A choline transporter in renal brush-border membrane vesicles: energetics and structural specificity

Abstract

Choline is a quaternary ammonium compound that is normally reabsorbed by the renal proximal tubule, despite its acknowledged role as a substrate for the renal organic cation (OC) secretory pathway. The basis for choline reabsorption was examined in studies of transport in rabbit renal brush-border membrane vesicles (BBMV). Although an outwardly directed H+ gradient (pH 6.0in: 7.5out) stimulated uptake of tetraethylammonium (TEA), a model substrate of the OC/H+ exchanger in renal BBMV, it had no effect on uptake of 1 microM choline. A 5 mM trans concentration gradient of choline did, however, drive countertransport of both TEA and choline, although trans TEA had no effect on choline accumulation in BBMV. A 20 mM concentration of unlabeled choline blocked uptake of both choline and TEA by greater than 85%, whereas 20 mM TEA blocked only TEA uptake. The kinetics of choline uptake into vesicles preloaded with 1 mM unlabeled choline appeared to involve two, saturable transport processes, one of high affinity for choline (Kt of 97 microM) and a second of low affinity (Kt of approximately 10 mM), the latter presumably reflecting a weak interaction of choline with the OC/H+ exchanger. An inside-negative electrical PD stimulated the rate of uptake and supported the transient concentrative accumulation of choline in BBMV. The high affinity transporter showed a marked specificity for choline and closely related analogues. A model of the molecular determinants of substrate-transporter interaction is described. We conclude that the electrogenic high affinity pathway plays a central role in renal reabsorption of choline.