EFFECTS OF LOCAL WARMING ON HUMAN CUTANEOUS ALPHA-ADRENERGIC VASOCONSTRICTOR RESPONSIVENESS

Abstract

Local warming causes cutaneous vasodilation via a non-sympathetic pathway. Local skin temperature can become quite high in thermally challenging environments especially when sweat function is compromised. However, the effects of local warming on cutaneous vaso-constrictor function in humans remains unknown. To investigate this question, in seven subjects (5 males, 2 females) forearm skin was locally warmed to 34, 37, 40, and 42○ C. At each local temperature α-adrenergic vasoconstrictor responsiveness was assessed by local delivery of 10−7 to 10−2 M norepinephrine (NE) via intradermal microdialysis. Skin blood flow was continuously monitored over each microdialysis membrane. Cutaneous vascular conductance (CVC) was indexed as skin blood flow / mean arterial pressure. Data were analyzed based upon a percentage reduction in CVC from steady-state values after local heating, but prior to administration of NE. Calculated norepinephrine concentration that caused a 50% reduction in CVC (EC50; reported as log NE concentration) for 34, 37, 40, and 42○ C local heating was −5.5 ± 0.4, −4.7 ± 0.3, −4.2 ± 0.3, −4.3 ± 0.2 M, respectively. Statistical analyses revealed that the EC50 for 40 and 42○ C was significantly greater (i.e. high NE concentration required to cause 50% reduction in CVC) than the EC50 for 34○ C, while no other statistical differences were identified between local temperatures. These data suggest that elevated local temperatures attenuate cutaneous α-adrenergic vasoconstrictor responsiveness. Supported by NASA:NAG9-1033 and NIH-HL61388