THE EFFECT OF MONOAMINE OXIDASE INHIBITORS ON GROWTH AND THE RAT TIBIA TEST

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Abstract
SUMMARY: A large number of monoamine oxidase (MAO) inhibitors: pivhydrazine (Tersavide), nialamide (Niamide), isocarboxazid (Marplan), mebanazine (Actomol), phenelzine (Nardil), pheniprazine (Catron) and tranylcypromine (Parnate), decrease cartilage growth in normal rats. This inhibition is not directly due to their properties as MAO inhibitors. Thus, for example, iproniazid (Marsilid), 2,5-dichlorophenylhydrazine, etryptamine (Monase) and amphetamine (Benzedrine) did not decrease epiphysial growth of the tibia in immature female rats. MAO inhibitors with a methyl residue on the α-carbon atom inhibited growth to a greater extent than those with a hydrogen in place of the methyl group. Pargyline (Eutonyl) did not depress cartilage growth in adrenalectomized rats, even when they received large doses of adrenaline. However, pargyline inhibited cartilage growth in adrenalectomized animals receiving hydrocortisone and in normal rats. Thus the growth-inhibiting effect of pargyline seems to be mediated by its potentiating effect on corticosteroids. Growth hormone prevented the inhibitory effect of pargyline on cartilage growth. Pargyline alone given to normal young male or female rats decreased weight gain for 2 weeks. Later the experimental animals caught up with the controls. It is assumed that the transient weight loss is due to the sympathomimetic effect of the MAO inhibitor, which depresses hypothalamic centres concerned with appetite, weight gain and growth.