Slide Set 9: Milk Secretion Flashcards
Milk components are secreted via:
1- Exocytosis (I) 2- Lipid secretion (II) 3- Transmembrane secretion (III) 4- Transcytosis (IV) 5- Paracellular pathway (V)
What are the milk components that are secreted via exocytosis?
Most of components in aqueous phase of milk are secreted via exocytosis (pathway I):
Ø Lactose
Ø Proteins
Ø Water
What type of secretion is exocytosis?
Constitutive secretion (continuous secretion, no storage)
What are the steps in lipid secretion?
Ø Triglycerides synthesized in the SER coalesce into large fat droplets
Ø Fat droplets are drawn to the apical membrane
Ø Fat droplets bulge against the apical membrane then separate as milk fat globule
What is the function of plasma membrane on fat globules?
1- Primary dietary source of phospholipids & cholesterol for the new born
2- Prevents fat droplets from coalescing to large droplets that may be difficult to secrete
What are the milk components that are secreted via transmembrane secretion?
Ø Limited to modest # of small molecules (some water, Na, K and monosaccharides)
Ø Drugs are efficiently transported into milk via pathway III
Ø No transport of Ca, PO4, citrate
What are the milk components that are secreted via transcytosis?
Ø Intact protein molecules can pass across the mammary cell from interstitial space via transcytosis
Ø e.g. immunoglobulins during colostrogenesis
Ø Many proteins, hormones, growth factors coming from plasma pass to milk via this pathway
What is paracellular transport?
Ø Allows molecules to pass between cells rather than across them
Ø During normal lactation, tight junction (TJ) do not allow for paracellular pathway
Ø During mammary infection (e.g. mastitis) & involution, TJ becomes leaky
What are the milk components that are secreted via paracellular pathway?
Ø Allow for protective molecules to pass in milk
Ø Allow for milk components to be absorbed by blood plasma
during involution
Ø When TJ are leaky, mammary secretion will be high in Na and Cl
What are the factors that affect drug transfer?
chemical (solubility) property, bioavailability
What are the sources of various materials found in milk?
- Environmental contaminants (pesticides, antibiotics)
- Some drugs (e.g. organochlorine & pesticides) may be fat soluble and found mostly in in the lipid phase
- Other drugs are water soluble and are found in protein fraction
Explain the passive diffusion of drugs into milk
- During lactogenesis I, mammary epithelial cells are small and intracellular space is large
- Maternal substances (drugs, lymphocytes, immunoglobulins) can transfer into milk via leaky junctions
- At parturition, intracellular gaps eventually close
- The transfer of drugs into human milk is usually facilitated by passive diffusion (down concentration gradient)
- As maternal plasma level of medication increases, the transfer of into milk increases
- As the mother metabolizes the medication (low plasma level), most medication diffuse of out of the milk
What does the mammary gland rely on for milk synthesis?
Ø Mammary gland rely on blood flow for energy & precursors for milk synthesis
The amount of metabolites available is dependent on:
Ø Rate of blood flow through the mammary gland
Ø Metabolite uptake by the mammary gland
You can divide the mammary epithelial tissue into two components in terms of milk secretion:
- Secretory tissue: this is the most active component. Alveolar tissue. Utilizes most of the energy. Uses the energy, especially during mamogenesis and for synthesis of lactose and for milk synthesis.
- Non-secretory is mostly connective tissue and fatty pad. Ductular system. Utilizes the minimum amount of energy.
