Monoamine oxidase, thyroid activity and hyperglycaemia in the frog.

Abstract

SUMMARY 1. Estimations of monoamine oxidase (m.o.) activity in suspensions of frog liver taken at monthly intervals showed that activity was relatively constant during the greater part of the year (mean value 8·6 ± 0·332 μl. O2/15 min/mg colloid nitrogen), but it rose to a significantly higher level from August to November inclusive (mean 11·7 ± 0·513 μl. O2/15 min/mg colloid nitrogen). 2. The pattern of seasonal variation in enzyme activity showed no correspondence with the annual cycle of change in the thyroid gland, nor was there any correlation between m.o. activity of the liver and the histological appearance of the thyroid in individual frogs. 3. Inhibition of m.o. in vivo by iproniazid elicited a hyperglycaemic response to excitement in previously unresponsive frogs which was almost identical with that obtained after treatment with thyroxine and iproniazid. 4. There was no evidence that treatment with thyroxine led to any alteration in the activity of liver m.o. 5. It is concluded that m.o. is not an important factor in the inactivation of catechol amines in the strongly stimulated frog, but the possibility is discussed that under natural conditions of lower stress the high enzyme activity from August to November might be one factor tending to limit mobilization of liver glycogen.