Four rats were implanted preflight with a biotelemetry transmitter capable of sensing and transmitting information on heart rate and deep body temperature. The telemetry data from the animals was monitored during the period which Spacelab was activated (approximately from eight hours after launch to six hours prior to re-entry). The average 24-hour waveforms of body temperature in preflight and flight conditions were examined. Four ground control animals were similarly implanted and monitored.

Results:

Microgravity altered the steady state regulation of heart rate and body temperature. During the preflight conditions, the animals demonstrated no net change in phase relationship with the mean period 23.9 ± 0.2 over the course of the pre- flight study; while inflight, the phase of all the animals consistently delayed such that the average period of the temperature rhythm inflight was 24.4 ± 0.3 hours. A longer flight will be necessary to identify whether or not the animals are freerunning with a period independent 24-hours or rather simply showing an internal phase angle shift which had not reached a steady state during the seven-day flight. Heart rate phase rhythm was stable in both conditions with a mean period of 23.9 ± 0.2 in either. The heart rate itself was, however, depressed inflight, the possible result of a reduced load on the cardiovascular system in space or a resetting of some other component within this regulatory system. Data suggest that normal expression of the circadian timing system is extensively modified in microgravity.