Forearm blood flow responses to handgripping after local neuromuscular blockade.

To test the hypothesis that acetylcholine "spillover" from motor nerves contributes significantly to skeletal muscle vasodilation during exercise, we measured the forearm blood flow responses during attempted handgripping after local paralysis of the forearm with the neuromuscular-blocking drug pipecuronium. This compound blocks postsynaptic nicotinic receptors but has no impact on acetylcholine release from motor nerves. The drug was administered selectively to one forearm by using regional intravenous drug administration techniques in five subjects. Pipecuronium reduced maximum forearm grip strength from 40.0 +/- 3.2 kg before treatment to 0.0 kg after treatment. By contrast, drug administration had no effect on maximum voluntary contraction in the untreated forearm (41.3 +/- 3.3 vs. 41.4 +/- 2.7 kg). During 2 min of attempted maximal contraction of the paralyzed forearm, the forearm blood flow increased from only 3.4 +/- 0.8 to 4.8 +/- 1.2 ml.100 ml-1.min-1 (P < 0.05). Heart rate increased from 63 +/- 3 to 73 +/- 8 beats/min (P > 0.05) during attempted contraction, and only three of five subjects showed obvious increases in heart rate. Mean arterial pressure increased significantly (P < 0.05) from 102 +/- 6 to 109 +/- 9 mmHg during attempted contractions. When these increases in flow are considered in the context of the marked (10-fold or greater) increases in flow seen in contracting forearm skeletal muscle, it appears that acetylcholine spillover from motor nerves has, at most, a minimal impact on the hyperemic responses to contraction in humans.