S4: Pharmacology of the Uterus

Question 1

Describe the layers of smooth muscle in myometrium

Answer 1

• An outer layer of longitudinal fibres.
• A middle layer of figure of eight shaped/pattern fibres.
• Inner circular fibres.
  This organisation of the smooth muscle is very important and means that on contraction of these fibres it causes an increase in uterine pressure, that forces contents towards the cervix. The figure of eight fibres can cause strangulation in 3 different directions to propel contents forward. The myometriums organisation (esp. figure of eight fibres) acts as a ligature (constricts) on blood vessels to prevent blood loss during birth.

Question 2

Describe the properties of contractions in the myometrium and its control

Answer 2

• The myometrium is spontaneously active; it is myogenic and able to initiate its own contractions. These spontaneous contractions are modulated by neurotransmitters and hormones (in particular oestrogen and progesterone).
• Progesterone inhibits contraction of the myometrium.
• Oestrogen induces contraction of the myometrium.
• At birth (parturition) the oestrogen:progesterone ratio increases (oestrogen increases) this means the uterus is more stimulated to contract and push the baby out.
• The myometrium as mentioned is myogenic, but is also innervated by the autonomic nervous system that modulate it. It is innervated by sympathetic nerves (not parasympathetic).
• The myometrium contains both alpha and beta adrenoceptors. Stimulation of a1 receptors produces contraction, while stimulation of b2 adrenoceptors causes relaxation of the myometrium. This is important clinically when trying to prevent an early birth (stopping contractions).

Question 3

Compare contractions in non-pregnant uterus and pregnant uterus

Answer 3

• In a non-pregnant uterus, the woman will have weak contractions early in her menstrual cycle but strong contractions during menstruation as there is a fall in progesterone and increase in prostaglandins.
• In the pregnant uterus, there are weak and uncoordinated contractions of the myometrium in early pregnancy when progesterone is high. But then strong and coordinated contractions at parturition, this is due to the high oestrogen.

Question 4

How Does the Myometrium Achieve Synchronous Contraction? Explain mechanism

Answer 4

• The myometrium is myogenic and this works by a pacemaker mechanism. The myometrium contains these pacemaker cells, called the interstitial cells of Cajal (ICCs) that initiate and coordinate contractions. They are also found in the gut.
• The ICCs generate electrical activity and pass this on to the smooth muscle cells via gap junctions. The electrical activity then passes from smooth muscle cell to smooth muscle cell via these gap junctions. The gap junctions are made of specialised proteins called connexins that make low resistance pathways of large pores that make them ideal for electrical activity to pass through them from cell to cell.
• This means from the ICC cell upon generating its electrical activity will quickly be spread to neighbouring smooth muscle cells and further on, in a synchronous and coordinated fashion. We want this coordinated contraction in the myometrium. Thus due to the gap junctions the uterus behaves as a syncytium, this is a group of electrically connected cells. So what happens electrically in one will happen in the other due to their low resistance link.
• These mechanisms described to achieve synchronous contraction are affected by hormones e.g. oestrogen increases the expression of gap junctions to promote contraction. Increased gap junctions means more ease of movement of electrical activity through cells and more coordinated contraction.

Question 5

Describe the relationship between the electrical and mechanical activity of the myometrium

Answer 5

• If we looked at the ICCs pacemaker cells we would see they generate slow-wave activity. The slow wave depolarisations generate action potentials on the top of them.
• These APs will spread from the ICCs to the smooth muscle cells via the gap junctions, the electrical activity then causes smooth muscle contraction.
  This is because the electrical activity conducted to the smooth muscle cells causes them to become depolarised, as a result vgCa2+ in the cell get activated. Ca2+ fluxes into the smooth muscle cells and cause contraction.
• The slow waves/smooth muscle cells are modulated by neurotransmitters and hormones.

Question 6

Describe cellular mechanism of uterine SM contraction and modulation using oxytocin as an example

Answer 6

The myometrium will contain oxytocin receptors that are GPCRs. The receptor for oxytocin is Gq/11 linked, therefore it is positively linked to PLC.

