Evidence for an organic cation-proton antiport system in brush-border membranes isolated from the human term placenta.
Uptake of guanidine, an endogenous organic cation, into brush-border membrane vesicles isolated from human term placentas was investigated. Initial uptake rates were manyfold greater in the presence of an outward-directed H+ gradient ([pH]o greater than [pH]i) than in the absence of a H+ gradient ([pH]o = [pH]i). Guanidine was transiently accumulated inside the vesicles against a concentration gradient in the presence of the H+ gradient. The H+ gradient-dependent stimulation of guanidine uptake was not due to a H+-diffusion potential because an ionophore (valinomycin or carbonylcyanide p-trifluoromethoxyphenylhydrazone)-induced inside-negative membrane potential failed to stimulate the uptake. In addition, uphill transport of guanidine could be demonstrated even in voltage-clamped membrane vesicles. The H+ gradient-dependent uptake of guanidine was inhibited by many exogenous as well as endogenous organic cations (cis-inhibition) but not by cationic amino acids. The presence of unlabeled guanidine inside the vesicles stimulated the uptake of labeled guanidine (trans-stimulation). These data provide evidence for the presence of an organic cation-proton antiporter in human placental brush-border membranes. Kinetic analysis of guanidine uptake demonstrated that the uptake occurred via two saturable, carrier-mediated transport systems, one being a high affinity, low capacity type and the other a low affinity, high capacity type. Studies on the effects of various cations on the organic cation-proton antiporter and the Na+-H+ exchanger revealed that these two transport systems are distinct.
Ganapathy
V
V
Department of Cell and Molecular Biology, Medical College of Georgia, Augusta 30912.
Ganapathy
M E
ME
Nair
C N
CN
Mahesh
V B
VB
Leibach
F H
FH
D002352:Carrier Proteins / Q000378:metabolism*; D002414:Cations, Monovalent; D002927:Cimetidine / Q000494:pharmacology; D003000:Clonidine / Q000494:pharmacology; D005260:Female; D019791:Guanidine; D006146:Guanidines / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008564:Membrane Potentials; D008565:Membrane Proteins / Q000378:metabolism*; D008871:Microvilli / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D012964:Sodium / Q000378:metabolism
D002352:Carrier Proteins; D002414:Cations, Monovalent; D006146:Guanidines; D008565:Membrane Proteins; D002927:Cimetidine; D012964:Sodium; D019791:Guanidine; D003000:Clonidine