Tetrapentylammonium (TPeA): slowly dissociating inhibitor of the renal peritubular organic cation transporter

The efflux of tetraethylammonium (TEA) from suspensions of rabbit renal proximal tubules is completely blocked by 500 μM tetrapentylammonium (TPeA) in the extracellular medium. The basis of this trans-inhibition of TEA transport by TPeA was examined in tubule suspensions. At TPeA concentrations < 10 μM, efflux of TEA was reduced by ∼ 50%, whereas at concentrations > 10 μM, TPeA reduced efflux an additional 50% to produce a near complete block of TEA efflux. Increasing concentrations of TPeA from 0-500 μM were found to produce a biphasic, concentration-dependent trans-inhibition of TEA efflux from tubule suspensions suggesting that TPeA may block efflux by binding to both a high and low affinity TPeA binding site. The trans-inhibition of TEA efflux by TPeA at low concentrations (< 10 μM) may result from a slow carrier turnover when TPeA is bound to the carrier site. To determine whether the inhibitory effectiveness of TPeA was also associated with its slow dissociation from the carrier site, the effect of a 10 s preincubation with 1 μM TPeA on TEA uptake was examined. The uptake of TEA by tubules preincubated for 10 s with TPeA was reduced by ∼ 30-50% compared to control tubules not preincubated with TPeA. A 10 s preincubation with 150 μM unlabeled TEA had no effect on TEA uptake compared to control tubules not preincubated with TEA. When the 10 s preincubation with 1 μM TPeA was followed by a 10 min recovery period, TEA uptake returned to control levels, indicating that the prolonged inhibition was reversible. This prolonged inhibition of TEA uptake after a 10 s preincubation with 1 μM TPeA, as suspected, may arise from a slow dissociation of TPeA from the OC transporter following a rapid association to the binding site. TPeA inhibition of TEA uptake into tubules was competitive in nature with a K_i of 1 μM. The ability of TEA to compete with TPeA for binding to the carrier suggests that the binding of TPeA to the carrier can be displaced by large concentrations of TEA. These observations suggest that the interactions of TPeA, and perhaps similarly large hydrophobic OCs, with the OC transporter are complex.