Decarboxylation of l-Dopa in the Rat Isolated Vascularly Perfused Small Intestine: Contribution to Systemic Elimination and Dose-dependent First Pass Effect

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Abstract
The contribution of the rat small intestine to systemic and presystemic elimination of l-dopa was studied. When l-dopa was administered into the vascular perfusate, a systemic extraction ratio of 0·38 was found, the major part being decarboxylated to dopamine. The intestinal l-dopa clearance was estimated to be 171 mL min−1 kg−1. Thus, l-dopa intestinal clearance in rat represents up to at least 20% of the total body clearance. After luminal administration of l-dopa 83–88% of the administered dose was absorbed within 60 min. The total amount of l-dopa appearing in the vascular perfusate increased more than proportionally to the increase in the dose. In contrast, the amount of dopamine increased less than proportionally to the dose. As a result, the intestinal first pass appeared to be strongly dose-dependent. Since the total percentage absorbed from the lumen was independent of the administered dose and the total amount that appeared in the vascular perfusate increased linearly with the dose, the dose dependency was probably due to saturation of intestinal l-dopa decarboxylation.