COLD DEFENSE-MECHANISMS IN VAGOTOMIZED RATS

Subdiaphragmatically vagotomized rats cannot mount a febrile response to pyrogens and are believed to have severe thermoregulatory deficienc ies. We addressed the issue of thermoeffector competence of vagotomize d rats by asking three questions. In Expt. 1 we asked, can vagotomized rats readily recruit tail skin vasoconstriction in the course of a mo derate cold exposure? In Expt. 2 the question was, can brown adipose t issue (BAT) thermogenesis readily be activated in vagotomized rats (e. g., in response to a tail pinch)? In Expt. 3, we investigated the ques tion: can vagotomized rats elevate their body temperature in response to ephedrine (a drug of high hyperthermizing potential) to the same ex tent as sham-operated controls? Rats were vagotomized or sham operated and implanted with a catheter into the jugular vein and a thermocoupl e into the interscapular BAT. To prevent the common complications of v agotomy, special perioperative care was given. During experiments, col onic, tail skin, and BAT temperatures (T-c, T-sk, and T-BAT, respectiv ely) were measured. The vagotomized animals were well nourished and ha d a body mass (325 +/- 6 g) similar to that of the controls (338 +/- 6 g). In Expt. 1, in response to external cooling (15 degrees C, 1 h), the vagotomized (n = 30) and sham-operated (n = 31) rats recruited tai l skin vasoconstriction at close values of both T-c (37.84 +/- 0.08 an d 37.97 +/- 0.07 degrees C) and T-sk (33.16 +/- 0.17 and 33.18 +/- 0.1 8 degrees C, respectively). In Expt. 2, tail pinch-associated stress i n vagotomized rats resulted in a sharp rise in the T-BAT - T-c gradien t by 0.3-1.0 degrees C. In Expt. 3, ephedrine administered intravenous ly (whether in a 5 or 35 mg/kg dose) evoked similar hyperthermic respo nses in the vagotomized and sham-operated rats: a moderate (similar to 2.5 degrees C) T-c rise in the low dose and a ''supramaximal'' (simil ar to 5.0 degrees C) rise in the high dose. In sum, the answer to all three questions asked is yes. Vagotomized rats, at least when well nou rished, exhibit no signs of thermoeffector deficiency. It is, therefor e, not effector incompetence but rather vagal deafferentation per se t hat can explain the febrile irresponsiveness of vagotomized rats.