RHYTHMIC DIURNAL VARIATION IN LIMB BLOOD FLOW IN MAN

Circadian variation in peripheral blood flow in relation to core temperature at rest

The aim of the present study was to examine the circadian variation in skin blood flow and its relationship to the rhythm of rectal temperature. Eight young men were confined to continuous bed rest for 24 h in a thermoneutral environment. Rectal temperature, oxygen consumption, and peripheral blood flow (laser-Doppler flowmetry and plethysmography) were measured at intervals of 2 h, except at 2400 hours and 0600 hours. According to the cosinor-method, the mean rectal temperature displayed a statistically significant circadian variation with a minimum at 0400 hours. A significant circadian rhythm was also observed in total finger blood flow, finger laser-Doppler flow, and forearm laser-Doppler flow, but not in total forearm blood flow and forehead laser-Doppler flow. The rhythms of rectal temperature and extremity skin blood flow were phased differently, i.e. the decrease in rectal temperature was preceded by an increase in extremity skin blood flow with an average of 4-h time difference. The results would support the idea that circadian rhythm of core temperature is, at least in part, regulated by variation in extremity skin blood flow.

Role of heat loss and heat production in generation of the circadian temperature rhythm of the squirrel monkey

To study heat production and heat loss in determination of the daily body temperature rhythm, we examined colonic temperature, skin (tail, foot and abdomen) temperatures and oxygen consumption in chair-restrained squirrel monkeys maintained in isolation in an environmental chamber with a 24-hr light-dark cycle (LD 12:12), maintained at a constant thermoneutral temperature (26 degrees C). In all experiments repeated high amplitude (2 degrees C) diurnal rhythms in colonic temperature were observed. Heat loss, estimated from changes in skin temperature, also displayed a circadian rhythm, although there was considerable variation in waveform. On average, a rhythm in heat production, indicated by changes in the rate of oxygen consumption, was also present. However, a large degree of variability was seen in oxygen consumption, and in several cycles from various animals there were no observable 24-hr rhythms. The circadian body temperature rhythm is thus not simply a consequence of daily changes in metabolism, but rather a regulated response that involves both heat production and heat loss.