Concentrations of steroid hormones, and of relaxin, in utero-ovarian venous plasma of periparturient sows

Abstract

In order to identify the sequence of hormonal changes surrounding farrowing in sows, concentrations of sex steroids (pregnenolone sulphate, pregnenolone, progesterone, dehydroepiandrosterone, androstenedione, testosterone, oestrone sulphate, oestrone and oestradiol), relaxin and cortisol were determined in blood samples collected via chronically implanted utero-ovarian (UOV) venous cannulae in seven animals between 120 h before and 120 h after the onset of delivery. The relaxin content of UOV plasma reached maximum values 1–2 days before delivery, at which time progesterone values were falling, or had fallen, to nadir. High concentrations of pregnenolone sulphate, an important luteal precursor to progesterone synthesis in pregnant sows, were also maintained in the UOV during the phase of progesterone decline, suggesting that progesterone secretion by ovaries of preparturient sows is constrained by enzyme activity (arylsulphatase and/or 3β-hydroxysteroid dehydrogenase), rather than by substrate availability. Amounts of other steroids in UOV plasma declined after the onset of delivery. Asynchronies in flux of progestagens and of relaxin in the UOV drainage of periparturient sows portray a sequence of changes in luteal secretory activity prior to delivery. During farrowing, times of expulsion of piglets did not consistently coincide with pulses of release of any particular steroid hormone, or of relaxin.