Abstract

Di- and tripeptides, as well as peptidomimetic drugs such as cephalexin (CFX), are absorbed by enterocytes via the oligopeptide transporter PepT1. We recently showed that the α₂-adrenergic agonist clonidine increases CFX absorption in anaesthetized rats. Herein, we investigated whether α₂-adrenergic receptors can directly affect PepT1 activity in a clone of the differentiated human intestinal cell line Caco-2 (Caco-2 3B) engineered to stably express α_2A-adrenergic receptors at a density similar to that found in normal mucosa. Measurement of CFX fluxes across cell monolayers cultured on transwell filters demonstrated that the α₂-agonists clonidine and UK14304 caused a 2-fold increase of CFX transport in Caco-2 3B cells, but not in Caco-2 (expressing PepT1 but not α₂-adrenergic receptors) or in the HT29 19A clone (expressing α₂-adrenergic receptors but not PepT1). The stimulatory effect of clonidine was abolished by glycyl-sarcosine (a competitor for the transporter) and blocked by yohimbine or RX821002 (α₂-antagonists). Analysis of the kinetics of CFX transport in control and clonidine-treated Caco-2 3B cells showed that clonidine increased V_maxof CFX transport without changing K_m. Clonidine action was abolished by colchicine but not altered by amiloride, demonstrating that microtubule integrity but not Na⁺/H⁺ exchanger activity is necessary for the effect of α₂-agonists to occur. In conclusion, clonidine can directly activate α₂-adrenergic receptors located on epithelial cells. The precise molecular mechanisms whereby these receptors modulate PepT1 activity remain to be elucidated but an increased translocation to the apical membrane of preformed cytoplasmic transporter molecules is likely to be involved.