Hormonal changes during lactation in sows: influence of spontaneous suckling on prolactin, oxytocin and corticoids concentrations.

Blood samples were collected from 8 lactating sows every 2 h for 24 h periods on the Days 5, 10, 15, 20, 25, 30 and 35 of lactation. Additional samples were drawn from four sows during morning spontaneous sucklings on the same days. Prolactin (PRL), oxytocin (OT), estradiol 17-beta (E2) and estrone (E1) were measured by RIA. Corticosteroids (CS) were determined by competitive protein binding assay (CPBA). In 4 of 8 sows, elevated concentrations of PRL were concomitant with greater concentrations of CS and slightly larger concentrations of OT. Mean concentrations of E1 were markedly greater than those of E2 in 6 of 8 sows. A response of PRL to nursing was observed in 90% of all suckling events studied. A CS increase was observed in 64% of suckling occurrences. During suckling OT surges appeared with peripheral concentrations greater on day 5 and 10 of lactation versus days 15, 20, 25, 30 and 35.