Mne Lecture 29 - The Posterior Pituitary

Front 1

Compare and contrast hormone secretion from the anterior and posterior pituitary !

Back 1

DIFFERENCES!!
-anterior pituitary:
-have closed portal system, have neuroendocrine neurons secreting releasing factors into portal system. This is different than a synapse. They’re trying to affect portal blood concentration of releasing factors to regulate these anterior pituitary cells that are bathed in this portal system.
-the anterior pituitary cells try to affect circulating concentration of hormones in the general circulation

posterior pituitary:
-hypothalamic neurons, many called magnocellular neurons because they’re so large, that send their axons through the infundibular stalk into the posterior pituitary and secrete their products directly into the blood.
-have neurons that are trying to affect the general circulation
-this is a category of neurons distinct from the classic pool of neurons
-LARGE amount of product secreted into the blood yet these hormones are at v. small concentrations
-they’re acting at receptors in the kidney, placenta, breast, vasculature, that have very high affinity for these low concentrations of hormones.
-two classic hormones of posterior pituitary: oxytocin and vasopressin

Front 2

How is the primary sequence of vaospressin similar to oxytocin? Different? How/from what might these two hormones have evolved?

Back 2

2 hormones are both
-9 amino acid peptide hormones
-internal disulfide bond
-share great deal of homology:
7/9 amino acids are identical
-note the differences, circled.
-Arginine vasopressin (to distinguish from other forms of vasopressin in pig) is very similar to oxytocin. 
-Probably arose from some progenitor gene that duplicated and then individually evolved into their distinctive physiologies
-these two genes are right next to each other on chromosome, probably the remnants of duplication domain

Front 3

Vasopressin

Back 3

-vasopressin is the major ADH of mammals
-involved in regulation of water balance in kidney
- vasoconstrictive

Front 4

Oxytocin

Back 4

-classic parturition hormone
-involved in uterine contractions in delivery, milk ejection in breast feeding, involved in lots of behavior involved with child rearing

-plays a classic role between breast feeding and milk ejection and parturition and childbirth
-part of classic suckling reflex where stimulation of nipple by infant leads to activation oxytocin secretion from posterior pituitary

Front 5

Vasotocin

Back 5

-hormone that probably gave rise to oxytocin and vasopresin
-has isoleucine at position 3 of oxytocin and arginine at position 8 of vasopresin
-we make this hormone in pineal gland, function uncertain
-present in amphibians
-does both roles: vasopressin agonist and governs mating/childrearing behaviors

Front 6

Why do vasopressin and oxytocin need to be concerned about preventing themselves form being broken down?

How do they accomplish this?

Back 6

-secreted into general circulation, reach nano-molar concentrations, need to protect from breakdown during circulation by carboxypeptidases and amino peptidases
-extends half life 
-extends concentration to be able to act on many of the receptors at which they function 


2 strategies: PROTECT THE AMINO AND CARBOXY TERMINALS: 

1. The amino terminal end is partially protected by the disulfide bond
--> not as likely to be eaten by aminopeptidases (catalyze the cleavage of amino acids from the amino terminus of protein or peptide substrates)

2. The carboxy-terminal end is protected by amidation
--> amide linkage at carboxy terminal. 
--> Remnant of a peptide bond 
--> Looks like carboxylic here, was linked to a glycine residue and then most of glycine was cleaved to leave amide linkage. 
--> Not as likely to be eaten by carboxypeptidases (a protease enzyme that hydrolyzes (cleaves) the peptide bond of an amino acid residue at the carboxy-terminal (C-terminal) end) in the blood

Front 7

The human precursors to vasopressin and oxytocin?

Back 7

Synthesized as part of large molecular weight precursors
-the amino terminal end of these hormones is revealed by signal peptidase activity in the ER
-that’s a bit unusual
-these other hormones (neurophysin in particular) don’t have any biological activity and probably play a role in the 3’ shape of these hormones for proper processing.

