Autocrine Control of Lactation

AAutocrineutocrine Control of Control of Lactation Lactation

Illinois Module : Lactation

Autocrine Control of Lactation

Control of lactation is clearly regulated by hormones,

However local factors are also important

Evidences

Cows and goats, frequent milk removal increases milk yield

Requires actual removal of milk from the gland, as hourly massage of the gland without milk removal does not have the same effect (Linzell and Peaker, 1971)

Evidences

One side of udder milked more frequently than the other side – Rate of milk secretion increases in the

gland milked more frequently – milk yield decreased in the less

frequently milked gland

These unilateral effects cannot be hormonal control, as both sides of udder exposed to the same concentrations of galactopoietic hormones

Evidences

In addition, it is not the result of increased pressure of the stored milkGoats where one gland was milked 2X/day and the other gland milked 3X/day, and replaced inert sucrose solution so that intramammarypressure was the same in both sides

EvidencesThe result, – secretory rate increased by 3X/day

milking

Thus, hypothesis is that a milk constituent acts as an inhibitor of milk secretion and removal of this inhibitor at milking regulates the rate of milk secretion

Feedback Inhibitor of Lactation:

FIL

Feedback Inhibitor of Lactation:FIL

A milk whey protein, ~7 kDa (Wilde et al., 1995 Biochem J 305:51-58)

Secreted by mammary epithelial cells to inhibits further milk secretion as its own concentration increases in the alveolar lumen

The exact mechanism of how this feedback inhibitor works is unknown

Feedback Inhibitor of Lactation:FIL

In vitro, FIL– reduce secretory rate and key enzymes

in mammary cells– stimulates intracellular degradation of

newly synthesized casein– reduces prolactin receptor numbers on

the cells – inhibit differentiation of mammary cell

function

Balance between systemic (hormonal)

and local (FIL) control of milk secretion

Each time milk is removed:

Prolactin secretedIntra-mammary pressure relievedFIL removed from alveoli

If milk is not removed:

No stimulation of PRL secretionAcute 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

Systemic and Local Factors in control of galactopoiesis as a

seesaw

gland under influence of systemic factors shortly after milking and maximal secretion rate achievedThen, gradually slows as the role of the local factors becomes dominant

If milk is not removed, then secretion rate drop to zero (see figure)

under normal nursing or milking intervals secretion rate does not go to zero

Once milk is removed, cycle begins again

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

Milk Secretion Rate

1) the amount of secretary tissue

2) the rate of milk secretion (per unit of time)

Milk yield depend on

Milk Secretion Rate

Secretion rate affected by accumulation of milk in alveolar lumen

Accumulation of milk in lumen increases intra-mammary pressure (see figure)

Milk Secretion Rate

Once the intra-mammary pressure reaches a certain level (8 to 10 hrs after last milking in dairy cow), secretion rate declines

If the pressure increases enough (in cow, about 70 mm Hg, 35 hrs after last milking), then secretion stops and milk starts to be resorbed

Milk Secretion Rate

Inhibition of milk secretion and increasing intra-mammary pressure caused by FIL rather than increased pressure

intra-mammary pressure measured in teat cistern using teat cannula reflect total gland pressure from accumulation of milk and not directly the intra-alveolar pressure

Adapted from Schmidt, G.H., 1971, Biology of Lactation, W.H. Freeman and Co., p. 150.]

Milking Interval andMilk Secretion Rate

For 2X/day milking, optimum interval is 12 hr – milk accumulation not significantly

lowered milk secretion rate, – but by ~14 hr secretion rate decline

Effect of long milking intervals on secretion rate greater in higher producing cows than lower producing cows

UI Dairy Research Facility (1996)

Milking Frequency

Nursing frequencymay be: Species

continuous kangaroo (joey)

or at intervals of:

.5 hr whale, dolphin

1 hr pig

4-6 hr cow

1X/day rabbit

1X/2 days tree shrew

1X/week Northern fur seal

Milking Frequency in Dairy Cattle

In most dairy cattle management schemes, cows are milked twice daily or three times dailyIn robotic milking systems, typically cows will enter 4 or less times per dayLittle additional benefit for milking more than 4 times per day

Summarized observations on milking frequency in dairy

cattle and effects on milk yield

A. 3 times/day vs. 2 times/day milking

3X/day milking increases milk production up to 25%

But, ~2/3 due to better feeding and management and ~1/3 due to decreased udder pressure

3X/day milking must be accompanied by a compensatory feeding program; if not, yield decline to that from 2X/d

3X/day milking more beneficial in late lactation,

Both first lactation and older cows increased yield in 3X/day vs. 2X/day

Mammary DNA , RNA and activities of key enzymes increased (Wilde et al., 1987, J. Anim. Sci. 64:533.)

