Effect of thyroxine and related compounds on monoamine oxidase activity

Effect of hyper- and hypothyroidism on platelet monoamine oxidase activity and serotonin metabolism

We evaluated platelet monoamine oxidase (MAO) activity in 13 hyperthyroid and 9 hypothyroid patients. The platelet MAO activity of these patients did not differ from that of age- and sex-matched euthyroid control subjects. Nineteen hyperthyroid and twelve hypothyroid subjects had serum serotonin concentrations similar to those of age- and sex-matched euthyroid control subjects. We also determined the urinary excretion of 5-hydroxy-indoleacetic acid (5-HlAA), serotonin, tryptamine, and tyramine in five hyperthyroid and four hypothyroid subjects. Two hyperthyroid patients had increased tyramine excretion; one of these patients also had increased tryptamine excretion. Three hypothyroid patients and one hyperthyroid patients had slightly decreased serotonin excretion. Both the hyper- and the hypothyroid subjects had normal urinary 5-HIAA excretion. There was no relationship between platelet MAO activity and monoamine excretion in the patients with thyroid dysfunction. We conclude that there is no systematic change in platelet MAO activity or serum serotonin concentration in patients with hyper- or hypothyroidism.

Physiological and pathological changes in tissue monoamine oxidase activity

This article explores the possibility that malfunction of the enzyme monoamine oxidase (E.C. 1.4.3.4., MAO) could lead to aberrations in the catabolism of biogenic amines in the central nervous system and give rise to certain mental abnormalities. No conclusive evidence could be presented to substantiate this. Data on the normal function of the enyzme (for example its existence in multiple forms, the control of MAO activity by hormones or the independent development of MAO activities towards different substrates during maturation) are reviewed.

The effects of thyroid hormones on monoamine oxidase in the rat heart

The administration of thyroxine to young male rats produced an increase in the specific activity of their cardiac monoamine oxidase (MAO). A reduction in the circulating concentrations of thyroid hormones, brought about by 2-thiouracil, led to a decrease. The relative change in activity produced was greater with tyramine than with benzylamine as substrate. By following the time-course of the return of enzyme activity, with tyramine as substrate, after a single injection of pargyline in vivo, it was concluded that both excess and lack of thyroid hormones cause their effects on MAO activity by changing the rate of synthesis of the enzyme and not its degradation rate constant. The degradation rate constant did change with the age of the animal. The MAO activity, which increased towards tyramine as substrate in hyperthyroid rat hearts, behaved in the same way as that of controls to heat treatment, irreversible inhibition by pargyline or by clorgyline and also in Km determinations. The pattern for benzylamine oxidation was similar, except for the effect of the inhibitor clorgyline which shifted the plateau region of the double sigmoid inhibition curve significantly using enzyme from hyperthyroid rat hearts. The plateau region was also shown to be affected by the age of the animal. The possibility is discussed that the increased cardiac MAO activity produced by thyroid hormones and by the growth of the animal is mediated by that form of the enzyme primarily responsible for the oxidation of tyramine. Mixed substrate experiments suggested that tyramine oxidation could be inhibited competitively by benzylamine.

Influence of thyroid hormones on the sensitivity of cardiac and smooth muscle to biogenic amines and other drugs

1. Rats and guinea-pigs were injected with 1-thyroxine or thyroidectomized. Sensitivity of isolated tissues and of the intact cardiovascular system to drugs was investigated.2. Thyroxine increased the sensitivity of the isolated perfused heart to noradrenaline, adrenaline, isoprenaline, acetylcholine and histamine.3. Thyroxine increased the sensitivity of the rat isolated uterus to noradrenaline, adrenaline, isoprenaline, acetylcholine, histamine and 5-hydroxytryptamine.4. In contrast, thyroxine decreased the sensitivity of the isolated ileum to acetylcholine, histamine and 5-hydroxytryptamine, but not to adrenaline.5. Thyroxine also decreased the sensitivity of the isolated aorta to noradrenaline, adrenaline, histamine and 5-hydroxytryptamine.6. In pithed rats, thyroxine potentiated pressor responses to (+)-amphetamine but not to noradrenaline or synephrine.7. In anaesthetized thyroxine-treated rats, blood pressure responses to (+)-amphetamine were potentiated while those to noradrenaline were unaltered. Responses to acetylcholine were increased, and the response to isoprenaline became purely pressor.8. Thyroxine-induced changes in the sensitivity of the isolated uterus and ileum to modification of the concentration of calcium in the physiological salt solution paralleled the changes in drug sensitivity of these tissues.9. In general, thyroidectomy produced the opposite effects to those of thyroxine.10. It is concluded that there is no specific interaction between thyroxine and sympathomimetic amines, the effect of thyroxine on drug sensitivity being non-specific.

THE METABOLISM OF CATECHOLAMINES IN HYPERTHYROID AND VITAMIN B12 DEFICIENT RATS

The effect of hyperthyroidism and vitamin B12 deficiency was studied on liver monoamine oxidase, liver catechol-O-methyl transferase, and the noradrenaline and adrenaline content of liver, heart, and brain. In the hyperthyroid rats, there was a 25% decrease in liver monoamine oxidase and this decrease does not appear to be related to a decrease in body weight. Liver catechol-O-methyl transferase activity was normal in hyperthyroid animals. The noradrenaline and adrenaline content of liver, heart, and brain were also not modified. Vitamin B12 deficiency did not affect catechol-O-methyl transferase activity but hepatic monoamine oxidase was lower by 20–30% in rats supplemented with the vitamin. In vitro, vitamin B12 and methyl cobalamin did not modify the activity of catechol-O-methyl transferase in concentrations ranging from 1 × 10−3 M to 1 × 10−6 M. Vitamin B12 also did not affect monoamine oxidase activity at concentrations of 1 × 10−4 M to 1 × 10−5 M.