INFLUENCE OF SUBSTRATE STRUCTURE ON TURNOVER OF THE ORGANIC CATION H+EXCHANGER OF THE RENAL LUMINAL MEMBRANE

We examined the influence of organic cation (OC) structure on the rate of turnover of the OC/H+ exchanger in rabbit renal brush-border membr ane vesicles (BBMV). The rate of efflux of [C-14]tetraethylammonium ([ C-14]TEA) from BBMV, measured in the presence of an inwardly directed chemical gradient for test agent, provided an indirect measure of acti vity of the OC/H+(OC) exchanger. The trans-stimulation of [C-14]TEA ef flux from BBMV was a saturable function of increasing extravesicular c oncentration of both unlabeled TEA and tetramethylammonium (TMA), with an apparent Michaelis constant (K-t) for the interaction of these com pounds with the OC/H+(OC) exchanger of 25 mu M and 1 mM, respectively. The effect on [C-14]TEA efflux of saturating extravesicular concentra tions of a series of n-tetraalkylammonium compounds was examined. Wher eas the short-chain compounds TMA and TEA markedly stimulated [C-14]TE A efflux (by 830% and 690%, respectively), the long-chain compounds te trapropylammonium and tetrabutylammonium were less effective, increasi ng efflux by only 40% and 120%, respectively. When the exchanger was s aturated with tetrapentylammonium, mediated efflux of [C-14]TEA was re duced. Increasing alkyl chain length was also correlated with an incre ase in the inhibitory effect (as measured by the apparent inhibition c onstant, K-i, or the IC50 value) that these compounds had against tran sport of [C-14]TEA by the OC/H+(OC) exchanger; i.e., there was a corre lation between decreasing IC,, and decreasing turnover of the OC/H+(OC ) exchanger. This same correlation was observed for a broader set of t est agents of diverse molecular structure, including a series of n-tet raalkylammonium and -phosphonium compounds and the OCs, choline, N-1-m ethyl nicotinamide, 1-methyl-4-phenylpyridinium, and amiloride. Becaus e high affinity of substrates for the OC/H+(OC) exchanger is correlate d with increasing substrate hydrophobicity, we conclude that the inter action of hydrophobic OCs with the renal OC/H+(OC) exchanger results i n the formation of a substrate-exchanger complex that has a comparativ ely low rate of turnover.