Influence of cold exposure on catecholamine depleting actions of hydroxylase inhibitors.

Abstract The importance of catecholamine secretion in the cold was studied by treating rats placed at 27°C or 4°C with alpha-methyl- p -tyrosine, a tyrosine hydroxylase inhibitor, H 22 54 , a tyrosine and phenylalanine hydroxylase inhibitor, or trimethyldopa, a phenylalanine hydroxylase inhibitor. Alpha-methyl- p -tyrosine produced the largest fall in tissue noradrenaline. H 22 54 and trimethyldopa, although causing a moderate fall in noradrenaline stores at 27°, were more effective in the cold room. Alpha-methyl- p -tyrosine and trimethyldopa depressed noradrenaline excretion and increased adrenaline release in the cold. The adrenaline served as a “second line of defense” evoked by the noradrenaline decrease. Adrenodemedullated rats given alpha-methyl- p -tyrosine or trimethyldopa at 4° showed a high rate of mortality. H 22 54 lowered tissue and urinary levels of noradrenaline in the cold but failed to produce hypothermia and death in intact or adrenodemedullated rats. It is suggested that other actions of H 22 54 , such as an inhibition of catechol- o -methyl transferase, may have contributed to the maintenance of normothermia.