Effects of enzyme inhibitors of catecholamine metabolism and of haloperidol on the pancreatic secretion induced by L-DOPA and by dopamine in dogs.

1. Effects of inhibitors of DOPA decarboxylase, dopamine beta-hydroxylase and monoamine oxidase, and haloperiodol on the secretion of pancreatic juice induced by L-DOPA and dopamine were studied in preparations of the isolated blood-perfused canine pancreas.2. The increased secretion induced by the infusion of L-DOPA (100 mug/min) was completely antagonized by Ro 4-4602 (300 mug), a DOPA decarboxylase inhibitor.3. The secretogogue effect of dopamine (1-10 mug) intra-arterially was not affected by Ro 4-4602, but was enhanced by the infusion of fusaric acid (100 mug/min), a dopamine beta-hydroxylase inhibitor.4. The increase in the secretion induced by dopamine (1-10 mug) was enhanced by treatment with nialamide (100 mg/kg), a monoamine oxidase inhibitor, given intravenously.5. Haloperidol (1 mg) intra-arterially attenuated the dopamine-induced pancreatic secretion.6. It is concluded that L-DOPA is converted to dopamine in the acinar cells which causes an increase in the secretion of pancreatic juice, thus the intracellular level of dopamine may be controlled by enzymatic equilibrium.