Seadragon VI: a 7-day dry saturation dive at 31 ATA. IV. Circadian analysis of body temperature and renal functions.

Circadian rhythms of body temperature and daytime rhythms of urine flow and urinary excretion of electrolytes were investigated in 4 male subjects before, during, and after a 7-d stay in a dry heliox 31 ATA environment. The chamber temperature was maintained at about 28 degrees C during pre- and postdive 1 ATA periods and was raised to 31.5 degrees C at 31 ATA. The circadian rhythm of the rectal temperature, as analyzed by the cosinor fitting method, showed the same mesor (the mean level of fluctuation) and the amplitude at 31 and 1 ATA. However, a reversible phase shift was noted at 31 ATA in which the acrophase shifted to 1435 h at 31 ATA from 1540 h (predive) or 1610 h (postdive) at 1 ATA. This shift was attributed to an early rise of rectal temperature during night at 31 ATA. A similar phase shift was observed at 31 ATA for the skin temperature of the forehead, a region not covered by clothing or blanket. The daytime rhythms of urinary excretion of water, Na, Cl, urea, and total osmotic substances were similar, with the acrophase at 1300-1500 h at both 1 and 31 ATA. On the other hand, the daytime rhythm for urinary excretion of K, which was similar to the above at 1 ATA, disappeared at 31 ATA. The urinary excretion of endogenous creatinine remained constant during both daytime and nighttime at both 1 and 31 ATA. These results suggest that exposure to 31 ATA may alter the underlying pattern of circadian or daytime rhythms for thermoregulatory and certain renal functions.