3 vs. 16 ± 2, 20 ± 4 vs. 19 ± 3 beats/min, respectively), and mean arterial pressure (22 ± 4 vs. 21 ± 4, 29 ± 5 vs. 26 ± 3, 28 ± 4 vs. 27 ± 4 mm Hg, respectively). Additionally, there were no differences between vehicle and naloxone trials during posthandgrip muscle ischemia. Thus, contrary to previous reports, we conclude that the endogenous opiate peptide system does not modulate cardiovascular and sympathetic responses to brief periods of static exercise or muscle ischemia in humans.