Can vasopressin induce milk ejection in the dairy goat?

Suckling increases plasma levels of both oxytocin (OT) and vasopressin (AVP) and intravenous infusions of AVP increase milk flow and milk fat concentration in goats. We hypothesized that vasopressin can cause contraction of the myoepithelial cells and thereby milk ejection. Eight goats were used in each of two series. They were kept together with their kids and were both suckled and hand milked. At experiments, one teat was first emptied by hand and after 3 min the same teat was milked again. On control days, cisternal milk (CM) was achieved at the first milking and at the second milking only a small volume of alveolar milk (AM) with modest fat content was received. On experimental days the goats were milked and immediately thereafter OT (10 mg) or AVP at the low dose 125 ng (AVPL) was injected intravenously followed by the second milking 3 min after the first (series I). In series II saline (0.9% NaCl) or vasopressin at the high dose 250 ng (AVPH) was injected. The plasma OT concentrations were unchanged after AVP injections. The CM volume did not differ between treatments within each series. Injections of AVPH or OT increased volume and fat content of AM. The proportion of AM to total milk volume was 15% in control experiments, 21% after saline and 29% after AVPL. However, after AVPH and OT the AM proportions were 56 and 57%, respectively, accompanied by an increased fat content. In conclusion, the high dose of AVP elicited a milk ejection reflex similar to that of OT.