Homeotherms

The Qxo, or temperature coefficient, is the factor by which the rate of a biologic process increases for a 10 °C increase in temperature. For the temperatures over which enzymes are stable, the rates of most biologic processes typically double for a 10 °C rise in temperature (Qjo = 2). Changes in the rates of enzyme-catalyzed reactions that accompany a rise or fall in body temperature constitute a prominent survival feature for cold-blooded life forms such as lizards or fish, whose body temperatures are dictated by the external environment. However, for mammals and other homeothermic organisms, changes in enzyme reaction rates with temperature assume physiologic importance only in circumstances such as fever or hypothermia. 

Nereistoxin (C H jNS2) kills fish and insects but is relatively non-toxic to homeotherms. The LD q value in mice varies from 30-1000 mg/kg depending on... 

Aveldano, M. I. and Bazan, N. G. Differential lipid deacylation during brain ischemia in a homeotherm and a poikilo-therm. Content and composition of free fatty acids and triacylglycerol. Brain Res. 100 99-110,1975. 

The ability of some organisms to control the pH and temperature of their cells and tissues represents a major biological development. Homeothermic animals (e.g. mammals) maintain a constant temperature of about 37 °C as this corresponds to the temperature of optimum activity of most enzymes. Poikilothermic or so-called cold-blooded animals (e.g. reptiles) have to sun themselves for sometime every morning in order to raise their body temperature in order to optimize enzyme activity within their cells. 

Metabolism - a final factor in need of comparative studies is the metabolism of xenobiotics. One obvious difference between mammalian and fish species is that their bodies usually function at temperatures at least 10°C different. This fact undoubtedly explains some differences in metabolic rate but even when in vitro incubations are run at optimal temperatures there is a 10 - 100 fold higher rate of mammalian vs. fish metabolism (14, 15). In other words, the level of the xenobiotic-metabolizing capacity, especially for oxidative pathways, of the poikilothermic animals is considerably lower than that of the homeothermic species. Elsewhere in this volume Dr. Bend has focused on this aspect of the handling of xenobiotics by fish (16). 

Diamond, L. and Clark, H.F. Comparative studies on the interaction of benzo(a)pyrene with cells derived from poikilothermic and homeothermic vertebrates. I. Metabolism of benzo( a)pyrene. J. Natl. Cancer Inst. (1970) k, 1005-1011. 

An elegant demonstration that the larval/adult transformation, resulting in sexual differentiation, is, in fact, associated with a neurosecretory mechanism has been made by Gustafsson and co-workers (207, 278, 280, 281,283). They showed that a clear activation of the peptidergic neurones took place when plerocercoid larvae of Diphyllobothrium dendriticum were transferred from the poikilothermic intermediate fish host to the final homeothermic bird host. The effect could also be reproduced by cultivating... 

The benefits of being an endothermic homeotherm, especially one with systemic regulation of body temperature, are readily apparent. By... 

In addition to these challenging physical constraints, the demands for food that an endothermic homeotherm faces are substantially greater than those for an ectotherm of the same body mass. At the same body temperature, on a unit mass basis, similar-sized endotherms and ectotherms differ in oxygen consumption on average by at least four- to fivefold (see Brand et al., 1991). It is obvious that many ectother-mic species may not have access to enough food, at all times, to make it possible, even in principle, for them to become endothermic homeotherms. An ecosystem in which all animals are endothermic homeotherms is probably impossible. 

Endothermic homeotherms are characterized by high mass-specific rates of oxygen consumption relative to similar-sized ectotherms. This difference can be shown by examining the allometric scaling relationship between rate of oxygen consumption (M) and body mass (IF). For ectothermic and endothermic animals, whole animal oxygen consumption rate is proportional to total body mass raised to approximately the 0.75 power ... 

Calder III, W.A. (1981), Scaling of physiological processes in homeothermic animals. Annual Review of Physiology, 43, 301-322. 

Although some primitive oiganisms inhabiting thermally stable environments have been referred to as nominal homeotherms, this is a misnomer as such organisms are not thermal regulators at all, much less precise ones. Some dinosaurs may have been inertial homeotherms, relying on their large thermal mass (McNab, 2002). 

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