3x/day requires 50% more labor than

Incidence of mastitis and reproductive performance not altered by 3X/d milking

Stage Time Mechanism Response

1Immediate(hours to days)

removal of chemical feedback inhibitor

increased milk secretion

2 Short Term (days to weeks)

stimulation of cell differentiation

increased milk secretion

3 Long Term (weeks to months)

stimulation of cell proliferation

increased milk secretion

Sequential Response to Thrice-Daily Milking

B. Milking udder halves

Milking one udder half 3X/day gives 16-32% more milk than udder half milked 2X/day, even though the halves are exposed to the same systemic stimulation

C. 4x/day milking

Milking 4X/day results in 5-10% more yield than 3x/day

But labor costs are doubled compared with 2X/day

D. bST to goats milked 3X/day

Greater milk yield than goats + BST and milked 2X/day or in goats (no BST) and milked 3X/day

So, the effects of 3X/day milking and bST apparently are additive

E. Massaging udders between milkings

increase of 1-1.5% in milk yield, although not statistically significant

F. Milking less than 2X/day

Milking 3X in 2 days (skipping 1 out of every 4 milkings)

– decreased milk yields of 18% (started in week 4 of lactation

– decrease of 11% started in week 20

(Eldridge and Clark, 1978, J. Dairy Res. 45:509)

1X/day milking at late lactation results in 12% less milk for the entire lactation

Length of lactation reduced by 12 days

1X/day milking for a complete lactation reduces milk yield by 50% in first calf heifers and by 40% in older cows

Milk Ejection

Milk Ejection

Milk in alveolar lumen out of glandMilk ejection can occur – under water – as whales,

porpoises, sea-cows, sea otters, hippopotamus

– While in flight as bat

Milk Ejection

Streak canal must be opened to remove milkOccur by: – negative pressure - such as with

the milking machine – positive pressure - such as with

hand milking – positive and negative pressure -

both occur during suckling

Milk Ejection Reflex

To get milk from the alveoli requires an active process called the milk ejection reflex

Milk Ejection Reflex

a neuroendocrine reflex with

• afferent pathway : neural • efferent pathway : hormonal, blood-borne

Afferent Pathway: neural

greatest amount innervation in mammary gland is in teatsStimulate teats activates pressure sensitive receptors in dermisnerve impulses via spinothalamic nerve to paraventricular nucleus and supraoptic nucleus in hypothalamus where oxytocin-containing neurons stimulated The efferent pathway starts at this point

Efferent Pathway: hormoneBegins with release of oxytocin and neurophysin

Oxytocin binds to receptors and cause myoepithelial cells to contract

Intramammary pressure increase and ejection of milk from the alveolar lumen

The biological mechanisms involved are complex[See J. Dairy Sci. 1983 66:2251]

Biological mechanisms of oxytocin on milk ejection

Manual stimulation of the teat or nipple is not required for oxytocinrelease or milk ejectionOxytocin can be released by sights and sounds of the milking parlorOxytocin is not always measurably elevated in blood during milk letdown

Oxytocin and Milk Ejection

Oxytocin

Peptide hormone, 9 amino acid longCys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Glymolecular weight 1007 daltonsa disulfide bond between the two cysteinesReduction of the disulfide bond inactivates oxytocin

Hypothalamic Nuclei and Oxytocin Synthesis

Oxytocin syntheized in paraventricularand supraoptic nuclei in hypothalamusInitially synthesized as a large MW precursor, consists of the oxytocin-carrier peptide neurophysinThen, cleaved in neuron to yield oxytocinbound to neurophysinOxytocin-neurophysin complex is the intracellular storage form of oxytocin

The oxytocin-containing vesicles transported down hypothalamo-neurohypophysial tract to store at posterior pituitary

The synthesis of oxytocin in the cell bodies and its transport to the axon endings occur separately from the milk ejection reflex

Oxytocin Surge

oxytocin increased within 1 to 2 min. after udder stimulation, but declining during milking

Why pre-stimulation of the cow needed before milking?

Hygiene – for prevention of mastitis and for

maximizing milk quality

Milk ejection - [see J. Dairy Sci. 1980 63:800]

Manual stimulation resulted in higher milking efficiency and higher peak and average milk flow ratesMean peak oxytocin was not different, but pre-stimulated cows' oxytocin peaked at 2 min. after stimulation, compared with 5 min. after machine-on time for the unstimulated cows

Why pre-stimulation of the cow needed before milking?

