Plasma vitamin D-binding protein and free 1,25-dihydroxyvitamin D3 index in pregnant ewes and their fetuses in the last month of gestation

Abstract

ABSTRACT A radioimmunoassay for ovine vitamin D-binding protein (DBP) has been developed. This assay can also effectively measure DBP in goat plasma. A suitable ovine DBP antiserum raised in a rabbit produced a single monospecific line of precipitation when reacted against purified sheep DBP and sheep plasma. The preliminary purification of 125I-labelled ovine DBP was carried out using adsorption chromatography, and the final purification immediately before addition to the assay tubes was achieved by high-pressure liquid chromatography. Displacement of 125I-labelled ovine DBP by dilutions of sheep and goat plasma or standard DBP gave parallel curves, and only weak competition was observed with calf and pig plasma. The assay detected as little as 26 pmol DBP/1 with intra- and interassay coefficients of variation of 3 and 14% respectively. The mean plasma concentration of DBP in nine pregnant sheep (110–120 days of gestation) was 8·7 ± 0·3 (s.e.m.) μmol/l. These levels were significantly (P<0·02; paired t-test) higher than those in matched fetal plasma (6·7 ±0·4 μmol/l) obtained in utero through a catheter in a carotid artery. Plasma DBP concentrations in pregnant sheep were also significantly (P<0·02) higher than in five normal non-pregnant sheep (6·8 ± 0·5 μmol/l). The mean concentrations of total 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) in maternal and fetal plasma were 92·0 ±8·7 pmol/l and 152·5±18·0 pmol/l respectively (P<0·05). The free 1,25-(OH)2D3 index, a measure of the level of free 1,25-(OH)2D3 in plasma, was calculated as the ratio between the molar concentrations of total 1,25-(OH)2D3 and DBP. The mean value of the free 1,25-(OH)2D3 index in the fetus (2·3 × 10−5) was significantly (P<0·01) higher than that in the dam (1·1 × 10−5), thus indicating a gradient of free 1,25-(OH)2D3 during the latter part of ovine pregnancy with a mean fetal/maternal ratio of 2·1. J. Endocr. (1987) 115, 7–12