alpha(2)-adrenergic receptors stimulate oligopeptide transport in a human intestinal cell line

Di- and tripeptides, as well as peptidomimetic drugs such as cephalexin (CF X), are absorbed by enterocytes via the oligopeptide transporter PepT1. We recently showed that the alpha(2)-adrenergic agonist clonidine increases CF X absorption in anaesthetized rats. Herein, we investigated whether alpha(2 )-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 alpha(2A)-adrenergic receptors at a density similar to that found in normal mucosa. Measurement of CFX fluxes across cell monolayers c ultured on transwell filters demonstrated that the alpha(2)-agonists clonid ine and UK14304 caused a 2-fold increase of CFX transport in Caco-2 3B cell s, but not in Caco-2 (expressing PepT1 but not alpha(2)-adrenergic receptor s) or in the HT29 19A clone (expressing alpha(2)-adrenergic receptors but n ot PepT1). The stimulatory effect of clonidine was abolished by glycyl-sarc osine (a competitor for the transporter) and blocked by yohimbine or RX8210 02 (alpha(2)-antagonists). Analysis of the kinetics of CFX transport in con trol and clonidine-treated Caco-2 3B cells showed that clonidine increased V-max of 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 alpha(2)-agonists to occur. In conclusion, clonidine can directly activate alpha(2)-adrenergic receptors located on epithelial cells. The precise mol ecular mechanisms whereby these receptors modulate PepT1 activity remain to be elucidated but an increased translocation to the apical membrane of pre formed cytoplasmic transporter molecules is likely to be involved.