Stereoselective transport of histidine in rat lung microvascular endothelial cells

1 Department of Pharmaceutics I, Tohoku Pharmaceutical University, Aoba-ku, Sendai 981-8558; and 2 Department of Pharmacy, Sapporo National Hospital, Sapporo 003-0804, Japan The transport characteristics of L - and D -histidine through the blood-lung barrier were studied in cultured rat lung microvascular endothelial cells (LMECs). L -Histidine uptake was a saturable process. The addition of metabolic inhibitors [2,4-dinitrophenol (DNP) and rotenone] reduced the uptake rate of L -histidine. Ouabain, an inhibitor of Na + -K + -ATPase, also reduced uptake of L -histidine. Moreover, the initial L -histidine uptake rate was reduced by the substitution of Na + with choline chloride and choline bicarbonate in the incubation buffer. The system N substrate, L -glutamic acid -monohydroxamate, also inhibited uptake of L -histidine. However, system N-mediated transport was not pH sensitive. These results demonstrated that L -histidine is actively taken up by a system N transport mechanism into rat LMECs, with energy supplied by Na + . Moreover, the Na + -independent system L substrate, 2-amino-2-norbornanecarboxylic acid (BCH), had an inhibitory effect on L -histidine uptake in Na + removal, indicating facilitated diffusion by a Na + -independent system L transport into the rat LMECs. These results provide evidence for there being at least two pathways for L -histidine uptake into rat LMECs, a Na + -dependent system N and Na + -independent system L process. On the other hand, the uptake of D -histidine into rat LMECs was not reduced by the addition of DNP, rotenone, or ouabain, or by Na + replacement. Although the uptake of D -histidine was reduced in the presence of BCH, the addition of L -glutamic acid -monohydroxamate did not significantly decrease uptake of D -histidine. These results suggest that the uptake of D -histidine by rat LMECs has different characteristics compared with its isomer, L -histidine, indicating that system N transport did not involve D -histidine uptake. sodium-dependent system N transport