N¹-methylnicotinamide transport by isolated perfused snake proximal renal tubules

N¹-methylnicotinamide (NMN) transport was studied in isolated perfused snake (Thamnophis spp.) proximal renal tubules. Unidirectional lumen-to-bath (J(NMN)(l→b)) and bath-to-lumen (J(NMN)(b→l)) fluxes saturated. Although J(NMN)(l→b) and J(NMN)(b→l) were similar, mean J(NMN)(l→b) tended to exceed mean J(NMN)(b→l) at all concentrations studied. Direct measurements confirmed a net reabsorptive flux equal to the difference between the unidirectional fluxes. This transport, opposite in direction to tetraethylammonium (TEA) transport, was not inhibited by TEA. Transport into the cells across both the luminal and peritubular membranes during flux measurements was apparently down an electrochemical gradient by a mediated process that was sodium dependent. Inhibition with NMN analogues suggested that transport into the cells across the luminal membrane during J(NMN)(l→b) was more specific than transport into the cells across the peritubular membrane during J(NMN)(b→l). Transport out of the cells across both the luminal and peritubular membranes during flux measurements was apparently against an electrochemical gradient.