Transport of milk constituents by the mammary gland.

This review deals with the cellular mechanisms that transport milk constituents or the precursors of milk constituents into, out of, and across the mammary secretory cell. The various milk constituents are secreted by different intracellular routes, and these are outlined, including the paracellular pathway between interstitial fluid and milk that is present in some physiological states and in some species throughout lactation. Also considered are the in vivo and in vitro methods used to study mammary transport and secretory mechanisms. The main part of the review addresses the mechanisms responsible for uptake across the basolateral cell membrane and, in some cases, for transport into the Golgi apparatus and for movement across the apical membrane of sodium, potassium, chloride, water, phosphate, calcium, citrate, iodide, choline, carnitine, glucose, amino acids and peptides, and fatty acids. Recent work on the control of these processes, by volume-sensitive mechanisms for example, is emphasized. The review points out where future work is needed to gain an overall view of milk secretion, for example, in marsupials where milk composition changes markedly during development of the young, and particularly on the intracellular coordination of the transport processes that result in the production of milk of relatively constant composition at a particular stage of lactation in both placental and marsupial mammals.