D-glucose transfer rates derived from single-injection and steady state tracer experiments in the isolated guinea-pig...

In isolated guinea-pig placentae the transfer of D-[3H]glucose and L-[14C]glucose was investigated in single injection (SIE) and steady state (SSE, n = 11, placental weight 4.5 +/- 1.14 g) experiments at constant perfusion flow rates (3 ml/min). In SIE, a mixture of D- and L-glucose was injected as a bolus into either the fetal or maternal side of the placenta, uptake curves were obtained and the maximal extraction values Umax were derived. From these the membrane rate constants Kmc and Kfc of either the maternal or fetal side of the trophoblast membrane were calculated. The specific placental transfer rate constants were computed for the maternal-fetal (Kspec,mf) and fetal-maternal (Kspec,fm) direction from the amount of label transferred to the acceptor side in either SIE or in SSE (where labelled D- and L-glucose had been added to the stock solution). The chemical concentration of D-glucose was changed (5, 50 and 100 mmol/l, n = 11) and it was found that all rate constants decreased with increasing D-glucose concentration. At a D-glucose concentration of 5 mmol/l, Kmc was 3.11 +/- 1.59 ml/min (n = 6) and Kfc 2.69 +/- 0.5 ml/min (n = 5), the combined membrane rate constant (which determines the total placental transfer) was estimated to be 1.44 ml/min. This value was not significantly smaller than the mean (both directions) transfer rate constant Kspec in SIE (1.51 +/- 0.89 ml/min) or SSE (1.52 +/- 0.56 ml/min). Thus the results from uptake and transfer experiments are consistent. From the rate constants D-glucose fluxes at different chemical D-glucose concentrations were estimated. Whereas the mean Km values for all fluxes based on the various rate constants were about 30 mmol/l, the maximal flux Vmax was highest at the maternal trophoblast membrane (159.1 +/- 70.2 mumol/min), it was 107.9 +/- 11.6 mumol/min at the fetal side and 67.3 +/- 7.9 mumol/min in SIE or SSE for placental transfer in both directions. It is concluded that D-glucose carriers predominate at the maternal side of the trophoblast, and that maternal-fetal glucose transfer is proportionate to the transplacental glucose concentration difference within the physiological range of glucose concentrations.