Milk lipid synthesis and secretion during milk stasis in the rat

The lipid content of milk expressed from rats decreased from 15.8% wt/vol within 1 h of the removal of the pups to 4.2% in samples obtained 12 h after the removal of the pups. The protein and lactose concentrations were unchanged. Analysis of gland homogenates showed that lactose and total lipid accumulated throughout the 12 h period of milk stasis. The rate of mammary fatty acid synthesis in vivo determined using tritiated water was not significantly altered by 12 h of milk stasis, and analysis of the milk and gland fatty acids showed that the relative proportions of de novo synthesized fatty acids (medium-chain fatty acids) and those derived from the circulation (16 and 18 C acids) had been maintained. Light and electron microscopy showed the accumulation of considerable quantities of lipid within the lumen of the alveoli even while the pups were still suckling. Twelve hours after the removal of the pups there was not only a marked increase in the lumenal lipid, but also considerable lipid trapped in the epithelial cells. In rats where the nipples were ligated unilaterally and allowed to be suckled for 12 h, the milk from the ligated glands contained significantly less lipid than that from the suckled glands but more lipid than from milk of rats removed from their pups for 12 h. Administration of oxytocin during 12 h of milk stasis significantly increased the milk lipid content.