Plasma relaxin concentrations in the pig during the periparturient period: Association with prolactin,estrogen and progesterone concentrations

Concentrations of relaxin, prolactin, unchromatographed estradiol 17_β (E_2β) and progesterone (P₄) were measured in serial samples of inferior vena caval blood, in three pigs, during late pregnancy, and parturition. Maximal relaxin concentrations occurred 60 to 24h before parturition, and ranged from 60 to 286ng/ml. Prolactin concentrations increased from 12.5ng/ml, 48 to 36 hours before parturition, to between 79 to 184ng/ml. At the time of the relaxin surge, E_2β levels were high, and P₄ concentrations were decreasing, thus raising the $\text{E}{}_{\mathrm{2\beta }}\text{P}{}_4$ ratio. A surge in prolactin concentrations followed upon a decline of P₄ to less than 10ng/ml, coinciding with the increase in relaxin concentrations in 2 gilts, and following the surge in relaxin in the third. Udder development occurred near the time of increased relaxin concentrations. ‘Milk let down’ followed maximal relaxin and prolactin concentrations in two gilts, and the increase in prolactin, rather than the increase in relaxin concentration, in the third.