Effect of changes in local skin temperature on postural vasoconstriction in man.

1. The effects of locally induced alterations in skin temperature on the postural changes in skin blood flow of the foot were assessed in 38 healthy subjects in a constant-temperature environment (22 +/- 0.5 degrees C, mean +/- SD). 2. Moderate local cooling and warming of the foot (26-36 degrees C) was induced by blowing cold or hot air. Higher ranges of temperature (38-44 degrees C) were achieved by a thermostatically controlled disc heater. 3. Skin blood flow was measured before and during each change in local skin temperature using a laser Doppler flowmeter with the foot maintained at heart level, and placed passively 50 cm below the heart. Blood flow was measured in two skin areas: (i) the dorsum on the foot, where arteriovenous anastomoses are absent, and (ii) the pulp of the big toe, where these anastomoses are relatively numerous. 4. It was found that within the physiological temperature range of 26-36 degrees C the normal postural fall in foot skin blood flow was preserved, whereas it was markedly attenuated or totally abolished at higher temperatures (38-44 degrees C). The pattern of response was quite similar in areas having or lacking arteriovenous anastomoses. 5. It is suggested that the failure of postural vasoconstriction observed at the higher skin temperatures might contribute to some of the problems of cardiovascular adaptations seen in a hot environment.