Physiological mechanisms of thermoregulation in reptiles: a review

The thermal dependence of biochemical reaction rates means that many animals regulate their body temperature so that fluctuations in body temperature are small compared to environmental temperature fluctuations. Thermoregulation is a complex process that involves sensing of the environment, and subsequent processing of the environmental information. We suggest that the physiological mechanisms that facilitate thermoregulation transcend phylogenetic boundaries. Reptiles are primarily used as model organisms for ecological and evolutionary research and, unlike in mammals, the physiological basis of many aspects in thermoregulation remains obscure. Here, we review recent research on regulation of body temperature, thermoreception, body temperature set-points, and cardiovascular control of heating and cooling in reptiles. The aim of this review is to place physiological thermoregulation of reptiles in a wider phylogenetic context. Future research on reptilian thermoregulation should focus on the pathways that connect peripheral sensing to central processing which will ultimately lead to the thermoregulatory response.     

Differential heating of trunk and extremities. Effects on thermoregulation mechanisms

Experiments in which the whole human body was heated or cooled are compared with others in which one extremity (arm or leg) was simultaneously cooled or heated. With a warm load on the rest of the body resulting in general sweating, a cold load on one extremity did not evoke local shivering; with general body cooling, heating one limb did not stop the shivering. Skin temperatures of the other parts of the body were not influenced by warming or cooling one extremity. Evaporative heat loss was influenced by local, mean skin and core temperature, whereas shivering did not depend on local temperature, and vasomotor control seemed to be controlled predominantly by central temperatures. A cold load on an extremity during whole body heating in most cases induced an oscillatory behaviour of core temperature and of the evaporative heat loss from the body and the extremity. It is assumed that local, mean skin and core temperatures influence the three autonomous effector systems to very different degree.     

Differences in the mechanisms of thermoregulation in rats in separate and combined adaptation to cold and heat

Combined adaptation of rats to heat and cold increasing mechanisms of thermogenesis enhances resistance to both factors and heat dissipation. Adaptive changes in thermogenesis are mainly a result of activation of adrenergic mechanisms, while the separate cold adaptation is accompanied by hyperfunction of thyroid glands. Mechanisms of heat dissipation in rats of the "combined" group increase even more than those of "heat" group.     

Cholinergic mechanisms of the hypothalamus medial preoptic area in control of thermoregulation and of wakefulness-sleep states in pigeons

The data obtained show that cholinergic mechanisms of the medial preoptic area of hypothalamus participate in control of wakefulness-sleep states and thermoregulation parameters in pigeons. Muscarinic and nicotinic cholinergic receptors are established to be involved in the wakefulness maintenance. The muscarinic cholinergic receptor activation of the medial preoptic area is accompanied by an elevation of the brain temperature, by development of peripheral vasoconstriction, and by an in increase in level of the muscle contractile activity. During the nicotinic cholinergic receptor activation of the area, a decrease in the brain temperature and an increase in level of the muscle contractile activity are found. A comparative analysis of experiments and early investigation suggests that during the cholinergic receptors activation changes in the brain temperature of pigeons depend on type of the cholinergic receptors but not on their localization in the preoptic area of hypothalamus.     

Mechanisms of thermoregulation in plants

Endothermic heating of floral tissues and even thermoregulation is known to occur in a number of plant species across a wide taxonomic range. The mechanisms by which flowers heat, however, are only just beginning to be understood, and even less is known about how heating is regulated in response to changes in ambient temperature. We have recently demonstrated that the alternative pathway of respiration, in which the alternative oxidase (AOX) rather than cytochrome C (COX) acts as terminal electron acceptor, is responsible for heat generation in one thermoregulating species, the sacred lotus (Nelumbo nucifera). In the March issue of the Journal of Experimental Botany we further demonstrated that AOX-mediated heat production in this species is regulated at both the level of gene expression and also post-translationally. Similarly, AOX has also been implicated in heat production in other thermogenic species. In this addendum we discuss the central role of AOX in heat production and how post-translational mechanisms may provide the fine control necessary for thermoregulation.Key words: alternative oxidase, Nelumbo nucifera, thermogenic plants, uncoupling proteins