The permeability of mammary ducts

1. The permeability of the teat mucosa of the goat has been studied in vitro and, as an isolated teat pouch, in vivo. In vivo the mucosa was found to be impermeable to radioactive Na, K, Cl and Rb isotopes and to labelled lactose, but permeable to 3 HOH, and to a lesser extent, urea. 2. The permeability of the intramammary ducts has been assessed by comparison of the rates of passage from the blood of ions and urea relative to 3 HOH into various fractions of milk in different parts of the gland. 3. Negligible quantities of ions were detected in the milk present in large ducts after an I.V. infusion or during a close arterial infusion to keep the venous blood concentrations constant. However, after approximately 25 min, milk in the alveoli had fully equilibrated with 3 HOH, and 24 Na + and was almost fully equilibrated with urea but not with 42 K + and 36 Cl − . 4. The quantity of milk in a fraction of milk was directly related to the amount of 3 HOH present in that fraction but not to the amounts of the other indicators (urea, labelled Na, K, Rb and Cl). Comparison of the concentration of these substances with 3 HOH in different milk fractions indicates that the mammary ducts are probably impermeable to ions and only slightly permeable to urea. 5. The injection of [ 14 C]lactose into the teat showed that after 10 and 20 min, 7 and 14% respectively of the radioactivity had reached the residual (alveolar) milk. Thus diffusion within the milk in the ducts could account for the small quantities of labelled ions found in milk within the ducts following intravascular injection. 6. It is concluded that the large and medium‐sized ducts in the mammary gland are impermeable to lactose and to the monovalent ions studied. 7. In non‐lactating animals, the ducts are more permeable to Na + and Cl − than in the lactating animals. 8. The relative rates of passage of ions into residual (alveolar) milk was Na > Cl > K.