Effects of volume and distribution of milk on the oxytocin-induced contraction of the lactating rat mammary gland in vivo

Milk was added intraductally in 0\m=.\2ml increments to previously emptied mammary glands in rats. Direct microscopic observations of the glands showed that each increment of milk did not distribute evenly either within a single lobule or among lobules. Increase of the intramammary pressure either by i.v. injection of oxytocin or by compression of the gland between thumb and forefinger, distributed the milk more evenly within the gland. These observations led to the routine injection of 1\m=.\6-3\m=.\2mu. oxytocin in order to distribute each increment of milk evenly throughout the gland. The intramammary pressure response of the gland to different doses of oxytocin was then determined. The rises in intramammary pressure above equilibrium pressure in response to 0\m=.\8and 1\m=.\6mu. oxytocin increased fourfold on average with increasing gland volume over the compliant portion of the static pressure-volume curve. The slopes of the dose oxytocin-intra-mammary pressure response lines became progressively steeper and shifted to the left as the volume of the mammary gland increased, indicating a progressive reduction in the dose of oxytocin required to elicit a given response. No differences were noted between rats lactating for 9-13 days and those lactating for 20-24 days. In a related study the duct system ofthe rat gland was found to contain little fluid even after 8 h ofnon-suckling, though it became filled swiftly after a single i.v. injection of oxytocin. Non-suckling for 16-24 h was required to overcome passively the resistance to entry of milk from the alveoli