Front 8

Gq

Back 8

-heterotrimeric G protein subunit that activates phospholipase C (PLC).
-PLC in turn hydrolyzes Phosphatidylinositol 4,5-bisphosphate (PIP2) to diacyl glycerol (DAG) and inositol triphosphate (IP3) signal transduction pathway.
-DAG acts as a second messenger that activates Protein Kinase C (PKC) and IP3 helps in phosphorylation of some proteins (increase intracellular Ca levels)

Front 9

Gs

Back 9

Gs alpha subunit (or Gs protein) is a heterotrimeric G protein subunit that activates the cAMP-dependent pathway by activating adenylate cyclase.

Front 10

Gi

Back 10

Gi alpha subunit (or Gi/G0 or Gi protein) is a heterotrimeric G protein subunit that inhibits the production of cAMP from ATP

Front 11

oxytocin

receptor is coupled to?
what is it involved in?

Back 11

-has a single receptor!
-Oxytocin acts at an oxytocin receptor that is coupled to Gq. It activates phospholipase c. 

-It is involved in smooth muscle contractions in the uterus, in smooth muscle contractions in the breast to eject milk into the suckling infant. It has a lot of CNS effects.

Front 12

Vasopressin

receptor is coupled to?
acts on?

Back 12

1. Vasopressin V2
-main one
-classic vasopressin in distal and collecting tubules of kidney
-It’s coupled to Gs: coupled to adenyl cyclase --> cAMP

2. Vasopressin V1b 
-coupled to Gq in the anterior pituitary
-it is on corticotrophs of the anterior pituitary
-Allows vasopresin to stimulate release of ACTH just like Corticotrophin releasing hormone does
-corticotrophin releasing hormone and vasopressin act synergistically to secrete ACTH from the anterior pituitary. 

3. Vasopressin V1a
-also coupled predominantly to Gq
-Present in the smooth muscle cells of vasculature
-It’s a vasoconstricting receptor so it’s involved in vasorestriction and increases in blood pressure; helps counteract losses of blood pressure during hemorrhage and position

Front 13

magnocellular neurons

Back 13

-big cells in hypothalamus
-have long axons
-you synthesize lots of hormone in these big cells 
-transport vesicles down the infundibular stalk into the posterior pituitary where they interact with capillaries associated with the main circulation
-during this transport get processing of precursors and release in response to a mix of signals 
-they integrate afferent signals that come in from different parts

Front 14

What affects vasopressin secretion?

Back 14

-->the CNS and circumventricular organs that sense, in the case of vasopressin for example, to osmotic signals (largely positive) so that vasopressin secretion will increase in response to increases in circulating solutes, to dehydration, to increases in salt concentrations, so that it’s very responsive to this.
-->vasopressin is also vasoconstrictive acting at V1a receptors and will respond to decreases in volume, to increases in blood pressure differentially. Barroreceptor inputs, mostly negative. As blood pressure goes up vasopressin secretion is inhibited 
-->these two physiological situations are NOT independent have changes in solutes, in blood volume, in blood pressure, in salt concentrations all going on at the same time. It’s the responsibility of vasopressin neurons to integrate this information and secrete vasopressin appropriately to do this. 

-increases in solutes
-dehydration
-increases in salt concentrations
-decreases in volume
-increases in blood pressure

Front 15

What do the vasopressin neurons do? How?

Back 15

-It is the responsibility of vasopressin neurons to INTEGRATE the inputs from various sources and secrete vasopressin Accordingly. 
-Does this through receptors scattered through hypothalamus, heart and its baroreceptors in the aorta and input into the magnacellular neurons
-there are osmoreceptor neurons
-there are baroreceptor inputs from the vasculature

Front 16

Circumventricular organs (CVO)

Back 16

- positioned at distinct sites around the margin of the ventricular system of the brain.

