Hi Barbara,
The role of prolactin is quite complicated but I will try to summarise what
I know of the research in this area. (also need to say I by no means
consider myself terribly knowledgeable in this area).

> Are prolactin receptor sites merely a theory for increased supply and
> early "milk surge" (lactogenesis ll)?

In vitro research (quoted in Lawrence) has shown " prolactin stimulates the
synthesis of mRNAs of specific milk proteins by binding to membrane
receptors of the epithelial cells"  so the prolactin receptors will be a
structural component of new epithelium within the breast and apoptosis
doesn't have any impact as new cellular growth with simply replace those
cells lost by apoptosis (whether recent or in a previous lactation).

I am aware of the "prolactin receptor theory" that is discussed in Riordin
based on the work done by DeCarvalho et al. Postulating that "frequent
feeding in early lactation stimulated a faster increase in milk output
because suckling stimulates the development of receptors to prolactin in the
mammary gland" This was due to the finding that women who fed more
frequently, compared to a control group who fed less frequently, had better
weight gains over the first 15 days. It should be said however that by one
month post partum there was no difference between the two groups.

However this early increase in milk supply can probably also equally be
explained by FIL (Feedback inhibitor of lactation). This is a small protein
present in milk which is thought to be the key to autocrine control in the
gland. The more milk present in the breast (eg: as a result of fewer feeds)
the higher the concentration of FIL and the slower the rate of milk
synthesis. The lower the volume of milk in the breast (eg. As a result of
frequent feeding) the lower the concentration of FIL, and the higher the
rate of synthesis. This mechanism initiates with the onset of lactogensis
II. Which is why the initial large milk supply seen at this time is
regulated (usually downwards) to infant demand.
Much of this work has been done in animal models, however FIL has been
demonstrated to be present in women. There are many references which can be
found in Cregan & Hartmann's paper: Computerised Breast Measurement from
Conception to weaning. 1999. JHL. 15(2). 89-96.
One paper however (in humans) is Prentice, AM et al. 1989.  Evidence of
local feedback control of human milk secretion. Transactions of the Society
of Biochemistry. 17: 122.

Also, in regard to prolactin, during established lactation the role of blood
prolactin is now considered to be permissive rather than regulatory.
Cox, DB et al. 1996. Blood and milk prolactin and rate of milk synthesis in
women. Experimental Physiology. 81: 1007-1020.

I haven't answered your question about numbers of prolactin receptors being
"set" in early lactation because I'm not familiar with the research. It was
a framework I believe developed on animal models. I believe autocrine
control and the role of prolactin as permissive rather than regulatory is
perhaps the more currently held "theory". Not that these theories
necessarily preclude the prolactin receptor theory. I will see what else I
can find out.

Regards
Cathy Fetherston CM MSc IBCLC

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