TETRAPENTYLAMMONIUM (TPEA) - SLOWLY DISSOCIATING INHIBITOR OF THE RENAL PERITUBULAR ORGANIC CATION TRANSPORTER

The efflux of tetraethylammonium (TEA) from suspensions of rabbit rena l proximal tubules is completely blocked by 500 mu M tetrapentylammoni um (TPeA) in the extracellular medium. The basis of this trans-inhibit ion of TEA transport by TPeA was examined in tubule suspensions. At TP eA concentrations < 10 mu M, efflux of TEA was reduced by similar to 5 0%, whereas at concentrations > 10 mu M, TPeA reduced efflux an additi onal 50% to produce a near complete block of TEA efflux. Increasing co ncentrations of TPeA from 0-500 mu M were found to produce a biphasic, concentration-dependent trans-inhibition of TEA efflux from tubule su spensions suggesting that TPeA may block efflux by binding to both a h igh and low affinity TPeA binding site. The trans-inhibition of TEA ef flux by TPeA at low concentrations(< 10 mu M) may result from a slow c arrier turnover when TPeA is bound to the carrier site. To determine w hether the inhibitory effectiveness of TPeA was also associated with i ts slow dissociation from the carrier site, the effect of a 10 s prein cubation with 1 mu M TPeA on TEA uptake was examined. The uptake of TE A by tubules preincubated for 10 s with TPeA was reduced by similar to 30-50% compared to control tubules not preincubated with TPeA. A 10 s preincubation with 150 mu M unlabeled TEA had no effect on TEA uptake compared to control tubules not preincubated with TEA. When the 10 s preincubation with 1 mu M TPeA was followed by a 10 min recovery perio d, TEA uptake returned to control levels, indicating that the prolonge d inhibition was reversible. This prolonged inhibition of TEA uptake a fter a 10 s preincubation with 1 mu M TPeA, as suspected, may arise fr om a slow dissociation of TPeA from the OC transporter following a rap id association to the binding site. TPeA inhibition of TEA uptake into tubules was competitive in nature with a K-i of 1 mu M. The ability o f TEA to compete with TPeA for binding to the carrier suggests that th e binding of TPeA to the carrier can be displaced by large concentrati ons of TEA. These observations suggest that the interactions of TPeA, and perhaps similarly large hydrophobic OCs, with the OC transporter a re complex.