Carbidopa inhibits the growth hormone- and prolactin-suppressive effect of L-dopa in acromegalic patients.
The plasma GH, PRL, TSH, and dopamine (DA) responses to an infusion of L-dopa were examined in six acromegalic patients before and after pretreatment with carbidopa, a drug which inhibits the peripheral conversion of L-dopa to DA. Carbidopa neither modified baseline DA nor induced changes in baseline GH, PRL, or TSH levels. The drug instead markedly reduced the L-dopa-induced DA rise, an effect which was concomitant to a striking reduction of the suppressive effect of L-dopa on plasma GH and a partial inhibition of the suppressive effect of L-dopa on plasma PRL. TSH levels did not change either after L-dopa alone or L-dopa plus carbidopa. These data demonstrate that in "responder" acromegalics, L-dopa inhibits GH secretion through its peripheral conversion to DA and not via activation of central DA neurotransmission. For the effect of L-dopa on PRL secretion, in addition to a peripheral dopaminergic component, a central component cannot be disregarded.
D000172:Acromegaly / Q000503:physiopathology*; D000328:Adult; D002230:Carbidopa; D004298:Dopamine / Q000097:blood*; D005260:Female; D013006:Growth Hormone / Q000097:blood*; D006801:Humans; D007700:Kinetics; D007980:Levodopa; D008297:Male; D008875:Middle Aged; D011388:Prolactin / Q000097:blood*; D013972:Thyrotropin / Q000097:blood*
D007980:Levodopa; D011388:Prolactin; D013972:Thyrotropin; D013006:Growth Hormone; D002230:Carbidopa; D004298:Dopamine