Physiological and pharmacologic effects on TH activity in rabbit and cat carotid body.

The carotid bodies, along with the superior cervical ganglia and the adrenal glands, were removed from rabbits and cats and the activity of tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis, was assayed by the method of Nagatsu (Anal. Biochem. 9: 122-126, 1964). The activities of the enzyme, in nmols tyrosine hydroxylated x h-1 x mg tissue-1, were: carotid body, rabbit 1.29, cat 0.84; superior cervical ganglion, rabbit 8.66, cat 4.97; adrenal gland, rabbit 0.95, cat 2.25. With respect to the carotid body, each of the following experimental procedures resulted in a long-term increase in TH activity in the rabbit but not in the cat: 1) severe hypoxia (5% O2 in N2 for 1 h, assay of TH 48 h later); 2) chronic transection of the carotid sinus nerve (assay of TH at 12-15 days); or 3) administration of reserpine (10 mg/kg at 0 and 24 h, assay of TH at 48 h). These observations are compared with our previous findings for rat carotid body and are discussed in relation to the role of catecholamines in chemoreception, and, in particular, to the reported differences in dopamine action in the carotid bodies of these different species. Our results also suggest species differences with respect to the participation of the sympathoadrenal system in response to reserpine and hypoxic stress.