The secretion of citrate into milk

Area of Science:

• Biochemistry
• Animal Science
• Lactation Biology

Background:

• Understanding milk component synthesis and transport is crucial for animal nutrition and physiology.
• Citrate plays a significant role in milk's metabolic pathways and overall composition.

Purpose of the Study:

• To investigate the time course of specific activity changes for citrate, lactose, and fatty acids in milk after administering labeled precursors.
• To determine the permeability of the mammary epithelium to citrate and other milk components.
• To elucidate the mechanism of citrate secretion into milk.

Main Methods:

• Intravenous administration of labeled glucose, acetate, and chylomicrons in goats.
• Intra-arterial infusions of labeled sodium (Na), chloride (Cl), and potassium (K).
• In vitro studies using lactating guinea-pig mammary slices and in vivo studies in goats.

Main Results:

• Peak specific activities for lactose and citrate in milk occurred at 2-3 hours, while fatty acids peaked at 5-7 hours.
• Labeled Na, Cl, and K showed rapid uptake into milk (≤1 hour), whereas the mammary epithelium was largely impermeable to labeled citrate.
• Oxytocin treatment increased mammary epithelial permeability to citrate, and citrate, protein, and calcium did not enter diluted milk, unlike ions.

Conclusions:

• Citrate is synthesized within secretory cells and secreted into milk via exocytosis of Golgi vesicles, similar to lactose and milk protein.
• The mammary epithelium's impermeability to citrate maintains its high concentration in milk.
• This mechanism prevents citrate from freely crossing the epithelium, distinguishing its transport from that of ions.