Mammary gland membrane transport systems.

The secretion of milk depends on the activity of a large number of membrane transport systems located on the apical and basolateral membranes of mammary secretory cells. It follows that a thorough knowledge of individual mammary tissue membrane transport systems is required if we are to fully understand the process of milk secretion. The distribution of the transporters between the apical and basolateral poles of the mammary epithelium must be asymmetrical given that the mammary gland is capable of vectorial transport. This is particularly evident in the case of glucose and amino acid transport systems: the transport mechanisms for these compounds are predominantly situated in the blood-facing aspect of the secretory cells. In addition. it is apparent that there is a polarized distribution of transport systems (carriers and channels) which accept sodium, potassium, chloride, phosphate, and calcium as substrates.