During lactogenesis in the goat, the onset of secretion of calcium
into milk occurs
at parturition (Thompson et al. 1995) at approximately the same
time
as the onset
of secretion of parathyroid hormone-related protein (PTHrP) by the mammary
gland (Ratcliffe et al. 1992); these events may be unrelated or
PTHrP
may be involved in calcium transport from blood to milk.
Parturition in goats is initiated by fetal secretion of cortisol (Flint
et al. 1978) and
maternal secretion of cortisol also increases (Paterson &
Linzell, 1971). Injecting
cortisol locally into the sinus of a mammary gland of the late-pregnant
goat when the
tight junctions between secretory epithelial cells appear to be
‘loose’, and injectate
can reach the basolateral surfaces of secretory cells, stimulates an early
tightening of
these junctions (Thompson, 1996) as occurs naturally at parturition. This
tightening
can be produced by an increased concentration of ionized calcium in the
extracellular
fluid of the gland (Neville & Peaker, 1981).
The experiments reported here were undertaken to determine if cortisol
injection
stimulates the mammary gland to secrete both PTHrP and calcium before
parturition.