EFFECTS OF ARTIFICIAL TEMPERATURE REGULATION ON SPLENIC NODULAR TISSUE IN THE EXTERIORIZED RAT SPLEEN: AN APPLICATION OF BIOENGINEERING PRINCIPLES *, †

Even to the casual observer, it has become axiomatic that all animals, although comprising two distinct groups, carry out their normal metabolic and physiologic functions through a wide range of environmental temperature. Awareness of this concept unquestionably dates back to ancient man. One group (poikilotherms), exhibits a remarkable parallelism between body temperature, inclusive of organs, with that of the ambient or environmental temperature. The other group (homeotherms), is characterized by an ability to generate and maintain a consistent body temperature irrespective of fluctuations that occur in environmental temperature. In general, homeothermic animals, including laboratory rodents, when subjected to an environmental temperature depression (general hypothermia) or an elevation (general hyperthermia), react to reestablish their body temperature norm. Allowing for species differencies, the type and rate of reaction varies according to the intensity and duration of the thermal stimulus, but invariably results in the activation of some or all of established temperature-regulatory mechanisms within the animal. These include the central and autonomic nervous systems, endocrine and exocrine glands, skin, cardiovascular system, and fluid depots (including blood). The regulatory mechanisms appear to be competent only within critical levels of environmental variation, beyond which body temperature progressively deteriorates in favor of an overwhelming ambient temperature. The composite of these subjects has been extensively reviewed by H e r ~ f e l d . ~ Students of comparative physiology have long utilized a temperature coefficient, or Qlo , to measure the rate of metabolic reaction and tissue response in poikilotherms, based on a 10°C decrease or increase in environmental temperature. The temperature coefficient was more useful with poikilotherms than with homeotherms until the advent of artificial temperature regulation as a technique or method. Attempts to regulate artificially the temperature of a single organ or a selected region of tissues of the homeotherm body are generally known as local hypothermia or hyperthermia, as the case may be. Within poorly defined limits, extreme local hypothermia tends to isolate a homeotherm organ from the influence of temperature-regulatory mechanisms when normal body temperature is induced to remain constant. In such instances, homeotherm organs approach a temperature-related circumstance that is natural t o poikil-