Date of Award

1991

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biological Sciences

First Advisor

Albert H. Meier

Abstract

Studies were made in Syrian hamsters and Holtzman rats to examine roles for circadian rhythms in the control of body fats stores. Baseline and circadian rhythms of plasma insulin varied seasonally in the Syrian hamster. Baseline concentrations of the hormone were almost 4 times greater in scotosensitive female hamsters held on short daily photoperiods when they were fat than in lean animals at other seasons and other photoperiodic schedules. Additionally, plasma insulin levels in fat hamsters, but not in lean animals, peaked at light onset when lipogenic responsiveness to insulin is greatest. The circadian rhythms of plasma insulin are not a result of an endogenous rhythmicity of B-cell secretion. Perifused Syrian hamster islets in the presence of glucose did not secrete the hormone in a circadian manner. Instead, islets lost their ability to respond to glucose when exposed to a constant level of the secretagogue over a 48-hour period. This islet desensitization was preventable if the glucose stimulus was provided in a pulsatile manner. The response to glucose pulses did not reveal a circadian periodicity. In Holtzman rats, daily 2-hour increases in the ambient temperature (thermopulses) provided 16 hours after light onset (TP-16) for 14 days consistently decreased retroperitoneal fat stores. Thermopulses provided at other times of day were largely ineffective. TP-16 treatment also decreased food consumption and basal levels of plasma insulin and increased glucose tolerance and tissue sensitivity to insulin. However, plasma glucose levels were unaltered. The robust circadian rhythms of plasma insulin and corticosterone present in controls (nonthermopulsed) were obliterated by the TP-16 treatment whereas those of prolactin, body temperature and locomotor activity were unaltered. Conversely, thermopulse treatment at light onset (TP-0) was ineffective in altering any of the parameters studied. These studies support a role for circadian neuroendocrine interactions in the control of lipid and glucose metabolism.

Pages

102

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