Very large range of baroreflex sympathetic control of vascular resistance in human skeletal muscle and skin.

We analyzed in the forearm of "comfortably warm" male volunteers 1) reflex sympathetic vascular resistance changes evoked by short-term graded [1.5-min exposure to 15, 40, 55, and 70 mmHg and high and barely tolerated (77-95 mmHg)] lower body negative pressure (LBNP) and 2) resistance changes evoked by abolition of control sympathetic vasoconstrictor tone (anesthetic axillary nerve block). Graded LBNP caused graded neurogenic vasoconstriction with pronounced average flow decline at high LBNP from 3.7 to 0.8 ml.min-1 x 100 ml-1 (77 +/- 2% decrease), corresponding to a drastic resistance increase from 25.4 to 127 mmHg.ml-1 x min x 100 ml (352 +/- 27% rise above control). Axillary nerve block caused marked increases in forearm blood flow from 4.3 +/- 0.4 to 15.7 +/- 1.4 ml.min-1 x 100 ml-1 [> 3-fold flow increase equivalent to an average resistance decline from 19.7 to 6.3 mmHg.ml-1 x min x 100 ml (72 +/- 5%)], reflecting a surprisingly high resting constrictor fiber vascular tone. The overall results indicate that the sympathetic skeletal muscle and skin resistance vessel control in men allows very large (almost 20-fold) alterations in blood flow and vascular resistance from complete inhibition of neurogenic vascular tone to maximal reflex nerve activation. This range of sympathetic control was clearly greater than that revealed in comparative experiments on the cat lower leg.