Radiotelemetry of core temperature in unrestrained, mature female rats revealed the existence of a 24-h rhythm that was bimodal. The principal peak occurred during the night under control conditions of 14 h light and 10 h darkness, and a less pronounced, secondary peak occurred 3-4 h after the onset of the light phase. Shifts in the phase of the photoperiod or alteration of the proportion of light per day revealed that the temperature rhythm was entrained by light, but that the two component peaks were governed by different aspects of the lighting regimen. Exposure of rats to continuous darkness, continuous light, or to a 20-h photoperiod revealed that the primary rhythm was endogenous, entrained by circadian photoperiods only, whereas the secondary rhythm was exogenous, requiring a circadian light/dark rhythm. A relationship between mean core temperature and ttion pressure, end-systolic L was constant, despite variations in filling and therefore independent of initial L and delta L; moreover, the L to which the ventricle shortened was determined by the course of the systolic force L-relation. Thus, irrespective of loading, delta L occurs within the confines of the contractile state-depdendent isovolumic force-L relation and where the latter is equivalent to the end-systolic force-length relation.
To define how the periodicity of light regulates reproductive cyclicity in the rat, animals were exposed to slightly altered photoperiods in which the light/dark ratio was maintained as in the control (14L + 10D) environment. Exposure to a 22.5-h photoperiod (13.125L + 9.375D) for 10-80 days induced a significant prolongation of the first and all subsequent vaginal cycles by about 30 h. Ovulation in these animals was normal. By contrast, exposure to a 26-h photoperiod (15.167L + 10.833D) had no significant effect on vaginal cyclicity. A comparison of the data with theoreticaly generated results revealed that a significant prolongation of the estrous cycle under conditions of a shortened photoperiod cannot be accounted for by a simple entrainment of the estrous cycle to the timing of the new circadian rhythm. Rather, the evidence suggested that a photoperiod-entrained rhythm may interact with a second rhythm of different periodicity to alter the duration of the reproductive cycle as measured in this study.
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