The content and turnover of norepinephrine in the rabbit brain regions and cardiovascular tissues

The content and turnover of norepinephrine (NE) in the hypothalamus, midbrain, pons-medulla, mesenteric artery, aorta and left ventricle were studied in the normal rabbit. Turnover rate of NE was determined by measuring the rate of decline in NE content of the tissue after blockade of synthesis with alpha-methyl-p-tyrosine. The NE content of the hypothalamus (1.44+/-0.03 microgram/g) was significantly higher than those of the midbrain (0.25+/-0.01 microgram/g) and the pons-medulla (0.36+/-0.01 microgram/g) (p less than 0.001), whereas the rate constant of NE turnover for the pons-medulla (0.213+/-0.009 hr-1) was significantly greater than those for the hypothalamus (0.164+/-0.008 hr-1, p less than 0.001) and the midbrain (0.180+/-0.008 hr-1, p less than 0.01). In the cardiovascular tissues examined, the NE content was highest in the mesenteric artery (6.33+/-0.19 microgram/g), moderate in the left ventricle (2.08+/-0.10 microgram/g) and lowest in the aorta (0.70+/-0.06 microgram/g). The differences among them were significant (p less than 0.001). However, the rate constant of NE turnover for the aorta (0.119+/-0.014 hr-1) was significantly greater than those for the mesenteric artery (0.059+/-0.008 hr-1, p less than 0.001) and the left ventricle (0.069+/-0.006 hr-1, p less than 0.005). The turnover rate of NE in the mesenteric artery was high, 0.372 microgram/g-hr, which suggests the very active NE synthesis. These results indicate that there are regional differences in content and turnover of NE of the cardiovascular tissues as well as of the brain.