Milk flow rate - [see J. Dairy Sci. 1985 68:1813] average milk flow rate increased with increasing duration of udder stimulation

However, oxytocin concentration was not different

Timing of oxytocinrelease relative to milk removal is an important

factor affecting milk ejection

A) Stimulate teats for 1 min prior to attaching the milking machine, or

B) Put milking machine on immediately without any prior manual stimulation

Machine-on-time is shorter for the pre-stimulated cow and the peak flow rate is higher for the pre-stimulated cowInitial rise and fall of flow rate during the first min of milking in B. In this case, the milking machine is initially removing the milk present in the cisterns (does not require milk ejection) and is providing the tactile stimulation necessary to elicit the normal release of oxytocin, which causes the second increase in milk flow rate

Results

Oxytocin & Milk ejection reflex

Sensitivity of the neuroendocrinereflex decline as lactation progressesPeak oxytocin come later after mammary stimulation as lactation progressesMaximum oxytocin concentration during milking also declines as lactation progresses

dry or nonlactating period may serve to restore the sensitivity of the neuroendocrinereflexNonlactating cows will release oxytocin in response to udder stimulation, but virgin heifers do not respond Maximum oxytocin in response to udder stimulation occurs only if the mammary gland is lactating or has lactatedMaximal prolactin release from the pituitary in response to tactile stimulation of the udder depends on the presence of a fully developed mammary gland

How much oxytocin is needed to elicit milk ejection?

Peak oxytocin is about 11to 65 microunits/ml serum; 40 liters of blood in a cow = about 0.4 to 2.6 IU. Normally inject 10 IU cause milk letdown, but as little as 0.02 IU into the jugular can result in milk ejection

(see Sagi et al. J. Dairy Sci. 1980 63:2006).

Oxytocin has a short half-life in the blood = 0.55 to 3.6 min

Thus, removal of milk by machine or by nursing must be closely timed with stimulation of the teats

Other Roles of Oxytocinin rats, induces maternal behaviorOxytocin has insulin-like activity and may be lipogenic (mother rapidly losing lipid when milk is removed)Both oxytocin and prolactin involve in osmoregulation (mother rapidly losing water when milk is removed)oxytocin acting as neurotransmitter

Other Roles of Oxytocin

interaction between oxytocin and prolactin release from the pituitary remains an area of investigation

(see also Mori et al., 1990, Endocrinology 125:1009)

Involvement of Autonomic Nervous System and Stress

Autonomic nervous system :ANS

Central nervous system

controls visceral function

ANS made up of parasympathetic and sympathetic nerves

Parasympathetic nerves

neurotransmitter is acetylcholine

There is no parasympathetic innervation in the mammary gland

Sympathetic nerves

epinephrine and norepinephrineEpinephrine (adrenaline) is primarily from adrenal medullaNorepinephrine is a neurotransmitter from peripheral nerves and nerves in the brain, and adrenal medulla

Effect of sympathetic nerves on milk ejection

depends upon the type of neurotransmitter receptor: alpha-receptors are vasoconstrictive- norepinephrine can stimulate milk ejection via brain alpha-receptorsbeta-receptors - norepinephrine can inhibit milk ejection via brain beta-receptors

Most sensory receptors (neurons) located in the teatpressure-sensitive neurons around the cisterns and the large ductsno direct innervation of alveoli or myoepithelial cellsNorepinephrine and epinephrine can inhibit oxytocin-induced contraction of myoepithelial cells

Stressful stimuli inhibit milk ejection

occurs via norepinephrine by the following mechanisms : reduces myoepithelial cell response to oxytocin;decreases mammary blood flow thus decrease oxytocin to the glandreduces oxytocin release from the pituitary

In bovine speciesnorepinephrine is the primary catecholamine

Injections of norepinephrine decrease milk yield, but Oxytocin is not altered

Emotional disturbances inhibit CNS in milk ejection reflex– especially in the first-calf heifer– oxytocin may be needed to remove milk to

prevent reduced yield through lactation

Other Mechanisms of Milk Ejection

Myoepithelial cells contract in response to vasopressin (ADH) though not of physiological significance in milk ejection Visual or auditory stimuli cause milk ejection as Milk ejection is a condition response

Stimulate genital tract, vaginal distention, release large amounts of oxytocin