No Evidence for Basolateral Secretion of Milk Protein in the Mammary Gland of Lactating Goats

¹ Lactational Physiology Programme, Dairy Science Group, AgResearch, Ruakura Research Station, Private Bag 3123, Hamilton, New Zealand

Recent research suggests that a small percentage of milk proteins may be secreted basolaterally, which would have implications for our work on the permeability of tight junctions in the mammary epithelium. In our work, the presence of -lactalbumin (LA) or lactose in plasma is used as an indicator of permeability. The aim of this study was to examine basolateral secretion by determining the presence of milk proteins in efferent mammary lymph. Five Saanen goats were fitted with mammary lymph catheters and were administered intramammary isosmotic bolus infusions of sucrose control solutions or ethylene glycolbis (ß-aminoethyl ether)-N,N,N',N'-tetraacetic acid to induce leaky tight junctions. Lymph samples were collected before and 5 h after infusion. Lymph was analyzed by Western blotting for the presence of -casein (CN), ß-CN, and -LA. No -CN or ß-CN was detected in lymph, but -LA was detected in all lymph samples. Moreover, the signal was much stronger in samples from goats that were treated with ethylene glycol-bis(ß-aminoethyl ether)-N,N,N',N'-tetraacetic acid, and concentrations of -LA in lymph were significantly increased with this treatment. These changes and the absence of casein in lymph suggest increased permeability of tight junctions rather than basolateral secretion. In summary, these data do not support basolateral secretion.

Key Words: casein • milk • tight junction • basolateral secretion

Submitted on April 22, 1997
Accepted on September 10, 1997

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