Oxytocin plasma level in the lactating sows--effect of suckling

Oxytocin concentration in the peripheral blood was measured by RIA during suckling period in lactating sows (n = 8). Blood samples were taken from the jugular vein around the clock for every 2 h on day 5, 10, 15, 20, 25, 30 and on day 35 of lactation. Besides that blood samples were collected more frequently during suckling periods. Oxytocin plasma concentration was very low and in most cases it was on a border of sensitivity of our method (3 pg/ml). Marked but short-lasting rise of oxytocin was observed only during a period of initial massage of the udders by the piglets. This rise observed in all studied pigs was higher (p less than 0.01) compared to the values before the massage on the onset of lactation only, and was 14.6 +/- 4.2 pg/ml and 6.4 +/- 1.2 pg/ml on day 5 and day 10 of lactation, respectively. In all other studied days in a few cases only suckling stimulated the release of oxytocin over its basic concentration. Mean values (+/- SEM) of oxytocin in blood samples collected during massage of udder on day 15, 20, 25, 30 and day 35 were 3.7 +/- 0.5, 4.2 +/- 0.8, 4.9 +/- 1.1, 3.2 +/- 0.4 and 3.0 +/- 0.6 pg/ml plasma, respectively. There was no relationship between the size of the litters and neither basic level of oxytocin nor its blood concentration during suckling (r = 0.13).