1. Upon oxytocin binding, it causes stimulation of PLC.
2. PLC hydrolyses PIP2 to secondary messengers IP3 and DAG.
3. IP3 binds to its LGICR receptor on the SR, this causes release of Ca2+ into the cytosol contributing to a rise in intracellular Ca2+.
4. DAG stimulates ion channels in the sarcolemma so that there is more depolarisation of the cell membrane that causes activation of VGCCs leading to influx of Ca2+ that further contributes to the intracellular Ca2+ rise.
5. The Ca2+ binds to calmodulin forming the Ca2+-Cal complex, this activates MLCK.
6. MLCK phosphorylates the myosin light chains, this allows cross-bridge formation between myosin and actin and thus contraction.

Question 7

Describe the principles of excitation and inhibition and its stimulants in the uterus

Answer 7

• Like in other smooth muscle tissue, contraction is caused by an increase in [Ca2+]I and the force of contraction is proportional to the increase in [Ca2+]i. Thus more Ca2+ means increased force.
• Each action potential will cause a rise in intracellular calcium, but also at the same time there will be mechanisms activated that are trying to lower intracellular Ca2+. This includes calcium extrusion and shoving it back into the SR. Thus the change in [Ca2+]I will be the result of these combined processes, it is a balance of inhibition and excitation.
• If there are low concentration of stimulants e.g. oxytocin on the ICCs, then there will be a certain amount of slow wave activity producing a certain frequency of SMC contraction.
• As the stimulant concentration is increased, it means there is a greater frequency of AP on top of the slow waves (i.e. higher peak in Ca2+), this will raise both the frequency and force of SMC contraction.
• At very high concentrations, the really high stimulation of Ca2+ influx will mean that the extrusion process struggles to keep up and there is a plateau of prolonged sustained SMC contraction.
• At maximal concentration, this can result in hypertonus where there is incomplete relaxation as extrusion process cannot keep up and the Ca2+ extrusion process is no longer effective. Although this could be useful to stop haemorrhage, if the baby is still in the uterus it is a bad thing as constant contraction without any relaxation will mean blood supply is cut off leading to foetal distress.

Question 8

What is oxytocin?

Answer 8

• Oxytocin is a nonpeptide hormone synthesised in the hypothalamus and released from posterior pituitary gland. It is released in response to suckling and cervical dilatation, so seems to have a role in parturition (birth).

Question 9

What is the synthetic versions of oxytocin that are used clinically?

Answer 9

Syntocinon/Pitocin

Question 10

Why is oxytocin actions dependent on oestrogen?

Answer 10

• Oxytocin’s action is dependent on oestrogen and this is because oestrogen (released in the later stages of parturition) produces an increase in oxytocin release, an increase in oxytocin receptors and an increase in gap junctions!
• Therefore if oestrogen isn’t around, levels of oxytocin will be low and there will be few receptors so there would be no use of giving oxytocin as it won’t work. The gap junctions are also required for oxytocin to work properly because the electrical activity needs to be conducted between cells in order to produce a coordinated contraction.
• Therefore oxytocin is only effective at term when oestrogen is high.

Question 11

What are the pharmacological actions of oxytocin?

Answer 11

Low concentrations of oxytocin analogues will increase both the frequency and force of myometrial contraction.
High concentrations of oxytocin analogues will cause hypertonus and this can cause foetal distress.

Question 12

What are the uses of oxytocin?

Answer 12

• To induce labour at term, if the mother needs help in delivery oxytocin increases contraction of her uterus. However oxytocin doesn’t soften the cervix which is an essential part of delivery, this is a downside. If the cervix isn’t softened properly oxytocin will not be as helpful.
• A combination of oxytocin and ergot can be used to treat/prevent post-partum haemorrhage. The mix of oxytocin and ergot will contract the uterus and ligature the vessels to prevent/reduce bleeding. The oxytocin is rapid acting whereas the ergot has more prolonged contractile action. The drug used is Syntometrine that contains both oxytocin and ergot.

Question 13

What are the prostaglandins in uterus?

Answer 13

Prostaglandins PGE2 (a vasodilator) and PGF2a (a vasoconstrictor) are synthesised in the myometrium and endometrium. This synthesis is promoted by oestrogen, so as oestrogens rise the prostaglandins synthesis also rises. Both increase myometrium contractions.

Question 14

What are the actions of prostaglandins PGE2 and PGF2a?

