Skeletal muscle and skin as targets for powerful homeostatic vasomotor baroreflexes in humans during prolonged circulatory stress: a study on the innervated and nerve blocked forearm.

Flow (vascular resistance) was followed in the innervated and axillary nerve blocked arm during prolonged low to high and barely tolerated circulatory stress [15-85 mmHg LBNP (lower body negative pressure) for 10 min; room temperature 24.8-25.7 degrees C]. With intact innervation LBNP caused initial graded and potent forearm vasoconstriction. At low LBNP, however, there was soon significant and maintained partial (50%) abolition of the early response. At high LBNP, the initial striking vasoconstriction remained constant throughout 10 min of pronounced circulatory stress [marked tachycardia; fall in systolic pressure but mean arterial pressure (MAP) normal]. Flow decreased in steady state by 15 +/- 4, 38 +/- 5, 63 +/- 2 and by pronounced 78 +/- 3% at 15, 40, 70, and 85 mmHg LBNP (resistance raised 27 +/- 7, 78 +/- 16, 192 +/- 18, and 387 +/- 55% above control), alterations ascribed to constriction in both muscle and skin. Comparison of LBNP responses with intact and blocked innervation revealed that the vasoconstriction was neurogenic with little or no humoral contribution. The overall observations show that under normal comfortable (thermoneutral) conditions the resistance vessels in muscle and skin, with haemodynamically important large tissue mass and great tolerance to even drastic and prolonged ischaemia, indeed are important targets in the homeostatic sympathetic control, especially when cardiovascular homeostasis is challenged by marked stress with urgent need for strong, maintained compensatory vasoconstriction. The study also demonstrated > three-fold (4.1 +/- 0.5 to 13.1 +/- 1.9 ml min-1 100 ml-1) forearm flow increases upon blockade of resting nervous vasoconstrictor tone. It thus appears that the sympathetic nerves not only can elicit prominent and maintained baroreflex limb vasoconstriction but also that, in humans, reflex inhibition of resting tone might allow surprisingly large resistance decline.