Abstract

To evaluate the first-pass hydrolysis of O-isovaleryl-propranolol (isovaleryl-PL), which was used as a model ester-compound, rat intestinal jejunum and blood vessels were perfused simultaneously. The membrane permeability of isovaleryl-PL was greater than that of PL because it was more lipophilic. Isovaleryl-PL was almost completely hydrolyzed to PL and isovaleric acid (IVA) in epithelial cells at a rate limited by its uptake. Based on pH partitioning, PL and IVA were transported into both vascular (pH 7.4) and luminal sides (pH 6.5). Therefore, when isovaleryl-PL was perfused into the jejunal lumen, more than 90% permeated into the blood vessel as PL. In addition, PL appeared in the lumen at a rate 6-fold greater than that in blood vessels. When isovaleryl-PL was perfused, its disappearance (50.5 ± 1.95 nmol/min) was the sum of the absorption and secretion rates of PL. In contrast, IVA was transported into blood vessels rather than the jejunal lumen. In addition, the calculated degradation clearance from in vitro hydrolysis (K_m 13.7 ± 1.71 μM, V_max 29.1 ± 3.81 nmol/min/mg protein) was 3.42 ml/min/10 cm jejunum, which was 24-fold greater than the observed degradation clearance (CL_deg) (0.14 ± 0.02 ml/min/10 cm jejunum). These findings indicate that in addition to the liver, the intestine markedly contributes to first-pass hydrolysis.