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Autocrine Control of Lactation


Autocrine
Control

Cows walking to the barn.

Conrtol of lactation is clearly regulated by hormones, however local factors are also important. More frequent milk removal in both cows and goats increases milk yield. Consider studies where cows or goats have had one side of the udder milked more frequently than the other side (for example 3X/day milking or hourly milking vs. 2X/day). Rate of milk secretion increases only in the gland which is milked more frequently.

Furthermore, if one udder side is milked 2X/day and the other side milked only once per day, or if milking is incomplete from one side, milk yield is decreased only in the less frequently emptied gland.

These unilateral effects cannot be attributed to systemic (hormonal) control because both sides of the udder are exposed to the same concentrations of galactopoietic hormones.

The response requires actual removal of milk from the gland. For example, hourly massage of the gland without milk removal does not have the same effect (Linzell and Peaker, 1971).

In addition, the response is not the result of increased pressure of the stored milk. Studies using goats where one gland (remember that goats only have two glands) was milked 2X/day and the other gland milked 3X/day have been used for this type of experiment. In glands milked 3X/day the volume of milk from the extra milking was replaced back into the gland by an equal volume of an inert sucrose solution (so, intramammary pressure was the same in both sides of the gland even though the milk was removed once more often on the one side). The result was that secretory rate was still increased by 3X/day milking, even though intra-mammary pressure was maintained the same as in the 2X/day milked gland.

These types of observations gave rise to the hypothesis that a milk constituent acts as an inhibitor of milk secretion and that removal of this inhibitor at milking regulates the rate of milk secretion.

A milk whey protein (~7 kDa molecular weight) has been identified as a Feedback Inhibitor of Lactation (FIL; see Wilde CJ, Addey CVP, Boddy LM, Peaker M (1995) Autocrine regulation of milk secretion by a protein in milk. Biochem J 305:51-58). This inhibitor is thought to be secreted by the mammary epithelial cells and in turn inhibits further milk secretion as its own concentration increases in the alveolar lumen. The exact mechanism of how this feedback inhibitor works is unknown. In vitro, the feedback inhibitor seems to reduce secretory rate and key enzymes in mammary cells, stimulates intracellular degradation of newly synthesized casein, reduces prolactin receptor numbers on the cells, and can inhibit differentiation of mammary cell function.

The balance between systemic (hormonal) and local (FIL) control of milk secretion is illustrated in the following discussion.

Each time milk is removed:

Prolactin release is stimulated
Intra-mammary pressure is relieved
FIL is removed from the alveoli

If milk is not removed:

There is no stimulation of PRL release

There is an acute accumulation of milk in the gland, resulting in:

Increased intra-mammary pressure
Activation of sympathetic nerves
Decreased mammary blood flow
Decreased availability of hormones and nutrients to the gland

Rate of milk secretion declines

This interplay between systemic and local factors in control of galactopoiesis can be thought of as a seesaw.

Diagram of relationship between systemic factors and local factors in control of milk secretion.

The gland is under the influence of the systemic factors shortly after milking and maximal secretion rate is achieved. This gradually slows as the role of the local factors becomes dominant. If milk is not removed, then the secretion rate will eventually drop to zero (see below), however, under normal nursing or milking intervals the secretion rate does not go to zero. Once milk is removed, the cycle begins again.

Diagram of how milking alters the relationship between systemic factors and local factors in control of milk secretion.

(see Wilde and Peaker 1990 J. Agric. Sci. 114:235)


 
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