The current thesis examined whether sex-differences in local and whole-body heat loss are evident after accounting for confounding differences in physical characteristics and rate of metabolic heat production. Three experimental studies were performed: the first examined whole-body heat loss in males and females matched for body mass and surface area during exercise at a fixed rate of metabolic heat production; the second examined local and whole-body heat loss responses between sexes during exercise at increasing requirements for heat loss; the third examined sex-differences in local sweating and cutaneous vasodilation to given doses of pharmacological agonists, as well as during passive heating. The first study demonstrates that females exhibit a lower whole-body sudomotor thermosensitivity (553 ± 77 vs. 795 ± 85 W•°C-1, p=0.05) during exercise performed at a fixed rate of metabolic heat production. The second study shows that whole-body sudomotor thermosensitivity is similar between sexes at a requirement for heat loss of 250 W•m-2 (496 ± 139 vs. 483 ± 185 W•m-2•°C-1, p=0.91) and 300 W•m-2 (283 ± 70 vs. 211 ± 66 W•m-2•°C-1, p=0.17), only becoming greater in males at a requirement for heat loss of 350 W•m-2 (197 ± 61 vs. 82 ± 27 W•m-2•°C-1, p=0.007). In the third study, a lower sweat rate to the highest concentration of acetylcholine (0.27 ± 0.08 vs. 0.48 ± 0.13 mg•min-1•cm-2, p=0.02) and methylcholine (0.41 ± 0.09 vs. 0.57 ± 0.11 mg•min-1•cm-2, p=0.04) employed was evidenced in females, with no differences in cholinergic sensitivity. Taken together, the results of the current thesis show that sex itself can modulate sudomotor activity, specifically the thermosensitivity of the response, during both exercise and passive heat stress. Furthermore, the results of the third study point towards a peripheral modulation of the sweat gland as a mechanism responsible for the lower sudomotor thermosensitivity in females.

The ventromedial hypothalamic nucleus (VMH) is an important central effector site involved in activating brown adipose tissue (BAT) or non-shivering thermogenesis. VMH neurons have previously been shown to be thermally responsive to changes in local temperature of the preoptic area/anterior hypothalamus (PO/AH). However, the thermoresponsiveness of VMH neurons of room temperature acclimatized and cold acclimatized rats to peripheral thermal stimulation has not been tested. In this thesis, a series of studies was designed to determine the thermoresponsiveness of VMH neurons to peripheral (scrotal) thermal stimulation of rats. Extracelluiar VMH neuronal activity was recorded from urethane anaesthetized male Sprague-Dawley rats, which were acclimatized either to room temperature (21°C for 4 weeks) or to cold (4°C for 4 weeks) prior to testing, during scrotal cooling and heating with glass-micropipettes filled with 0.5 M sodium acetate containing 2% pontamine sky blue. The rats' colonic temperatures were kept at normothermia (37°C), or hypothermia (33-35°C) during scrotal thermal stimulation. In the room temperature acclimatized rats, VMH neurons were temperature responsive to scrotal heating and cooling and were classified as warm responsive (WRN), cold responsive (CRN) and temperature non-responsive neurons (TNRN), based on their thermal coefficients. The ratio of VMH WRNs and CRNs was similar to that of thermoresponsive neurons observed in other brain regions (eg, the PO/AH and thalamus). VMH WRNs and CRNs were further classified as biphasic or monophasic in nature according to their thermal responses to scrotal heating and cooling. VMH neurons sustained their thermoresponsiveness to repeated trials of scrotal thermal stimulation with colonic temperatures maintained at 37°C or when colonic temperatures were acutely lowered from 37°C to 35°C and 33°C. In addition, scrotal thermal signals specifically altered neuronal activity of VMH thermoresponsive neurons, as changes in EEG activity did not occur with changes in VMH neuronal activity. Scrotal thermal inputs were functionally shown to be transmitted via the medial preoptic nucleus (MPO) prior to reaching the VMH nucleus because thermoresponsive VMH neuronal activity was blocked with the pretreatment of lidocaine into the MPO. Scrotal heating or cooling to 21°C-acclimatized rats did not increase IBAT temperatures, inferring that scrotal cooling had not evoked IBAT thermogenesis in this group. In cold acclimatized (CA) rats, prolonged (over 2 hours) and transient localized scrotal cooling caused IBAT temperatures to increase, inferring that scrotal cooling activated BAT thermogenesis. Mean basal firing rates of all recorded VMH neurons of CA-groups significantly increased, compared to those of VMH neurons observed in room temperature acclimatized (RA) groups. More VMH CRNs than WRNs were recorded in the CA-group and the thermoresponsiveness (ie, thermal coefficient) of VMH CRNs significantly increased during localized scrotal cooling in the cold acclimatized group, compared to thermal coefficients of VMH CRNs of the RA-groups. (Abstract shortened by UMI.)