-They are among the few sites in the brain which have an incomplete blood–brain barrier.

-As a result, neurons located in circumventricular organs can directly sense the concentrations of various compounds, particularly peptide hormones, in the bloodstream, without the need for specialized transport systems which move those compounds across the blood–brain barrier.

Front 17

Where are the osmotic receptors? How do they work?

Back 17

-Osmotic receptors are in the CIRCUMVENTRICULAR organs
-these are like windows through the BBB
-focus on measuring different qualities of blood

-OVLT and SFO are two particular circumventricular organs that are rich in osmoreceptors. 
-->Sample solute concentration
-->salt concentration in blood
-Volume of these neurons in the circumventricular organs changes in response to osmotic pressure!
-->As osmotic volume goes up they shrink and visa versa.
-Sends + signals to magnocellular neurons saying “osmolalitiy is going up !! Let’s stimulate neurons secrete more vasopressin!!!"

-also baroreceptors that are responding to changes in blood pressure sending negative inputs into these cells to affect vasopressin secretion 

OVLT = organum vasculosum laminae terminalis
SFO = subfornical organ

Front 18

Describe osmotic control of vasopressin

Back 18

osmotic control of vasopressin is very straightforward and linear

-plasma vasopressin [] go up linearly in response to plasma osmolality
-as osmolality gets greater and greater, that exerts a greater + effect on magnocellular neurons and that causes an increased secretion of vasopressin
-if you extrapolate the line down to 0 vasopressin, you get to a point called the OSMOTIC THRESHOLD which is defined as that osmolality above which you get vasopressin secretion, and its linear above that point-make equation of a line predictive of what vasopressin concentrations will be

y=mx+b 0.3(plasma osmolality – osmotic threshold) = get a decent sense of what plasma vasopressin levels might be

SAME with urine:
-as vasopressin goes up, conserve water, urine osmolality goes up as you try to recover water and concentrate solutes in urine
plasma vasopressin goes up, urine osmolality goes up, get nice equation for a line.

Front 19

OSMOTIC THRESHOLD

Back 19

that osmolality above which you get vasopressin secretion, and its linear above that point

Front 20

solute [] go up from salt from a huge bag of potato chips you just beasted

what happens now ?!

Back 20

want to go to set point for plasma osmolality

several things happen:
-increase thirst,
-conserve water,
-concentrate urine,
- diltute solutes so they’re back to set point for a normally hydrated person,

plasma osmolality might be 287 mosm/kg,this would predict a
Pavp (vasopressin) = 2 pg/ml urine
osmolality = ~500 mOsm/kg

Front 21

What happens if you have a 1% increase in total body water?

Back 21

-volume increases
- inhibit vasopressin, so your plasma osmolality goes down by almost 3 mOSM/kg to about 284
-you inhibit vasopressin secretion by ½ and secrete more dilute urine as you try to get rid of extra extracellular fluid

Front 22

What happens if you have a 2% increase in total body water?

Back 22

Maximal vasopressin suppression at about 2% increase in body water 

At about 2% increase in body water, which is modest but big change

turn off vasopressin,
get maximally dilute urine,
plasma osmolality approaches osmotic threshold

Front 23

What happens if you have a 2% decrease in total body water?

Back 23

2% decrease
– dehydrated eat salt, increase solutes

-solute goes up
-you increase vasopressin by a factor of 2 
-secrete very concentrated urine

Front 24

What happens when you get a 1 mOsm rise in plasma osmolality?

Back 24

1 mOsm rise in plasma osmolality 

go from 287 --> 288

-predicts 0.3 pg rise in vasopressin
-slight increase in your ability to concentrate urine
-this is more in response to physiological action or a small dietary change, but over 24 hour period represents liters of volume difference


6 liters at 100 mOSm/kg
3 liters at 200 mOsm /kgThis represents a big change in thirst, in regulation of extracellular fluid, in secretion of solutes even though it’s linear it’s very sensitive to modest changes in ECF volume and solute concentration

Front 25

Maximal antidiuresis occurs at

Back 25

[AVP]p = 5 pg/ml

Front 26

Is additional AVP secreted at greated osmolality than [AVP]p = 5 ?

