The transfer of glycine across the placenta during pregnancy plays an important role in fetal growth and development (Dicke & Verges, 1994). Vesicle studies of the microvillous membrane of the human placental syncytiotrophoblast have revealed predominant Na+-dependent transport systems for small, neutral amino acids (systems A, gly and ASC) (Dicke et al. 1993). However, the transport of glycine into the intact syncytiotrophoblast has not been characterised. In this study we measured glycine and methylaminoisobutyric acid (MeAIB; a non-metabolisable substrate of system A), uptake rate into human placental villous fragments (Greenwood et al. 2000) and estimated the contribution of system A to glycine uptake in intact tissue.
Fragments of villous tissue (5-10 mg wet weight) were dissected from placentas delivered at term following normal pregnancy. Uptake of [3H] glycine or [14C] MeAIB was initiated by immersing the fragments in Tyrode solution containing 0.5 µCi ml-1 [3H] glycine or 1 µCi ml-1 [14C] MeAIB at 37 °C. Uptake was terminated by washing with ice-cold Tyrode solution after specified times to give a time course of uptake. Fragments were put into distilled water and the lysate measured for radioactivity. Afterwards, the tissue was transferred to NaOH to dissolve. A sample of each NaOH lysate was assayed to establish the protein content of each fragment. Uptake was measured in the presence and absence of Na+ (choline replaced Na+) to determine the Na+-dependent component of uptake.
These results show that a component of both glycine and MeAIB uptake into villous fragments is Na+ dependent but also suggests that both system A and Na+-independent routes contribute to glycine uptake into the intact syncytiotrophoblast.
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| Figure 1. A, Na+-dependent [3H] glycine ([fullcircle]; n = 8 placentas) and [14C] MeAIB ([opencircle]; n = 5 placentas) uptake into placental villous fragments was linear over 5-30 min. Values are means ± S.E.M. Glycine: r 2 = 0.20, P < 0.01; MeAIB: r 2 = 0.56, P < 0.0001; least-squares linear regression. B, in the presence of Na+, MeAIB inhibited [3H] glycine (0.5 µCi ml-1) uptake (*P < 0.05 vs. control) and glycine inhibited [14C] MeAIB (0.1 µCi ml-1) uptake (**P < 0.01 vs. control). In the absence of Na+, MeAIB inhibited [3H] glycine uptake (##P < 0.01 vs. control-Na+). Glycine had no effect on [14C] MeAIB uptake in the absence of Na+. Values are means ± S.E.M.; n = 7 placentas; paired t test. |
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Dicke, J.M., Verges, D., Kelley, L.K. & Smith, C.H. (1993). Placenta 14, 85-92.
Greenwood, S.L., Lambert, K.D. & Sibley, C.P. (2000). J. Physiol. 528.P, 27-28P.