Cardiac sympathetic activity during rat pregnancy

The turnover of injected tracer [³H] norepinephrine (NE) was determined in heart and interscapular brown adipose tissue of virgin and 10-day and 20-day pregnant rats. In two experiments fractional [³H]NE turnover in heart was 87% and 92% higher in 20-day pregnant animals compared to virgin controls, but did not differ between 10-day pregnant and control animals. NE turnover in brown adipose tissue did not differ between control and pregnant animals at either gestational age. Twenty-four-hour urinary excretion of NE, epinephrine (E), and dopamine (D) was measured serially in six pregnant rats and compared to virgin controls. NE excretion during pregnancy was significantly higher than the controls and showed a progressive increase during the last third of pregnancy. At term the excretion rate was 2.6-fold greater than that of controls. Excretion of E and D did not differ between pregnant and nonpregnant animals. It is concluded that cardiac sympathetic nervous system activity increases during rat pregnancy. That this change in sympathetic activity is not global is indicated by the finding of unchanged NE turnover in interscapular brown adipose tissue. Urinary excretion data are consistent with increased sympathetic activity during late gestation, with no change in adrenal medullary function.