Catecholamine concentrations and the activity of tyrosine hydroxylase after an increase in the concentration of tyrosine in rat tissues

The concentrations of tyrosine in rat plasma and brain were increased 2-7 fold by the administration of either L-tyrosine or cycloheximide. Under these conditions catecholamine concentrations in the brain and the heart remained unchanged even when the rats were maintained in a cold environment to increase catecholamine turnover. The data are interpreted to mean that an increase in the tyrosine concentration in the tissues does not result in an in vivo substrate inhibition of tyrosine hydroxylase.