EFFECTS OF PROLONGED HEAD-DOWN TILT (HDT) BED REST ON SWEAT GLAND FUNCTION AND MAXIMAL FOREARM CUTANEOUS VASCULAR CONDUCTANCE

Abstract

The magnitude of cutaneous vasodilation and sweating is reduced during a thermal challenge following simulated or actual microgravity exposure. The effects of microgravity exposure on cutaneous vasodilator capacity and on sweat gland function are unknown. The purpose of this study was to test the hypothesis that simulated microgravity exposure using the 6° HDT model reduces maximal forearm cutaneous vascular conductance (CVC) and sweat gland function. To test this hypothesis six subjects were exposed to 14 days of strict HDT bed rest. Prior to and following HDT bed rest maximal forearm CVC was measured, via plethysmography, by heating the entire forearm to 42° C for 45 min. Forearm CVC was calculated as forearm blood flow/mean arterial blood pressure. Sweat gland function was assessed by administering 10−6 to 2 M acetylcholine (9 doses) via intradermal micordialysis while simultaneously monitoring sweat rate over microdialysis membranes. Maximal forearm CVC was reduced in five of the six subjects following HDT bed rest, however the overall reduction was not significant (21.3 ± 1.2 to 19.3 ± 2.3 units). Neither the concentration of acetylcholine causing 50% of the maximal sweating response (EC50; log acetylcholine concentration: −1.3 ± 0.2 to −1.3 ± 0.03 M; P = 0.04) nor maximal sweat rate (0.92 ± 0.06 to 0.79 ± 0.10 mg/cm2/min; P = 0.2) were significantly altered by HDT bed rest. These data suggest that 14 days simulated microgravity exposure using the HDT bed rest model may reduce maximal forearm CVC without altering sweat gland function. It remains unclear whether alterations in maximal cutaneous vasodilation, if present, are responsible for impaired cutaneous vasodilation observed after space flight or after simulated microgravity exposure. Supported by NASA:NAG9-1033 and NIH-HL61388