Plasma Progestins in Pregnant, Postpartum and Cycling Mares3

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Abstract
Jugular vein plasma from 13 mares was extracted with diethyl ether and chromatographed on Sephadex LH-20 columns (.5 × 9 cm) after which progesterone and 17α-hydroxyprogesterone (170HP) were quantified by a competitive protein binding radioassay. During pregnancy, progesterone increased (P < .05) from 1.1 ± .4 ng/ml (mean ± SE) on day 0 to 7.5 ± 1.2 ng/ml on day 8 followed by a transient (nonsignificant) decrease to 4.8 ± .4 ng/ml on day 28. From days 28 to 44 progesterone again increased (P < .05) attaining a maximum concentration of 15.2 ± 1.4 ng/ml on day 64. Thereafter progesterone gradually declined and remained between 1 and 2 ng/ml from days 180 to 300. During the last 30 days of gestation, progesterone increased to 4.4 ± 1.1 ng/ml 5 days prepartum and was less than .5 ng/ml 1 to 3 days postpartum. Less than .2 ng/ml 170HP was found in plasma during gestation, except between days 40 and 120 (2 to 4 ng/ml) and the last 30 days of gestation (.5 ng/ml). Two unidentified compounds, one eluting slightly ahead and one behind progesterone on the Sephadex LH-20 columns, first appeared in the plasma between 30 and 60 days of gestation, increased gradually to day 300, with a significant increase to approximately 8 ng/ml (progesterone equivalents) 5 days prepartum. These compounds were not detected during the postpartum period nor during the estrous cycle. Preliminary results indicate these compounds cross react in at least two different progesterone radioimmunoassays. Pregnant mare serum gonadotropin, measured by a rat ovarian weight bioassay, was first detected at day 40, attained a maximum concentration of 67 IU/ml on day 56, followed by a gradual decline and was non-detectable ( < 1.0 IU/ml) by day 150. During the estrous cycle, progesterone was less than 1 ng/ml during estrus and was 2.2 ± .6, 5.2 ± 1.0, 5.0 ± 1.0, 5.4 ± 1.2, and 3.9 ± 1.2 ng/ml (mean ± SE) on days 2, 4, 6, 8, 10 and 12 of diestrus, respectively. Progesterone levels were significantly elevated by day 4 of diestrus but were not different (P > .05) during days 4 through 12. Copyright © 1975. American Society of Animal Science . Copyright 1975 by American Society of Animal Science.