| Abstract: | New England Deaconess Hospital rats implanted with a pheochromocytoma P259 became hypertensive and showed high concentrations of plasma dopamine (42.0 +/- 14.6 ng/ml) and norepinephrine (45.7 +/- 8.4 ng/ml). However, the norepinephrine content of several peripheral tissues of these rats did not differ from those of the New England Deaconess Hospital control rats, and their dopamine content, although slightly higher, was much lower than would have been expected from the plasma dopamine levels. Methylation by catechol-O-methyltransferase did not appear to play a major role in the inactivation of tissue catecholamines since there were no noticeable increases of normetanephrine or 3-methoxytyramine in the tissues of the rats with pheochromocytoma. There was also no increase in conjugated dopamine, in either the sulfate or glucuronide form, in the plasma or tissue of the hypertensive rats, although injection of L-dopa induced a large increase in dopamine sulfate in the plasma and urine of these rats. This finding indicated that, although their sulfoconjugation mechanism was intact and not affected by the pheochromocytoma, it did not participate in the metabolism of dopamine released by the tumor into the blood. On the other hand, plasma and urine of tumor-bearing rats exhibited abnormally high concentrations of homovanillic acid, the main metabolite of dopamine resulting from monoamine oxidase action. In contrast to the control rats, intravenous infusion of free dopamine in rats with pheochromocytoma had no effect on plasma free dopamine levels but increased homovanillic acid levels considerably. The present data underline the important role of monoamine oxidase in the removal of excessive quantities of catecholamines released by the tumor in New England Deaconess Hospital rats with the pheochromocytoma implant. |
