Decrease in adrenal tyrosine hydroxylase and increase in norepinephrine synthesis in rats given L-dopa.

When large doses of L-dopa (1000 milligrams per kilogram, given subcutaneously) were administered to rats, the rate of catecholamine synthesis was increased in both the heart and adrenal gland. It is likely that increases also occur in other sympathetically innervated tissues. When the same dose of L-dopa was given daily for 4 or 7 days, the levels of tyrosine hydroxylase in the adrenal were lowered in comparison to controls. These findings may be a further indication of the existence of a regulatory mechanism which modifies endogenous levels of tyrosine hydroxylase in response to changes in the biosynthetic demand for norepinephrine.