Carbohydrate metabolism in the sheep placenta

Abstract

The accumulation of hexitols and a substance tentatively identified as glycerol has been observed in the recirculating perfused placental preparation of the sheep. Glucose injected into the maternal circulation or added to the perfusion circuit was without apparent effect on hexitol or glycerol production. Fructose added to the perfusion reservoir was similarly without effect. Hypoxia did not alter the rate of accumulation. In different experiments generally labelled [ 14C] glucose, fructose and sorbitol were added to the perfusion reservoir of the recirculating perfused placenta. Fructose and sorbitol were shown to be only slowly, if at all, metabolished and placental permeability to these subtances was shown to be very limited. Glucose, on the other hand, was rapidly metabolised and also crossed the placenta into the maternal circulation. of the radioactive counts administered as glucose 10–20% appeared in the fructose during the perfusions and the results were consistent with glucose beinig the only precursor of the fructose. The hexitol was strongly labelled in the [ 14C]glucose experiments and in one case the glycerol was also labelled. In none of the experiments with labelled compounds was there significant labelling of the insotiol. Umbilical arterial and venous concentrations for hexitol and glycerol have been measured in the exteriorised foetal preparation. No significant arteriovenous differences were observed. Determinations of lactate in the perfusate of the recirculating placental preparation showed that lactate accumulated despite the existence of large transplacental gradients. The results are discussed particularly in relation to the importance of sorbitol production by the placenta as a source of foetal fructose and to the pathway of fructose formation in the placenta. It is suggested that placental fructose is possibly formed from glucose via sorbitol within placental cells impermeable to sorbitol.