title: Effect of whole body heat stress on peripheral vasoconstriction during leg dependency.
abstract: The venoarteriolar response (VAR) increases vascular resistance upon increases in venous transmural pressure in cutaneous, subcutaneous, and muscle vascular beds. During orthostasis, it has been proposed that up to 45% of the increase in systemic vascular tone is due to VAR-related local mechanism(s). The objective of this project was to test the hypothesis that heat stress attenuates VAR-mediated cutaneous and whole leg vasoconstriction. During normothermic conditions, measurements of cutaneous blood flow (laser-Doppler flowmetry) and femoral artery blood flow (Doppler ultrasound) were obtained from both legs during supine and leg-dependent conditions. These measurements were repeated following a whole body heat stress (increase in internal temperature of 1.4 +/- 0.2 degrees C). Before leg dependency, cutaneous (CVC) and femoral vascular conductances (FVC) were significantly elevated in both legs during heat stress relative to normothermia (P < 0.001). During leg dependency the absolute decrease in CVC was attenuated during heat stress (P < 0.01) while the absolute decrease in FVC was unaffected (P = 0.90). When CVC and FVC data were analyzed as a relative change from their respective baseline values, heat stress significantly attenuated the magnitude of vasoconstriction due to leg dependency in the cutaneous and femoral circulations (P < 0.001 for both variables). These data suggest that an attenuated local vasoconstriction, evoked via the venoarteriolar response, may contribute to reduced blood pressure control and thus reduced orthostatic tolerance that occurs in heat-stressed individuals.
authors.affiliation: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas; and 2University of Texas Southwestern Medical Center at Dallas, Texas 75231, USA.
authors.forename: R Matthew
authors.initials: RM
authors.lastname: Brothers
authors.forename: Jonathan E
authors.initials: JE
authors.lastname: Wingo
authors.forename: Kimberly A
authors.initials: KA
authors.lastname: Hubing
authors.forename: Juan
authors.initials: J
authors.lastname: Del Coso
authors.forename: Craig G
authors.initials: CG
authors.lastname: Crandall
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D001783:Blood Flow Velocity / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology; D001831:Body Temperature; D005260:Female; D005263:Femoral Artery / Q000000981:diagnostic imaging / Q000502:physiology*; D006358:Hot Temperature; D006801:Humans; D007866:Leg / Q000098:blood supply / Q000000981:diagnostic imaging; D008297:Male; D012039:Regional Blood Flow / Q000502:physiology*; D012867:Skin / Q000098:blood supply / Q000000981:diagnostic imaging; D013312:Stress, Physiological / Q000502:physiology*; D013686:Telemetry; D018608:Ultrasonography, Doppler; D014661:Vasoconstriction / Q000502:physiology*