In vivo assessment of decarboxylase inhibition or potentiation: urinary dopamine and L-dopa output after L-dopa administration.

In the L-Dopa treated rat, a decreased urinary output of free and conjugated dopamine and an increase in free and conjugated L-Dopa excretion after administration of decarboxylase-inhibiting drugs provide a good in vivo index of Dopa decarboxylase inhibition. With the exception of free dopamine output, which showed an equivocal change, these measurements appear to provide a good yardstick of decarboxylase status in man also. Using this approach, it was not possible to find any evidence of facilitation of decarboxylase action, in L-Dopa-treated parkinsonians given pyridoxine supplements, to account for the ability of this compound to neutralize the beneficial effect of L-Dopa.