Answer 14

• Prostaglandins are thought to have a role in dysmenorrhea (severe menstrual pain through uterine contraction) menorrhagia (severe menstrual blood loss though excessive vasodilation) and in parturition. This is why we use NSAIDs to help treat menstrual pain as NSAIDs inhibit COX and stop production of prostaglandins.
• As well as their vascular actions they also have non-vascular actions. They act together to coordinate an increase in frequency and force of uterine contractions. They also increase presence of gap junctions, they increase synthesis of oxytocin and they soften the cervix!

Question 15

What are some PGs analogues?

Answer 15

PGs are effective in early and middle pregnancy. Thus the PGs analogues we use are most effective in the same period! In brackets is what they are an analogue of.

• Dinoprostone (PGE2)
• Carboprost (PGF2a)
• Mistoprotol (PGE1)

Question 16

What are the uses of prostaglandins analogue?

Answer 16

The good thing is we aren’t relying on the receptors being there or not like we do with oxytocin, as the PG receptors are always there.
This allows us to use PGs in more/different ways including:
- Induction of labour before term, oxytocin would be unlikely to do this as the no. of receptors would not be high enough.
- Induce an abortion.
- Postpartum bleeding.
- Softening the cervix.

Question 17

Problems with the use of PG analogues.

Answer 17

• If they get into the circulation they can have harmful effects, dinoprostone can cause systemic vasodilatation and this can cause a huge drop in BP and result in cardiovascular collapse. For this reason these PG analogues are given as a cervical gel/vaginal insert so that they only act locally.
• PGs may also cause hypertonus and foetal distress if they cause too much contraction with too little relaxation.

Question 18

Describe Ergometrine

Answer 18

• Ergot is a fungus that grows on some cereals (e.g. rye) and grasses. It is a combination of different substances and contains an array of potent agents e.g. ergometrine, ergotamine (very similar to LSD), histamine, tyramine and Ach. When ergot is ingested it causes gangrene (due to vasoconstriction), myometrial convulsions and abortion.
• However if used in a safe clinical way, it can be used to produce powerful and prolonged uterine contractions. However it tends to only work when the myometrium is relaxed.
• Its mechanism of action is though to be stimulation of a-adrenoceptors, that cause contraction of the uterus. It may also act on 5-HT receptors.
• Its main use is in treating/preventing post-partum bleeding, where it is used alongside oxytocin as Syntometrine. It is NOT used to induce labour due to too powerful and long contractions.

Question 19

When may myometrial relaxants be used?

Answer 19

Myometrial relaxants may be used in premature labour. Importantly they can be used to delay delivery by 48hrs, this gives time for the mother to be transferred to specialist unit and given antenatal corticosteroids to aid foetal lung maturation and increase likelihood of survival.

Question 20

Describe some myometrial relaxants

Answer 20

• b2-adrenoceptor agonists e.g. salbutamol can be used for relaxation. They will relax uterine contractions by direction action on the myometrium and used to reduce the strength of contractions in premature labour. If a woman was having a normal labour but they also had asthma and took an inhaler, these effects may occur as side-effects.
• Ca2+ channel antagonists e.g. nifedipine (used in hypertension) or magnesium sulfate (a membrane stabiliser, prevents membrane depolarisation) would be useful as it is Ca2+ influx that causes contraction of the smooth muscle.
• We can also use oxytocin receptor antagonists e.g. Retosiban, these would compete with oxytocin for their receptors and block them and this would reduce the effect of oxytocin at birth.
• COX inhibitors e.g. NSAIDs will reduce levels of PGs, this is why they are useful to treat dysmenorrhea and menorrhagia. However NSAIDs can get into the foetal circulation and this can cause fetal renal dysfunction.

Question 21

What would be used for induction of labour at term?

Answer 21

For induction of labour at term we would use oxytocin, as there has been high oestrogen meaning we have lots of oxytocin receptors and gap junctions. So oxytocin analogues will work well.

Question 22

What would be used for induction of labour before term or a termination?

Answer 22

For induction of labour before term or a termination, we would use prostaglandins as there no oxytocin receptors yet.

Question 23

What would be used to reduce or prevent post partum bleeding?

Answer 23

To reduce or prevent post-partum bleeding we would use prostaglandins, oxytocin/ergot (Syntometrine) that gets the vessels ligatured.

Question 24

What would be used to prevent a premature birth?

Answer 24

Finally to prevent a premature birth, we could use b2-adrenoceptor agonist, Ca2+ blockers, Mg sulfate or oxytocin inhibitors!