Distinct transport activity of tetraethylammonium from l-carnitine in rat renal brush-border membranes

Abstract

We investigated the contribution of the Na+/l-carnitine cotransporter in the transport of tetraethylammonium (TEA) by rat renal brush-border membrane vesicles. The transient uphill transport of l-carnitine was observed in the presence of a Na+ gradient. The uptake of l-carnitine was of high affinity (Km=21 μM) and pH dependent. Various compounds such as TEA, cephaloridine, and p-chloromercuribenzene sulfonate (PCMBS) had potent inhibitory effects for l-carnitine uptake. Therefore, we confirmed the Na+/l-carnitine cotransport activity in rat renal brush-border membranes. Levofloxacin and PCMBS showed different inhibitory effects for TEA and l-carnitine uptake. The presence of an outward H+ gradient induced a marked stimulation of TEA uptake, whereas it induced no stimulation of l-carnitine uptake. Furthermore, unlabeled TEA preloaded in the vesicles markedly enhanced [14C]TEA uptake, but unlabeled l-carnitine did not stimulate [14C]TEA uptake. These results suggest that transport of TEA across brush-border membranes is independent of the Na+/l-carnitine cotransport activity, and organic cation secretion across brush-border membranes is predominantly mediated by the H+/organic cation antiporter.