NMN transport by snake renal tubules: Choline effects, countertransport, H⁺-NMN exchange

N¹-methylnicotinamide (NMN) transport by isolated perfused snake (Thamnophis spp.) proximal renal tubules was examined for effects of choline and evidence of countertransport of organic cations and H⁺-NMN exchange. Choline had no effect on NMN transport, supporting the concept of multiple organic cation transport processes. A transconcentration of unlabeled NMN had no effect on unidirectional transepithelial fluxes of labeled NMN. Low pH in the bath enhanced the lumen-to-bath flux of labeled NMN and, at the same time, reduced the intracellular concentration of labeled NMN, but low pH in the perfusate had no effect on the bath-to-lumen flux of labeled NMN. To examine transport at single membranes more directly in intact tubules, we examined efflux of labeled NMN across the luminal membrane of tubules covered with oil and across the peritubular membrane of tubules with oil-filled lumens. Neither unlabeled NMN nor low pH in the perfusate had any effect on efflux across the luminal membrane, providing no evidence of countertransport or H⁺-NMN exchange across this membrane. However, unlabeled NMN and low pH in the bath both significantly stimulated efflux across the peritubular membrane, providing evidence that both countertransport of organic cations and H⁺-NMN exchange may play a role in transport of NMN across this membrane.