Catecholamine Synthesis in Rabbits with Neurogenic Hypertension

Norepinephrine synthesis was determined in the heart and adrenals of rabbits made hypertensive by complete denervation of the carotid sinuses and aortic arch. There was a 24% reduction of norepinephrine concentration in the left ventricle 3 weeks after denervation, despite evidence of enhanced synthesis as determined by three methods: (1) an 18% increase in apparent synthesis rates (from 0.76 to 0.90 mμmoles/g/hour) calculated from norepinephrine turnover rates after infusion of DL-norepinephrine-3H; (2) a 124% increase in synthesis rates (from 2.1 to 4.7 mμmoles/g/hour) estimated from the incorporation of label from infused tyrosine-3H; (3) a 50% increase (from 4.0 to 6.0 mμmoles/g/hour) in the activity of the regulatory enzyme tyrosine hydroxylase. Additionally, in the denervated rabbits there was a 43% reduction in the amount of epinephrine in the adrenal gland 2 days after denervation and a return to normal values at 3 weeks. Adrenal tyrosine hydroxylase, phenylethanolamine-N-methyl transferase, and the incorporation of label from tyrosine-3H into epinephrine were increased at 3 weeks. A smaller number of hypertensive rabbits studied at 2 days showed a reduction in left ventricular norepinephrine concentration. Catecholamine reductions, due only in part to dilutional hypertrophy, may be related to increased utilization and may serve to enhance synthesis of the neurotransmitter and maintain the increase in blood pressure.