Uptake of bicarbonate/CO2 in the isolated guinea pig placenta

Experiments were performed to investigate the mechanisms of bicarbonate/CO2 transport in the isolated guinea pig placenta. The maximal uptake of [14C]-bicarbonate/CO2 (reference [3H]-L-glucose) was about 60 per cent on both the fetal and maternal side, using the method of paired tracer single injection dilution technique. Carbonic anhydrase inhibitors such as acetazolamide (0.1 and 1 mmol/l) and the poorly permeant aminothiadiazole (0.5 mmol/l) reduced bicarbonate/CO2 uptake on both sides by 25-50 per cent. Inhibition was significantly higher on the fetal side. When lactate (5 mmol/l) was added to the acceptor side solution, the uptake of bicarbonate/CO2 increased 10-20 per cent. DIDS (1 mmol/l) decreased bicarbonate/CO2 uptake additionally under the simultaneous influence of acetazolamide. It is concluded that at least two mechanisms for placental bicarbonate transfer exist. A portion of bicarbonate, predominantly on the fetal side and outside the trophoblastic cells, is converted to CO2 which then diffuses into the tissue and crosses the placental barrier. Another part of bicarbonate is transported as ion using an anion exchange system, which exists in both the multivillous and the basal membranes of the syncytium. The first system is important for the excretion of fetal CO2 into the mother.