Patterns of release of oxytocin, prolactin, insulin and LH in lactating sows, studied using continuous blood collection technique.

To describe the patterns of release of oxytocin, prolactin, insulin and LH in relation to suckling and to investigate relationships between them, blood was collected continuously from four sows at a rate of 1 ml per min for 12 h (0900-2100 h) on day 10 (D10) and day 20 (D20) of lactation. The 12-h period was divided into four smaller periods: P1 (0900-1200 h), P2 (1200-1500 h), P3 (1500-1800 h) and P4 (1800-2100 h). The number of nursings occurring during a 12-h period tended to decrease between D10 (17.3 +/- 0.3) and D20 (16.0 +/- 0.3). About 40-50% of all sucklings induced a significant rise in plasma oxytocin, while prolactin gradually increased following each suckling. Decrease in the levels of oxytocin (14.1 +/- 0.4 vs 7.1 +/- 0.4 pmol/l, P < 0.001), prolactin (12.9 +/- 0.3 vs 7.9 +/- 0.1 micrograms/l, P < 0.001) and insulin (24.8 +/- 1.7 vs 14.4 +/- 1.7 mU/l, P < 0.001) were observed between D10 and D20. Average and basal levels of LH and the number of LH pulses increased (P < 0.01) between D10 and D20. A negative correlation between number of sucklings and number of LH pulses was demonstrated on D20 (r = -0.59, P = 0.07). There was a positive correlation between levels of LH and oxytocin on D10 (r = 0.54, P = 0.001) and between prolactin and insulin levels on D20 (r = 0.34, P = 0.05).