Circadian rhythms, ∼24-h oscillations in physiological variables, are pervasive throughout nature. Synchronization of these oscillations to the 24-h day enables organisms to take advantage of environmental cycles. In mammals, a circadian pacemaker located in the hypothalamus coordinates the overt rhythms. / The present study investigates aspects of the 24-h organization of breathing, and its chemical control, and of thermoregulation in mammals. Pulmonary ventilation (V˙E), oxygen consumption ( V&d2;O2 ), body temperature (Tb), and locomotor activity were monitored by non-invasive means, in freely-moving male adult rats. Rats are nocturnal, and it is well known that their Tb, activity and metabolic rate are highest during the dark hours of the day. / Hypoxia inhibits thermogenesis, and the Tb and V&d2;O2 circadian patterns are contributed to by changes in thermogenesis, implying that hypoxia blunts the daily oscillations of these variables. Indeed, the amplitude of both oscillations was smaller, due to a decrease in the dark phase values. Evidence supports an action of hypoxia on the hypothalamic thermoregulatory mechanisms, rather than on the clock itself. / Metabolism is well known to be a major determinant of V˙E, and of the V˙E response to changes in inspired gases. Using a custom-designed system to monitor breathing continuously, during air breathing, V˙ E was found to oscillate, with higher values during the dark compared to the light hours of the day; these changes were almost in proportion to those of V&d2;O2 , and did not depend on those of activity. / The depressant effect of hypoxia on the high values of the V&d2;O2 oscillation predict that the hypoxic V˙E response would be blunted at this time. Indeed, the response was lower during the dark compared to the light hours; however, the daily changes in the V˙E response were in proportion to those of V&d2;O2 , such that the hyperventilatory response (% increase in V˙ E/ V&d2;O2 ) was similar throughout the day. The V˙E/ V&d2;O2 response was also similar throughout the day in hypercapnia, even though the metabolic response to hypercapnia differed from that in hypoxia. / Globally taken, these results indicate that (1) breathing and its control mechanisms accompany the daily oscillations of many physiological variables, and (2) the advantages of a biological clock do not compromise the adequacy of the hyperventilatory responses to chemical challenges.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.37657 |
| Date | January 2002 |
| Creators | Seifert, Erin. |
| Contributors | Mortola, J. P. (advisor) |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Physiology.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001864143, proquestno: NQ78769, Theses scanned by UMI/ProQuest. |
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