Back 26

-Additional AVP is secreted at greater osmolality, but this AVP does not conserve more renal water

-Doesn’t mean in response to greater osmotic stimulation that vasopressin secretion will not go up, but it won’t conserve more renal water, won’t conserve urine more

Front 27

Additional AVP is secreted at greater osmolality, but this AVP does not conserve more renal water


What happens then if you’re really dehydrated?

Back 27

Adaptive/Behavorial responses. Homoestasis. Once vasopressin is not enough, need to do something to help. Go get a drink of water, do something else to help body accommodate high levels. Stimulate thirst and other behaviors. Same is true for temperature

Front 28

Dehydration is avoided by stimulation of

Back 28

thirst osmoreceptors.

Front 29

The osmotic threshold for thirst activation is

Back 29

5-10 mOsm/kg greater than for AVP secretion.

Front 30

Why wouldn’t you stimulate thirst at same time you stimulate vasopressin?

Back 30

-Trying to regulate something by drinking and you’re always going to overshoot, secrete.
- Same reason why you have a bladder.
-Have system that allows for homeostatic control of extracellular solutes.
-Can do that physiologically.
-There is no reason we have to be drinking continuously in order to regulate body fluid.

**Once you start hitting extremes then you augment homeostatic controls by behavior mod (drinking, abstaining from drinking)
--> Augmenting our regulation

Front 31

Drinking ameliorates thirst and inhibits

Back 31

AVP secretion before changes occur in fluid volume or osmolality.

Front 32

AVP secretion is inhibited by

Back 32

activation of cold-sensitive oropharyngeal receptors

Oropharyngeal receptor in throat sense drinking of cold water, and body shuts off vasopressin secretion in anticipation that you’re going to start diluting some of ECV

Front 33

cardiovascular control of vasopressin

Back 33

It becomes more difficult to secrete vasopressin as bp goes up and much easier as it goes down, but you’re integrating all these at the same time anywhere from posture changes to blood losses to changes in blood volumeall of that changes

Now going from linear system to exponential control
relationship between vasopressin levels and mean arterial levels

easiest way to approach: at any given BP the relationship between vasopressin levels in blood remains linear between osmolality and vasopressin whether you’re hypo- or hyper-tensive 
BUT
as you move through changes in BP it’s exponential and complicated


BIG PICTURE: as you increase BP, the osmotic threshold goes up
--> larger ECV, more pressure on baroreceptors, increase vasopressin secretion you retain water which contributes to ECV and that’s counterproductive so secretion of vasopressin become more difficult as BP goes up.
-->As bp goes down, as it goes below normal, vasopression secretion is increased, easier to secrete.

Osmotic control of vasopressin is the dominant regulator of vasopressin, the most powerful is HEMMORAGE get a severe drop in blood volume, severe drop in blood pressure and pressure dump vasopressin to try to get some vasoconstriction to stem that blood loss.

Front 34

Where does water recovery in the kidney occur?

Back 34

-at these distal collecting ducts there are vasopressin receptors (V2) that are concentrated in the cortical collecting and inner medullary collecting ducts

-these receptors permeabilize membrane to water

Front 35

How does vasopressin interact with the kidney?

Back 35

-apical and basolateral membrane 
-v2 receptors on basolateral membrane in response to activation by vasopressin activate Gs --> cAMP --> protein kinase a --> accentuate pathway that activates insertion of aquaporin 2 channels into the apical membrane. By inserting water channels you’re completing circuit 

-aquaporin 3 and 4 on the basolateral membrane
-aquaporin 2 on apical membrane

-water is able to flow and recover water that way
-essentially open up one end of a closed system.

