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The Influence of Zinc-Binding Ligands in Fetal Circulation on Zinc Clearance Across the In Situ Perfused Guinea Pig Placenta^1,2,

Divisions of Clinical Nutrition and Biochemistry Research, Research Institute, The Hospital for Sick Children, Toronto, Ontario, Canada, M5G 1X8, and Departments of Nutritional Sciences, Pediatrics, and Biochemistry, University of Toronto, Toronto, Ontario, Canada, M5S 1A8

Although zinc is essential for normal fetal growth and development, little is known about factors that influence its transfer across the placenta. The in situ perfused guinea pig placenta model was used to study the influence of zinc-binding ligands in fetal circulation on maternofetal placental zinc transfer. A placenta of each anesthetized sow was perfused (on the fetal side) with a physiological perfusate via the umbilical vessels, with the fetus excluded. The sow was infused intravenously with ⁶⁵Zn as a tracer of placental zinc clearance and with antipyrine as an indirect indicator of maternal placental blood flow. Maternal plasma and placental effluent samples collected at intervals were counted for ⁶⁵Zn with a gamma counter, and the absorbance of nitrosated antipyrine was measured at 350 nm. The addition of physiological levels of zinc-binding ligands (albumin, L-histidine and L-cysteine) to the perfusate increased the relative maternofetal clearance of zinc across the placenta calculated as zinc clearance/antipyrine clearance [mean ± SEM; 0.113 ± 0.016 vs. 0.062 ± 0.012; ligands vs. no ligands; n = 8; P < 0.05]. The results suggest that the availability of zinc-binding ligands in fetal circulation is one determinant factor of placental zinc transfer.

KEY WORDS: • zinc • placenta • transport • trophoblast • guinea pigs

¹ P. G. Paterson was supported by a Medical Research Council of Canada postdoctoral fellowship.

² This work was presented in part at the 33rd Annual Meeting of the Canadian Federation of Biological Societies, Halifax, Nova Scotia, June 1990 [Paterson, P. G., Sarkar, B. & Zlotkin, S. H. (1990) The effect of fetal zinc levels and zinc-binding ligands on clearance across the placenta of the guinea pig. CFBS Proc. 33: 103 (abs. 395)].

³ Current address: Division of Nutrition and Dietetics, College of Pharmacy, University of Saskatchewan, Saskatoon, Sask., Canada S7N 0W0.

Manuscript received 19 April 1990. Revision accepted 5 October 1990.