Thermoregulatory consequences of long-term microwave exposure at controlled ambient temperatures.

This study was designed to identify and measure changes in thermoregulatory responses, both behavioral and physiological, that may occur when squirrel monkeys are exposed to 2450-MHz continuous wave microwaves 40 hr/week for 15 weeks. Power densities of 1 or 5 mW/cm2 (specific absorption rate = 0.16 W/kg per mW/cm2) were presented at controlled environmental temperatures of 25, 30, or 35 degrees C. Standardized tests, conducted periodically, before, during, and after treatment, assessed changes in thermoregulatory responses. Dependent variables that were measured included body mass, certain blood properties, metabolic heat production, sweating, skin temperatures, deep body temperature, and behavioral responses by which the monkeys selected a preferred environmental temperature. Results showed no reliable alteration of metabolic rate, internal body temperature, blood indices, or thermoregulatory behavior by microwave exposure, although the ambient temperature prevailing during chronic exposure could exert an effect. An increase in sweating rate occurred in the 35 degrees C environment, but sweating was not reliably enhanced by microwave exposure. Skin temperature, reflecting vasomotor state, was reliably influenced by both ambient temperature and microwaves. The most robust consequence of microwave exposure was a reduction in body mass, which appeared to be a function of microwave power density.

Localization of intra-abdominal thermoreceptors in the ewe.

1. In neutral environmental temperatures, shivering was induced in sheep by intra-ruminal cooling. Shivering was then depressed for a period by intra-abdominal heating. Intra-abdominal heating without ruminal cooling induced panting and a reduction of caval temperatures. Posterior caval temperatures were found to be inappropriate to the responses observed.2. Intra-abdominal heating of sheep in cold and warm environments depressed shivering and augmented panting respectively. Unilateral splanchnotomy abolished these responses on the ipsilateral side.3. The results are interpreted to indicate that the thermoreceptors stimulated lie within the walls of the rumen and intestine, and possibly the mesenteric veins.4. The splanchnic nerves are indicated as the afferent pathway for these receptors, and differential splanchnic innervation of the gut is suggested to explain unilateral abolition of the response to warming.

Thermoregulatory responses to intra-abdominal heating of sheep.

When electrical heat sources were implanted in the abdominal cavities of sheep and heated to dissipate 20 to 22 watts of additional endogenous heat in the animal, a rapid increase in respiratory frequency and respiratory water loss occurred 3 to 5 minutes after the initiation of heating. The response was accompanied by a marked decline of the temperature of the hypothalamus, with an increase of less than 1.0 degrees C in skin temperature over the location of the heaters in the abdomen. When the same skin area was heated externally in the absence of internal heating, no significant response was seen. The results support the concept of the existence of thermoreceptors, located in deep tissues or veins, which play a role in the regulation of body temperature.

Visceral tissue vascularization: an adaptive response to high temperature.

Electrical heat sources implanted in the abdominal cavities of sheep were heated to give initial temperatures of 42 degrees and 45 degrees C at the surfaces of the heaters. During 18 days of constant heating, a vascularized connective-tissue envelope encapsulated the heat sources, and the temperatures at the surfaces of the heaters declined 0.8 degrees and 1.8 degrees C, respectively. The degree of vascularization and the magnitude of the decrease in the surface temperature appeared to be related to the proximity of the tissue's initial temperature to 45 degrees C, a temperature ordinarily considered detrimental to cell structure. The vascularization thus appears to be adaptive.