Front 36

Aquaporins

Back 36

Aquaporin channels are specific for water 

-6 transmembrane domains --> 4 of which form a channel for water
-this is a good example of facilitate diffusion: facilitating passive transport
- water will normally cross the membrane (but it does it slowly)
-but if you can generate a lot of urine quickly (think beer), there is a lot of volume moving across here
-in absence of aquaporin puts a lot of pressure on membranes for passive diffusion
-get facilitated diffusion with aquaporin transporters, they get inserted and you recover that water.

Front 37

The maintenance of salt homeostasis, circulatory volume, and blood pressure requires the integrated actions of the

Back 37

renin-angiotensin system, the adrenergic nervous system, vasopressin, atrial natriuretic hormone, kinins, endo-thelins, prostaglandins, and the nitric oxide system.

KEV NAP (the) RA

Front 38

What happens in response to hypovolemia?

Back 38

start with hypovolemia then follow what happens:

-decrease in blood volume that is sensed at level of kidney which secretes renin, an enzyme that processes angiotensinogen to angiotensin I.

-angiotensin convertin enzyme converts that into angiotensin II which acts back on the adrenals to secrete aldosterone which acts back on the kidneys to stimulate sodium reabsorption.

-As you stimulate sodium reabsorption water will follow, which increases serum tonicity or osmolality which as you increase plasma osmolality that is sensed by hypothalamus secrete vasopressin from posterior pituitary which acts on kidney to cause water reabsorption which raises blood volume.

-Circuit brings you back. Blood volume increases raises GFR (glomulular filtration rate), and that will cause secretion of sodium and decrease in volume

Front 39

Diabetes Insipidus

Back 39

Characterized by excretion of large volumes of dilute urine that cannot be reduced by decreased fluid intake; a decreased ability of the kidney to concentrate urine

1. Central Diabetes Insipidus
2. Neprhogenic Diabetes Insipidus
3. Dipsogenic Diabetes Insipidus
4. Gestitational Diabetes Insipidus

Front 40

Central diabetes insipidus:

Back 40

-Mutation in ability to secrete bio-active vasopressin

-Usually mutation in vasopressin gene
-->Usually mutation prevents proper processing of precursor

-Will lead to apoptosis of magnocellular neurons
--> do autopsy, no magnocellular neurons. Individuals born normal and quickly develop diabetes insipidus

-Central diabetes insipidus is sometimes genetic, but other times product of trauma or cancer – have a tumor of the hypothalamus or of the pituitary that impacts the posterior pituitary; whip lash that severs the infundibular stalk which can lead to loss of vasopressin secretion 
-in some cases get sprouting of neurons get increased secretion of vasopressin from median eminence or areas nearby
-can resolve DI that way after a few months
-if it’s central can usually take long lasting vasopressin agonist can snort it, gets absorbed in mucus membranes, regulates vasopressin that way

Front 41

Nephrogenic diabetes insipidus

Back 41

-Means the disfunction is at level of kidney

-Typically because there is a mutation in V2 receptor or aquaporins

Front 42

Dipsogenic diabetes insipidus

Back 42

-More transient
-Typically associated with Psychiatric disorders --> chronic water drinkers

Schizophrenics

Front 43

Gestational diabetes insipidus

Back 43

-More transient

-Have secretion of enzymes that degrade vasopressin from the uterus

-Usually resolves following delivery

Front 44

AVP V2 Receptor mutations can result in

Back 44

nephrogenic DI

-v2 receptor Different mutations identified in humans with nephrogenic diabetes incipitus
-note many are associated with transmembrane domains

Front 45

Aquaporin-2 mutations result in?

Back 45

nephrogenic DI

frequently associated with transmembrane domains

Front 46

What stimuli do oxytocin nerves respond to?

