Effect of tilting on adipose tissue vascular resistance and sympathetic activity in humans

To study baroreceptor reflex control of human adipose tissue circulation, head-up tilting (to 80 degrees for 10 min) was performed in eight healthy volunteers. Blood flow in subcutaneous adipose tissue and skeletal muscle on the arm was evaluated by local clearance of 99mTcO4-. The isotope depots were maintained at heart level throughout the experiment. Tilting induced rapid increases in plasma norepinephrine and epinephrine, heart rate, and diastolic but not mean arterial pressure. There was rapid vasoconstriction in both adipose tissue and skeletal muscle followed by vasoconstrictor escape during tilting and a brief period of hyperemia after tilting. The vascular resistance index [mean arterial pressure/disappearance rate (kappa) of 99mTc (%/min)] increased from 16 +/- 2 to 41 +/- 9 U (P less than 0.05) in adipose tissue and from +/- 3 to 44 +/- 15 U (P less than 0.05) in skeletal muscle. Tilting caused an initial decrease followed by an increase in arterial plasma glycerol without concomitant changes in plasma immunoreactive insulin. The vascular and glycerol responses to tilting closely resemble canine subcutaneous adipose tissue responses to sympathetic nerve stimulation. The present experiments show baroreceptor reflex-induced vasoconstriction in human adipose tissue, which presumably is mediated by sympathetic nerve activity.