RELAXIN INHIBITS THE PULSATILE RELEASE OF OXYTOCIN BUT INCREASES BASAL CONCENTRATIONS OF HORMONE IN LACTATING RATS

The effects of porcine relaxin were examined in urethane-anesthetized, lactating rats to clarify the actions of relaxin on basal levels and the pulsatile release of oxytocin during suckling. Baseline plasma oxy tocin concentrations were 27.6 +/- 2.9 pM in unsuckled rats. They were significantly increased in rats after the onset of suckling (34.5 +/- 2.4 pM) compared with unsuckled animals. Oxytocin concentrations were increased further during reflex milk ejection (41.2 +/- 2.7 pM). Trea tment with porcine relaxin (5 mu g in 0.1 ml saline) caused a signific ant (p < 0.05) increase in plasma oxytocin compared with pretreatment concentrations or saline injection in all three groups. Frequent blood samples were taken before and during milk ejection to confirm pulsati le oxytocin associated with reflex milk ejection. Short-lived (5-10-se c) pulses with peak concentrations of oxytocin 392.4 +/- 122.1 pM were observed shortly before or at the time of the peak rise in intramamma ry pressure associated with reflex milk ejection. Relaxin (5 mu g i.v. ) completely suppressed the pulsatile release of oxytocin. These data suggest that relaxin increases basal secretion of oxytocin but inhibit s the pulsatile secretion associated with reflex milk ejection.