Autonomic blockade and cardiovascular responses to static exercise in partially curarized man.
Level 3 - non-randomized controlled study
Controlled within-subject physiological experiment with pharmacological blockade in humans
PubMed 2600859 · doi:10.1113/jphysiol.1989.sp017662
What was done
Seventeen human subjects performed static handgrip contractions for 2 min at 15% of initial maximal voluntary contraction strength before and during partial curarization with tubocurarine. Heart rate, mean arterial pressure, and rate of perceived exertion were measured. Subsets of subjects also performed exercise during curarization with pharmacological autonomic blockades: atropine (n=7), propranolol (n=7), phentolamine (n=7), atropine plus propranolol (n=14), or atropine plus phentolamine (n=5).
What was found
In control contractions (perceived exertion 10 units), mean arterial pressure increased by 6 mmHg from a baseline of 89 mmHg with no change in heart rate (resting 68 beats min-1). With tubocurarine, perceived exertion rose to 16 units for maintained contractions and 20 units for non-maintained contractions. Correspondingly, mean arterial pressure increased by 11 mmHg and 15 mmHg, and heart rate increased by 8 beats min-1 and 16 beats min-1, respectively. Atropine increased resting heart rate to 107 beats min-1 and mean arterial pressure to 98 mmHg, leaving the exercise pressor response unaffected while reducing the exercise heart rate increase from 15 to 4 beats min-1. Propranolol lowered resting heart rate to 56 beats min-1 and phentolamine lowered resting blood pressure to 80 mmHg, but neither altered exercise responses. Combined blockades showed similar patterns to the individual agents.
Why it matters
This study demonstrates that central command directly drives cardiovascular responses during static exercise when perceived effort is elevated relative to muscle force. It indicates that the heart rate increase is largely mediated by vagal withdrawal, whereas the blood pressure response relies on redundant autonomic mechanisms not blocked by single or dual receptor antagonists.
Limits
The sample size was small (17 participants overall, with drug subgroups ranging from 5 to 14). Demographics such as age and sex were not reported in the abstract. Variance measures and statistical significance values were omitted from the abstract, and partial curarization is an artificial experimental model of neuromuscular weakness.
Cited by
- supports Experiments by Jere Mitchell and Niels Secher using curare nerve blocks demonstrated that attempt to contract paralyzed muscle causes heart rate to rise through Central Command independent of actual muscle contraction.