POSTURAL VASCULAR-RESPONSE VS SYMPATHETIC VASOCONSTRICTION IN HUMAN SKIN DURING ORTHOSTASIS

Sympathetic activation and local vascular smooth muscle reactions to v essel distension contribute to the increase in vascular resistance in the skin during orthostasis. The relative contribution of these two me chanisms to the changes of skin blood flow along the body axis on stan ding was investigated in healthy male subjects by laser-Doppler (LD) f luxmetry. Compared with recumbency, LD flux (LDF) in the standing subj ects was reduced by -19.6 +/- 7.2% at the forehead and by -69.6 +/- 9. 6% in the leg. In the absence of hydrostatic pressure changes, the LDF changes on standing averaged -29 +/- 13%, independent of skin region, reflecting the effect of vasoconstriction due to sympathetic activati on. The postural vascular response, elicited by lowering the arm or th e leg from heart level, was significantly attenuated in orthostasis co mpared with recumbency. The vessel reaction to local alteration of tra nsmural pressure was studied in the skin of the forehead and lower leg by application of external pressure in supine subjects. No difference in vessel responsiveness to changes of transmural pressure was found between these skin sites. The findings suggest that the changes of ski n perfusion in orthostasis result from a nonadditive interaction of he ight-dependent and -independent mechanisms.