L-Thyroxine and monoamine oxidase activity in the kidney and some other organs of the rat.

L-Thyroxine (T4) (10,30 and 300 mug/kg. S.C. for 5 days) increased kidney monoamine oxidase (MAO) activity and decreased that of the liver, using tryptamine as the substrate. These changes were dose-dependent. Time-dependent changes were found with the 30 mug/kg dose. Heart MAO activity was increased by the high dose while small decreases were found with the lower doses. No changes were found in the whole brain, vasa deferentia, the thyroid or adrenal glands. Hypophysectomized rats showed a 50% reduction in kidney MAO which was fully restored following T4 (10 mug/kg/day) treatment. Evidence was obtained that both T4 and L-triiodothyronine can activate the deaminating activity of MAO in vitro. Thus, T4 and/or its metabolites may play a physiological role in the maintenance of kidney MAO activity. The mechanism(s) whereby T4 and/or its metabolites alter kidney MAO activity is not resolved but direct effects upon the various forms of MAO and/or influences upon regulating substances seems likely to be involved.