Plasma levels of vasoactive intestinal polypeptide and oxytocin in response to suckling, electrical stimulation of the mammary nerve and oxytocin infusion in rats.

The aim of the present study was to measure plasma levels of vasoactive intestinal polypeptide (VIP) and oxytocin following suckling, electrical stimulation of the mammary nerve and oxytocin infusion in lactating rats. Trunk blood was collected by decapitation after 5 and 20 min of suckling in conscious lactating rats. Repeated blood samples were drawn from the carotid artery in anesthetized rats, in connection with suckling, oxytocin infusion (0.22 nmol/l/kg/h) and electrical stimulation of the mammary nerve (5 V, 5 Hz, 2 ms). VIP and oxytocin levels were measured by radioimmunoassay. In conscious rats, VIP levels rose significantly from 18 +/- 5 to 102 +/- 30 pM after 5 min of suckling and to 123 +/- 25 pM after 20 min of suckling when milk ejection occurred. Oxytocin levels rose significantly from 90 +/- 24 to 269 +/- 45 pM during milk ejection. Suckling, oxytocin infusion and mammary nerve stimulation in anesthetized rats raised VIP levels significantly from 13 +/- 2, 18 +/- 5 and 10 +/- 2 to 43 +/- 8, 45 +/- 16 and 53 +/- 22 pM, respectively, whereas oxytocin levels rose from 111 +/- 34 to 294 +/- 66 pM after 20 min of suckling and to a peak value of 500 +/- 70 pM after oxytocin infusion. This study shows that VIP is elevated in plasma in lactating rats when the pups are suckling. The results showing that VIP levels rise following mammary nerve stimulation and oxytocin infusions indicate that both neurogenic and hormonal mechanisms can contribute to the regulation of VIP levels in plasma.