Back 46

-these oxytocin neurons respond to lots of stimuli not just suckling
-input also from visual system, auditory system, olfactory system
-the site of a baby or hearing crying child or smelling your baby will stimulate milk ejection in preparation for breast feeding
-go to baby store where new mothers/new parents are.. New mom/babies one baby starts to cry and all mom’s breasts start to explode until things settle down a bit

-stimuli are very powerful, equally or more powerful than suckling response-stimulating milk ejection and also prolactin to make more milk for next feeding.

Front 47

How are oxytocin and vasopressin similar?

Back 47

-oxytocin is vasoconstrictive
-Oxytocin and vasopressin share this vasoconstrictive process
-vasopressin is vasoconstricting in vasculature, oxytocin is vasoconstricting in the mammary glands, uterus
-acts on myoepithelial cells in breast to cause ejection of milk into the suckling infant

Front 48

we use oxytocin for other things, so you don’t want something that’s going to stimulate oxytocin to initiate premature contractions/labor

How do we prevent this from happening?

Back 48

-the way the system works is that in the myometrium of the uterus express low levels of oxytocin receptors 
-once you start to reach term then levels of oxytocin levels go up and preexisting oxytocin levels become effective at higher concentration of receptors
-oxytocin levels don’t go up that much in delivery but they’re acting on a much higher [] of receptors
-very common thing to give to mothers whose delivery is not progressing -->can give oxytocin
-can be effective, probably overused
'
*not many cases where system is regulated this way

note: y axis is Concentration of oxytocin receptors (fmol/ml protein)

Front 49

Tocolysis:

Back 49

The delaying or inhibition of labor during the birth process

-Given the role of oxytocin in parturition, you would think that oxytocin antagonists (like ATOSIBAN) would be a good solution for premature labor/delivery
-premature labor/preterm delivery is a risk factor for mom and baby and drain of resources
-if we could control premature labor it would be a benefit
-people thought oxytocin antagonists would help, but they’ve been disappointing

-the most common oxytocin antagonists are no more effective than placebo
-shouldn’t be surprising because parturition is so complicated, one of most complicated endocrine things we could do. 
-Oxytocin is just one player in a whole slew of things that coordinate and stimulate smooth muscle contraction and priming cervix and getting everything ready for this big event 
-getting rid of oxytocin isn’t sufficient to tame premature laborbed rest continues to be required.

Front 50

Drugs used clinically to induce or augment lactation

Back 50

1. domperidone
2. TRH
3. Metoclopramide
4. Sulpiride
5. Chlorpromazine

Front 51

Adoptive breast feeding

Back 51

Prevalence is new:
with assisted reproductive strategies in place and foster moms giving birth to infants for parents there are a category of mothers who want to breast feed but didn’t give birth to childso there are pharmacological strategies to try to induce lactation in mom

-->sometimes it works, other times it doesn’t 
-->depends on goal:
a. if the goal is to nurture mom/child relationships and get suckling, not particularly nutritive that can work 
b. if you’re trying to feed infant and give large supply of milk that’s more difficultside effects, sometimes chronic nipple suckling or stimulation will do this

Front 52

domperidone

side effects:

Back 52

well tolerated

action:

Hint 52

blockage of pituitarydopamine receptors

Front 53

TRH

side effects:

Back 53

Well tolerated; no clinical signs of hypothryoidism

action:

Hint 53

Direct stimulation of lactotrophs

Front 54

Metoclopramide 

side effects:

Back 54

Well tolerated extrapyramidal; drowsiness

Action:

Hint 54

blocks dopamine-mediated inhibition of prolactin secretion

Front 55

Sulpiride

side effects:

Back 55

Well tolerated extrapyramidal; drowsiness

Action:

Hint 55

blocks dopamine-mediated inhibition of prolactin secretion

Front 56

Chlorpromazine

side effects:

Back 56

Well tolerated extrapyramidal; drowsiness

Action:

Hint 56

blocks dopamine-mediated inhibition of prolactin secretion