Uptake of Cimetidine into Syncytial Microvillus Membrane

Uptake of the H2-receptor antagonist, cimetidine, into syncytial microvillus membrane vesicles of human term placenta was investigated to clarify whether an active transport mechanism can be responsible for the observed barrier of the human placenta for cimetidine. Imposition of an outwardly directed H+-gradient stimulated cimetidine uptake, resulting in a small transient overshoot. The H+-gradient-dependent peak uptake was decreased under voltage-clamped conditions by carbonyl cyanide p-trifluoromethoxy-phenylhydrazone, suggesting the presence of an organic cation-proton exchange mechanism. Uptake was partially, but significantly, inhibited by organic ca­ tion transport inhibitors, H2-receptor antagonists and several other cationic drugs, providing further evidence for mediated uptake. H+-qradient-dependent cimetidine uptake was satura­ ble and characterized by a low-affinity (Km) of 6.3 mM and Vmax of 17.5 nmol/mg protein/10 sec. We conclude that the system cannot play an important role in the barrier function of the human placenta in the transport of cimetidine. Rather than active transport, other factors, as for instance the degree of ionization of cimetidine at physiological pH, seem to be a more likely explanation for the low clearance of cimetidine across the human placenta. H2-receptor antagonists, like cimetidine and ranitidine, are frequently used therapeutic agents in the treatment of peptic ulcer in humans. Additionally, cimetidine is some­ times used in the prevention of gastric acid aspiration for women at labor under general anesthesia (McGowan, 1979). The guanidine analogs, ranitidine (Mihaly et al., 1982) and cimetidine (Mihaly et aL, 1983) cross the placenta in preg­ nant ewes at near term. Fetal plasma concentrations, how­ ever, were much lower than m aternal plasma concentrations after maternal dosage. This transplacental gradient was re­ ported to be due to the placenta itself (e.g\, active transport from fetus to mother, irreversible placental metabolic elimi­ nation), because fetal renal elimination did not play an im­ portant role (Mihaly et al., 1983). In a follow-up study the transplacental gradient could not be explained by a low pla­ cental permeability or differential plasma protein binding of cimetidine between mother and fetus (Ching et al., 1985). Cimetidine, administered to women at labor, showed a Received for publication October 20, 1994. 1 These investigations were supported by the Netherlands Organization for Scientific Research (NWO). 2 Parts of this paper were presented at the 5th Meeting of the European Placenta Group, Manchester, UK, 8-11 September 1993. Abstract: Placenta 14, A79, 1993. slow placental transfer finally achieving equal concentra­ tions in maternal and fetal plasma. Protein binding or pla­ cental metabolism was of minor influence. Because the con­ centration of cimetidine in the amniotic fluid increased together with a decreasing concentration in the cord blood, a role for fetal renal elimination seemed likely (Howe et al., 1981). However, because the fetal kidney begins to excrete waste products approximately 5 months after conception, the placenta could play a role in the elimination of cationic drugs before 5 months. In the perfused human placental cotyledon, the clearance of cimetidine was low, providing evidence for a barrier func­ tion of the placenta in cimetidine transfer (Ching et al. , 1987). Because fetal and maternal plasma concentrations did equilibrate, active transport appeared to play an insignifi­ cant role in maintaining a barrier across the human pla­ centa. The absence of mediated transport was confirmed by Schenker et al. (1987) by use of the same experimental tech­ nique. The maternal to fetal transfer of cimetidine showed no signs of accumulation, saturability or susceptibility to inhi­ bition by structural analogs. In this study, no evidence was found also for the saturation of cimetidine uptake in microvil­ lus membrane vesicles of human term placenta. However, ABBREVIATIONS: FCCP, carbonyl cyanide p-trifluoromethoxyphenylhydrazone; Hepes, N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid; Mes, 2-(N-morpholino)ethanesulfonic acid; M0, starting mince; NMN, n-methylnicotinamide; pH,, pH inside the vesicle; pH0, pH outside the vesicle; SMMV, syncytial microvillus membrane vesicles; TEA, tetraethylammonium; Tris, 2-amino-2-hydroxymethylpropane-1,3-dioi; val, valinomycin.