INTERRELATIONS BETWEEN SYMPATHOADRENAL SYSTEM AND HYPOTHALAMO-PITUITARY-ADRENOCORTICAL THYROID SYSTEMS IN RATS EXPOSED TO COLD STRESS/

The interrelations between sympathoadrenal (SA) system and hypothalamo -pituitary-adrenocortical (HPA) or hypothalamo-pituitary-thyroid (HPT) system during cold stress were examined by measuring plasma levels of dihydroxyphenylalanine (DOPA), catecholamine and their metabolites in adrenalectomized (ADX) and thyroidectomized CTX) rats exposed to cold stress (-3 degrees C). Plasma levels of adrenocorticotropic hormone ( ACTH), corticosterone (CORT), thyroid-stimulating hormone (TSH) and th yroid hormones in cold-stressed rats were measured also. Plasma ACTH l evels were increased transiently after 1 h of cold exposure, after whi ch the circadian rhythm and plasma levels of ACTH were similar to thos e of normal rats. Plasma CORT levels were also elevated after 1 h of c old exposure; the increased levels of CORT tended to return to normal levels after 9 h of cold, but remained higher than those of normal rat s during at least 24 h of cold exposure. Plasma ACTH levels of 5 day c old-stressed rats were no longer elevated above those of control rats and plasma CORT levels were only slightly higher than in control anima ls. However, plasma levels of TSH and free thyroid hormones were eleva ted after 1 day and remained elevated after 5 days of cold exposure. T hus, cold stress appears to activate chronically the HPT system, but o nly transiently activates the HPA system. ADX rats had higher basal pl asma levels of dihydroxyphenylglycol (DHPG), methoxyhydroxyphenylglyco l (MHPG), DOPA and homovanillic acid (HVA) than those of sham-operated (SHAM) rats, but norepinephrine (NE) levels were not significantly gr eater than in SHAM animals. TX rats had higher basal plasma levels of NE, epinephrine (EPI) and dopamine (DA), as well as much higher plasma levels of the metabolites. Exposure to cold increased plasma NE level s in both ADX and TX rats, but the increments in TX rats were much gre ater than in SHAM and ADX groups. Plasma EPI levels were not significa ntly elevated during cold exposure in SHAM rats, but were highly eleva ted in TX rats exposed to cold. TX rats had much larger increments in plasma levels of DHPG, MHPG, DA, dihydroxyphenylacetic acid (DOPAC) an d HVA during cold exposure than those of SHAM and ADX rats. These resu lts are consistent with the view that endogenous glucocorticoids restr ain responses of catecholamine synthesis, release, reuptake, and metab olism in sympathetic nervous system of cold-stressed animals, but that in the absence of an effective HPT system, there is enhanced sympatho adrenal medullary function and augmentation of their responses to cold as a means for maintaining body temperature when the HPT thermogenesi s system is impaired.