12- Pharmacology of the Uterus Flashcards
What is the myometrium?
A layer of smooth muscle that forms the wall of the uterus – located between the endometrium and the perimetrium
How many layers of smooth muscle does the myometrium have + what are these layers?
3 smooth muscle layers
- Outer longitudinal muscle fibres
- Middle: figure-of-eight/mesh-like fibres
- Inner: circular fibre
Why is the myometrium a highly muscular organ?
In order to provide contractions during the menstrual cycle and during delivery of the foetus
What are the mechanical properties of the myometrium?
Rhythmic contractions
- Spontaneously active
- Vary during menstrual cycle and pregnancy – indicates that they are sensitive to different hormones
- Force content towards the cervix and out of the body
Contractions within the uterus originate in the muscle itself
- Doesn’t require neuronal or hormonal input – factors that regulate the contractions
- But highly sensitive to e.g. sex hormones
What initiates contraction in the uterus?
- Mechanism is kind of like that of the heart
- Spontaneous depolarisation of ‘pacemaker’ cells - give rise to action potentials - APs spread to different cells
- Electrical communication between (smooth muscle) cells
- Gap junctions spread depolarisation
- Myometrium can function as a syncytium - entire organ functions as a single unit i.e. entire uterus contracts simultaneously when delivering a pregnancy
Initiated by depolarisation of pacemaker cells
Explain excitation-contraction coupling?
- Slow waves of pacemakers and smooth muscle responses are modulated by neurotransmitters and hormones
- Depolarisation initiates firing of APs and therefore contraction
- Depolarisation spreads to other smooth muscle cells throughout the uterus and enables contraction and the allows the uterus to function as a syncytium
- Depolarisation is conducted to other cells
What facilitates smooth muscle contraction in the uterus?
- similar to other smooth muscle tissues
- increased intracellular [Ca2+]
- incremental increases in [Ca2+]i causes incremental increases in the forces of contraction
- the amount by which intracellular calcium levels increase by determines the force of contraction
What is the effect of an influx of Ca2+?
Causes the opening of voltage gated calcium channels, Ca2+ floods into the cell and causes membrane depolarisation of smooth muscle
What is a mechanism for lowering [Ca2+]i ?
Ca2+ extrusion
How is contraction modulated by hormones and neurotransmitters?
Oxytocin acts on a G protein coupled receptor and activates the Phospholipase C chain of activity ( breakdown of PIP2 into IP3 and DAG , IP3 binds to a receptor to cause intracellular release of Ca2+ from internal calcium stores causing contraction
Ca2+ binds to calmodulin and activates myosin light chain kinase which interacts with actin to cause a contraction
How is uterine contraction measured?
Isometric tension recording
- Measure tension generated with diameter of the muscle ring remains constant
Oxytocin induced contractions
Ion channel modulators
- K+ channel activator (Higher K+ concentrations inhibit depolarisation and therefore reduces the frequency and amplification of contractions)
- Ca2+ channel blocker (inhibit Ca2+ influx)
How are smooth muscles of the uterus regulated by neurotransmitters?
Uterus has sympathetic innervation expression of alpha and beta adrenoreceptors
alpha adrenoreceptor agonist - contraction
- this is via the a1 pathway à breakdown of phospholipase C, production of IP3 and DAG from PIP3
- causes release of intracellular Ca2+ à smooth muscle contraction
beta2 adrenoreceptor agonist - relaxation
- Gs coupled GPCRs
- Activation of adenylate cyclase à causes breakdown of cyclic cAMP
- Causes relaxation within smooth muscle
How are smooth muscles of the uterus regulated by sex hormones?
Progesterone - inhibits contraction
Oestrogen - increases contraction
Both act on both nuclear and membrane receptors
Describe the regulation of a non pregnant uterus by sex hormones
Week contractions early in the cycle- oestrogen low
Strong contractions during menstruation - progesterone low
Describe the regulation of a pregnant uterus by sex hormones
Weak and uncoordinated contractions in early pregnancy (high progesterone – inhibits contractions)
7 month till term – Oestrogen increases towards, progesterone stays constant
Progesterone: oestrogen ratio significant during pregnancy
Oestrogen / progesterone ratio increases throughout last trimester culminating with strong, coordinated contractions for delivery (low progesterone to oestrogen ratio)
Entire uterus functions as a syncytium for delivery as different parts of the uterus are co-ordinated via oestrogen