The Transport Mechanism of an Organic Cation,Disopyramide, by Brush-border Membranes. Comparison Between Renal Cortex and Small Intestine of the Rat

Abstract— The characteristics of disopyramide uptake in brush-border membrane vesicles isolated from rat renal cortex and small intestine were investigated. Transport of disopyramide into an osmotically reactive intravesicular space was observed with notable binding to the membrane surface. An outwardly directed H⁺ gradient stimulated disopyramide uptake, resulting in a transient uphill transport in both brush-border membranes. As for the renal brush-border membrane, the H⁺ gradient itself appeared to be the driving force for this stimulation of uptake. These findings suggest that disopyramide-H⁺ antiport is the mechanism of disopyramide action in renal cell membrane. The initial uptake was saturable (K_m and V_max of 680 μm and 1·25 nmol (mg protein)^?1/30 s, respectively). The stimulation of disopyramide uptake by an outward H⁺ gradient in rat intestinal brush-border membrane was due to an interior negative H⁺-diffusion potential. A K⁺-diffusion potential (interior negative) enhanced disopyramide uptake. These results suggest that there are different mechanisms of disopyramide uptake for renal and intestinal brush-border membrane vesicles.