During lactation, prolactin is essential for the maintenance of secretory activity in many species, including rats. Interestingly, in the lacrimal gland the translocation of AQP5 to the apical membrane during tear secretion involves binding to the prolactin-inducible protein. The specific role of prolactin in the mammary exocytosis process remains elusive, although a role of SNARE proteins has been suggested. Although primarily associated with metastatic mammary cells, PIP is also found in milk and is sometimes used as a marker of apocrine secretion. Hence, prolactin may regulate water flux into milk through up-regulating PIP and hence translocation of AQP5. It is important to remember that this translocation may not be direct to the apical membrane. Water first enters the Golgi vesicle, and it will be important in future studies to identify whether AQP5 are present on 
the Golgi. In addition to variation in expression and translocation, it has also been proposed that water flux may be regulated by gating of AQPs. Structurally, AQP5 can be shown to possess the necessary characteristics for computer-simulated gating. The whole area of AQP gating remains somewhat controversial, and since gating of AQP5 has not been shown to occur in vivo it will not be discussed further here. In Epimedoside-A agreement with previous observations in mammary tissue, we observed AQP1 mainly localized to the capillaries. However, the most abundant AQP expression and largest increase that we observed during lactation was associated with APQ3, which localized to the basolateral membrane of secretory cells. This also agrees with previous observations in mouse and bovine. Western blotting confirmed significant presence of AQP3 throughout lactation. Lactating mammary tissue synthesizes and secretes large amounts of triglyceride, and as a consequence has a significant requirement for glycerol. Several AQP including AQP3 selectively transport glycerol in addition to water. In adipose tissue and liver the predominant glycerol transporters are AQP7 and AQP9, respectively. There is limited evidence of expression of mRNA for both AQP7 and AQP9 in lactating rat mammary tissue but no attempt has been made to detect the proteins themselves. AQP7 appears to be regulated by insulin in a negative fashion, implying that is more involved with efflux of glycerol from hepatocytes during lipolysis than with influx during adipogenesis, and in this context the mammary gland would be unlikely to have a requirement for AQP7. AQP3, on the other hand, does have a potential role in mammary glycerol uptake and this could explain the high amount of this protein located at the basolateral membrane of the secretory cells. We observed preferential localisation of AQP3 to the lateral membrane of some secretory cells. The reason for this is unclear, but could be related to localized differences in extracellular fluid substrate concentration. The contraction of myoepithelial cells and hence secretory alveoli during milk ejection will induce considerable movement of extracellular fluid in the vicinity of the alveoli, but the Ginsenoside-Ro consequences are unknown. The importance of mammary AQP expression for physiological as well as pathological mammary function has recently been reviewed. This includes discussion of the possible role of AQP in “diluting” milk during storage within the duct system. It is important to recognize that the accepted mechanism for regulating the composition of the aqueous phase of milk is based on the principle that milk.