Transport of clonidine across cultured brain microvessel endothelial cells.

Brain penetration of clonidine, an alpha-2 adrenoceptor agonist, was studied using an in vitro cell culture system consisting of primary cultures of porcine brain capillary endothelial cells. Uptake of clonidine was measured as a function of its concentration in the incubation mixture. Saturation of uptake was apparent and could be described by Michaelis-Menten-type kinetics (K(M) = 1.34 mM; Vmax = 0.099 nmol/min/cm2). Saturation was not observed at a low temperature (4 degrees C). Transendothelial transport experiments revealed that translocation of clonidine cannot be attributed solely to paracellular leakage. Uptake was reduced at low extracellular pH or by using an incubation buffer that contained the K+ ionophore valinomycin. Time-dependent uptake of clonidine and transendothelial transport were slower than expected considering the high octanol-to-buffer partition coefficient of this compound. On the basis of transendothelial transport experiments, we concluded that the carrier system responsible for active transport of clonidine is located at both the apical and the basolateral membrane domain.