Download Ch.V-2 GalactopoiesisHormone

Galactopoiesis
Part II : Roles of Hormones
Part I: Overview
Part III: Milk removal & Autocrine
Illinois Module : Lactation
Dynamic changes in mammary gland of lactating goat
Knight, C . and M. Peaker (1984, Quarterly J. Exper. Physiol. 69:331).
Weeks of
Lactation
Milk Yield
Cell Numbers
Yield per Cell
1
Phase of the
Lactation
Curve
Ascending
Increasing
Increasing
2
Ascending
Increasing
Increasing
3
Ascending
Increasing
Increasing
Increasing
4
Ascending
Increasing
Static
Increasing
5
Ascending
Increasing
Static
Increasing
6
Ascending
Static
7
Peak
Static
8
Peak
Static
Declining
8 - 11
Peak
Declining
Declining
11 - 23
Declining
Declining
Declining
23 - 36
Late lactation,
pregnant
Declining
Static
Declining
Dynamic changes in mammary cell
regulated by
„
galactopoietic hormones
and
„
local mammary factors
Role of Prolactin
Role of Prolactin
„ primary
component of the galactopoietic
complex of hormones
Species variability in PRL role
„
lactating rabbit, PRL is galactopoietic
„
PRL + glucocorticoid restore lactation in
hypox. rats
„
GH + PRL + glucocorticoid improve lactation
„
PRL + GH + glucocorticoid + T4 restore
lactation in hypox. goat
PRL maintain lactation in nonruminants
„ suppression PRL
by bromocriptine or
L-DOPA reduce lactation performance
„ antisera
to PRL receptors inhibits PRL-
induced casein synthesis and milk yield
decreased
PRL in ruminants
„ Galactopoietic of PRL is less clear in
ruminants
– Suppression of PRL in cows and goats by
bromocriptine has minimal effects on milk yield
– In lactating ewe, bromocriptine markedly
reduces milk yield
„ But, PRL is necessary for full lactogenesis
in cattle
Suckling or milking induces PRL surge
•PRL surge (green line)
3-fold increase over
non-stimulated (blue
hatched line)
• PRL surge occurs
over half an hour after
milking or nursing
• oxytocin surge only
lasts about 5 to 10
minutes (red box)
• milking-induced PRL surge
small compared with peripartum
(Tucker,
surge of PRL associated
with 1994)
lactogenesis
Suckling or milking induces PRL surge
„
milking-induced PRL surge, a direct link between
nursing or milk removal and galactopoietic
hormones
„
galactopoietic response to nursing intensity (litter
size) in pigs or rodents may involve amount of
PRL released at nursing
„
effect of suckling on PRL declines with advancing
lactation, even if nursing stimulus is kept
equivalent throughout lactation
Suckling or milking- induced PRL
surge may work by …
„ decreasing
„ increase
PIF from hypothalamus
pituitary response to PRL-RF
milk yield & PRL in dairy cattle
„
no correlation between milk yield and PRL
levels 2-4 hr before milking or 1 hr after
milking, but 5 min. post-milking
Many stimuli alter PRL levels,
especially TEMPERATURE and LIGHT
„ temperature increases, PRL increase
„ day-length increases, PRL increase
– response lags a few days after day-length
change
„ PRL also respond to stress
– Great care must be taken when collecting
blood samples , not to stress animal
Growth Hormone - bST
Growth Hormone - bST
„
Nursing increased GH in rat and goat, but not in
lactating cow or lactating woman
„
GH decreases with advancing lactation
„
In rats - GH has little effect on established lactation
„
In ruminants - GH is clearly galactopoietic
Bovine Somatotropin (bST)
„
bST increase MY, 10% in early to mid-lactation,
and up to 40% during late lactation
„
bST-treated cows partition nutrient to support
increased milk output
„
bST seem not to affect cow's health other than
problems normally with high milk production
„
Mastitis incidence is not affected by long term
administration of bST
„ bST
increases synthesis of lactose,
fat and protein in mammary gland
„ bST
slightly alters milk composition
Photoperiod on Growth and
Milk Production in Cattle
Dahl et al. (2000
Light on growth in heifers:
16 hr of light daily increased
„ rate of body growth 10-15%
„ w/o proportional increase in feed
intake
„ due to protein gain, not body fat
„ increase PRL, but not GH
Light on in heifers:
16 hr of light daily
„ stimulate mammary growth in
prepubertal and postpubertal heifers
„ increased
milk yields by about 10%
during fall and winter
„ intake
increased sufficiently to
account for increased milk yield
Placental Lactogen
Placental Lactogen: PL
„
Cattle often lactating and pregnant at the
same time, So, Placental hormone may
affect mammary function
„
bPL given during lactation increase MY
„
mechanism of PL as galactopoietic effect
differ from BST [See Byatt et al., 1992, J. Dairy Sci.
75:1216.]
„ goat
is not lactating and pregnant at
the same time (except in very late
lactation)
„ So,
very high levels of caprine-PL
found in pregnant goat's blood
„ Thus,
PL have a dramatic effect on
mammary development in goat
„ PL
found in at least 9 species, but
presence of PL in cattle is doubtful
„ Where
PL found, it may mimics PRL
or GH actions
Adrenal and Thyroid
Hormones
Adrenal corticoids
„ essential
„ high
for maintenance of lactation
doses inhibit lactation
„ rats,
lower doses stimulates MY in early
lactation and prevents decline MY in
later lactation
„ ruminants,
high doses inhibitory
effects, but lower doses ambiguous
adrenalectomized rat
„
impaired milk secretion and casein mRNA
decreased by 85-92%, Cortisol reverse
„
Adrenalectomy-ovariectomy stops
lactation completely
„ In rat, adrenal secretions are limiting
– ACTH declines in extended lactation
– Corticoid binding globulin declines during
early lactation, increases during later
lactation
– So, effective corticoids decline
„ In cattle, no change in CBG during
lactation
– so Corticoids are not limiting to milk yield in
cows, but still essential
Thyroid Hormones
*** are essential for maximal
secretion of milk
„ Thyroidectomy cattle decreased MY
„ Injection for 7 weeks at 25% above
normal thyroid secretion increase MY by
27%
„ continued at 150% for 7 more weeks
had little or no effect
„Feeding thyroprotein (iodinated
casein) to cows
– increases MY by 10 % in early lactation
and by 15-20% in late lactation
– effect lasts 2-4 mos. and subsequent
yields are below normal
„no net benefit in feeding thyroprotein
over entire lactation
Ovarian Steroid Hormones
Ovarian Steroid Hormones
„
Ovariectomy has no effect on postpartum
mammary growth or lactation, suggesting
that ovarian steriods are not necessary for
maintenance of lactation
„
On the other hand, increased estrogen
may affect milk production
„
concurrent pregnancy influence persistency of
milk yield in declining phase of lactation, dairy
cattle
„
Inhibitory effects of pregnancy on lactating cows
apparent about mid pregnancy
„
mechanism of this effect is not fully understood
„ P4 seems no effect on MY in lactating cow
because
– P4 has higher affinity for milk fat than glucocorticoid
– No P4 receptors in the mammary gland during lactation
„ However, timing of inhibition of milk yield in
cattle coincides approximately with the period of
increasing placentally-derived plasma estrogen
„
pharmacological doses of E2 decrease MY
„
E2 in late lactation enhance mammary
gland undergoes involution at drying off
„
A role for E2 in mammary gland involution
in dairy cattle has been indicated
Further reading
„ HorLactation.doc