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Field observations indicated a difference in the ability to locomote at low body temperatures in two closely related species of lizards from very different habitats and with radically different seasonal behavior. I measured the critical thermal minimum (the body temperature at which a cooling lizard just loses the ability to right itself) in both species. The winter-active, montane Sceloporus jarrovi had a significantly lower critical thermal minimum in both summer and winter than the winter-hibernating, lowland S. magister. Critical thermal minima were significantly lower in winter than in summer for both species. To determine a physiological basis for these differences, I examined the activity of myosin ATPase, which plays the limiting role in the velocity of muscle contraction, and the energetics of muscle as reflected by high energy phosphate compounds. Microenvironmental conditions were correlated with behavior, constraints on winter activity, and muscle physiology. Ca²⁺-activated myosin ATPase activity in S. magister of valley bottoms is greater than that in the vertical rock-dwelling S. jarrovi. No seasonal acclimatization occurs in myosin ATPase activity in either species. Changes in the muscle metabolism of hibernating animals has been attributed to the lack of muscular contractions in the dormant animals. I measured levels of phosphorylated compounds in a hindlimb muscle from summer and winter lizards of both species. Significant seasonal changes occur in some of the phosphate compounds in both species even though, within a given season, respective levels of phosphorylated compounds are similar in both species. Phosphorylcreatine and total acid-soluble phosphate levels increased in winter animals of both species. Apparently the high levels of phosphorylcreatine in winter S. magister are not simply due to inactivity; winter-active S. jarrovi contain similar amounts. Seasonal cycling of phosphate compounds may relate more to parathyroid status than to muscle activity. Winter activity in S. jarrovi was site-specific and highly dependent on a favorable microclimate. Winter dormancy in S. magister apparently is not dictated by the severity of the microclimate nor physiological limitations of skeletal muscle, but may be strongly influenced by the thermal inertia of that relatively large species.

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Saccostomus campestris display sexual disparity in the use of summer daily torpor in response to energy stress. The hypothesis that males may compensate for a limited heterothermic capacity with lower normothermic body temperatures by maintaining lower resting metabolic rates relative to females was tested. Furthermore, the influence of testosterone on torpor incidence in males was investigated. Body temperature (T[b]) and oxygen consumption (VO₂) were measured at various ambient temperatures (Tₐ) and were compared between the sexes under food ad libitum and food restriction treatments. There were no significant differences in T[b] and VO₂ between sexes under food ad libitum treatment. Under food restriction there were pronounced sex differences in the employment of heterothermy. Females defended a lower setpoint T[b] for torpor (ca. 25°C), than males (ca. 29°C), and also employed torpor more frequently than males. Non-torpid males did, however show slight reductions in VO₂ under food restriction. The effect of testosterone on daily torpor was investigated by comparing minimum T[b]and torpor frequency of castrated mice implanted with testosterone-filled (experimental) and saline-filled (control) silastic capsules in response to food ad libitum and food restriction treatments. Testosterone inhibited torpor in males. The majority of control animals employed torpor under both food ad libitum and food restriction diets. It was concluded that although the animals were capable of shallow, summer torpor, it was confined to moderate ambient temperatures and was not used at low Tₐ's where several animals became pathologically hypothermic. Females derive energetic benefits from the use of torpor whereas males may partially compensate for their limited heterothennic capacity by a reduction in resting metabolic rates, accompanied by moderate reductions in body temperature during energetically stressful periods. The difference in the capacity for daily heterothenny between sexes was attributed to differences in their reproductive physiology. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2000.

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Bibliography: leaves 162-197. / vii, 216 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / To determine the effects of melatonin on sleepiness and body temperature under conditions that better approximated the endogenous melatonin profile. / Thesis (Ph.D.)--University of Adelaide, Dept. of Obstetrics and Gynaecology, 1998?

Thesis (M.S.)--University of Michigan, 1977.