Interrelations between Sympathoadrenal System andHypothalamo‐Pituitary‐Adrenocortical/Thyroid Systemsin Rats Exposed to Cold Stress

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<jats:p>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 (TX) rats exposed to cold stress (−3 °C). Plasma levels of adrenocorticotropic hormone (ACTH), corticosterone (CORT), thyroid‐stimulating hormone (TSH) and thyroid hormones in cold‐stressed rats were measured also. Plasma ACTH levels were increased transiently after 1 h of cold exposure, after which the circadian rhythm and plasma levels of ACTH were similar to those of normal rats. Plasma CORT levels were also elevated after 1 h of cold exposure; the increased levels of CORT tended to return to normal levels after 9 h of cold, but remained higher than those of normal rats during at least 24 h of cold exposure. Plasma ACTH levels of 5 day cold‐stressed rats were no longer elevated above those of control rats and plasma CORT levels were only slightly higher than in control animals. However, plasma levels of TSH and free thyroid hormones were elevated after 1 day and remained elevated after 5 days of cold exposure. Thus, cold stress appears to activate chronically the HPT system, but only transiently activates the HPA system. ADX rats had higher basal plasma levels of dihydroxyphenylglycol (DHPG), methoxyhydroxyphenylglycol (MHPG), DOPA and homovanillic acid (HVA) than those of sham‐operated (SHAM) rats, but norepinephrine (NE) levels were not significantly greater 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 levels in both ADX and TX rats, but the increments in TX rats were much greater than in SHAM and ADX groups. Plasma EPI levels were not significantly elevated during cold exposure in SHAM rats, but were highly elevated in TX rats exposed to cold. TX rats had much larger increments in plasma levels of DHPG, MHPG, DA, dihydroxyphenylacetic acid (DOPAC) and HVA during cold exposure than those of SHAM and ADX rats. These results are consistent with the view that endogenous glucocorticoids restrain responses of catecholamine synthesis, release, reuptake, and metabolism in sympathetic nervous system of cold‐stressed animals, but that in the absence of an effective HPT system, there is enhanced sympathoadrenal medullary function and augmentation of their responses to cold as a means for maintaining body temperature when the HPT thermogenesis system is impaired.</jats:p>

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