Tissue-specific noradrenergic activity during acute heat stress in rats

The purpose of this study was to determine whether nonexertional heat stress alters the behavior of internal organ and skeletal muscle sympathetic nervous system (SNS) activity in unrestrained conscious rats. Norepinephrine (NE) synthesis was blocked with α-methyl-p-tyrosine, and the rate of decline in tissue NE concentration after synthesis blockade was used to estimate SNS activity in the left ventricle, kidney, liver, adrenal gland, and soleus and extensor digitorum longus muscles of the hindlimb. Male Sprague-Dawley rats (250-320 g) were maintained in thermoneutral control conditions (ambient temperature = 24°C, n = 10) or exposed to an ambient temperature of 42°C until a colonic temperature (T(c)) of 39.5 or 41.0°C (n = 10 each) was attained. During heating, as T(c) rose from control levels (~38°C) to 41.0°C, mean arterial pressure and heart rate increased from 120 ± 3 to 141 ± 3 mmHg and from 381 ± 4 to 420 ± 5 beats/min, respectively (P < 0.05). There was a strong trend for increased NE turnover rates in the left ventricle, liver, and adrenal gland in NE synthesis-blocked rats attaining a T(c) of 39.5°C compared with the normothermic values, whereas the turnover rate in the kidney was significantly elevated at this level of hyperthermia (126%) vs. the control condition. In animals heated to a T(c) of 41.0°C, the NE turnover rate was markedly increased in the left ventricle (590%), kidney (531%), liver (262%), and adrenal gland (602%) compared with normothermic control values. In contrast to the internal organs, the rate of NE turnover in the soleus and extensor digitorum longus muscles did not change significantly from control levels in animals heated to 39.5 or 41.0°C. These findings indicate that hyperthermia can be a potent stimulus for increasing SNS activity, as estimated from tissue NE turnover rates, in the conscious rat. Moreover this apparent sympathetic neural activation is region specific and is dependent on the degree of hyperthermia.