TRANSPORT OF GUANIDINE IN RABBIT INTESTINAL BRUSH-BORDER MEMBRANE-VESICLES

The characteristics of guanidine uptake were studied in brush-border membrane vesicles isolated from the rabbit proximal intestine. Guanidine uptake was manyfold greater in the presence of an outward-directed H+ gradient (intracellular pH = 5.5; extracellular pH = 7.2) than in the absence of a H+ gradient (intracellular and extracellular pH = 7.2). The time course of guanidine uptake exhibited an overshoot phenomenon in the presence of the H+ gradient, indicating occurrence of uphill transport. This H+ gradient-stimulated guanidine uptake was not due to an inside-negative H+-diffusion potential because carbonyl cyanide 4-trifluoromethoxyphenylhydrazone, a protonophore, failed to have any effect on guanidine uptake. Moreover, the transient uphill transport of guanidine was observed even in voltage-clamped membrane vesicles. However, under the conditions that effectively dissipated the H+ gradient, there was no active transport for guanidine. This + gradient-dependent transport mechanism for guanidine is distinct from the Na+-H+ exchanger, because amiloride did not inhibit guanidine uptake even at a concentration as high as 100 .mu.M. These data provide evidence for the presence of a guanidine-H+ antiport system in the rabbit intestinal brush-border membrane. The presence of a carrier for guanidine in these membranes is further substantiated by the trans-stimulation of the uptake of radiolabeled guanidine by unlabled guanidine and by the inhibition of guanidine uptake by imipramine under equilibrium exchange conditions. This transport system is different from the well-known organic cation-H+ antiporter of the renal brush-border membrane that has been described by other investigators because the typical substrates of the renal system such as tetraethylammonium, N1-methylnicotinamide, cimetidine, and choline failed to inhibit guanidine uptake in the intestine, whereas many other organic cations, including polyamines inhibited the uptake.