Jonathan writes:
<< There are both endocrine and exocrine regulatory mechanisms.
 Stimulation of the breast (by suckling, pumping, manual expression, etc.)
 produces prolactin, which stimulates the breast to produce milk.  If milk
 remains in the breast, milk production is reduced; the more milk remaining,
 the more reduction.  The first is most important in the early postpartum
 period, the second after production is established.   >>

Not exactly, but close. Endocrine and autocrine, yes - but not exocrine.

Endocrine: prolactin is present in high levels in early postpartum anyway -
the "stimulation" of direct physical contact on the nipple will cause a rise
in prolactin about 20-30 minutes later, but only in the first few weeks and
diminishing to almost no blip by around 3 months. This apparently "finishes"
cell growth and differentiation that began in pregnancy, before the breast
was stimulated.

Autocrine: Milk remaining in the breast is the most important
downward-regulator of supply throughout lactation, even in the early days. By
day 3-5, lack of removal of milk already has a negative effect on the rate of
synthesis of new milk. The retained milk suppresses further synthesis by two
mechanisms: (1) autocrine (biochemical) feedback and (2) mechanical pressure
on the cell structures.  "Autocrine" means that the product of cell
metabolism acts on the tissue that produced it in the first place. Milk
produced by the alveolar cells acts on the SAME cells.

Linda Smith, private practice in Dayton OH