Differential effects of L-thyroxine on cardiac and hepatic monoamine oxidase activity toward benzylamine and serotonin.

Male and female rats of 2 age groups were given subcutaneous injections of L-thyroxine (500 mug/kg) at 12 h intervals for a period of 10 days. The activity of monoamine oxidase (MAO) in normal and thyrotoxic rats was studied with two substrates: benzylamine and serotonin. The results showed that a thyroxine effect on cardiac and liver MAO activity is dependent on the substrate used in the assay. Kinetic studies of cardiac and liver MAO after thyroxine-treatment showed an unaltered Km for benzylamine but a change in Km for serotonin. Both findings may indicate a discriminative action of thyroid hormones on different forms of MAO. The possible presence of a soluble activator and inhibitor for MAO was investigated.