24A10: Exam Report

(a) Explain the excitation contraction mechanism as it relates to the smooth muscle of the myometrium including a gravid uterus (50% of marks).
(b) For each tocolytic agent: salbutamol, nifedipine, magnesium sulphate, provide the following information:
(i) List the class (10% of marks)
(ii) Provide a dose (10% of marks)
(iii) Describe the mechanism of action (20% of marks) (iv) Outline the adverse effects (10% of marks)

42% of candidates passed this question.

  1. This question required an explanation of the process of contraction in smooth muscle, as well as specific information pertaining to the smooth muscle of the uterus. Specific information included: stimuli for contraction (i.e., hormones such as oxytocin, oestrogen, and prostaglandins and stretch); possessing an unstable membrane potential which plays a greater role than nervous input; as well as the uterus smooth muscle functioning as a syncytia and having baseline tone. The key steps of contraction (influx of calcium, formation of calcium-calmodulin formation, activation of MLCK, creation of myosin-actin cross bridges with utilisation of ATP) are those generic to any smooth muscle and marks were awarded accordingly. Descriptions of striated muscle did not attract marks. A description or explanation was required rather than a list of steps (e.g., the sources of calcium rather than a simple statement of calcium influx occurring).
  2. Use of these drugs as tocolytics is acknowledged as a less common reason for their utilisation – the fractionation of this question was to breakdown into more achievable parcels of information. This allowed candidates to score more marks for relating the mechanism of action specifically to the uterus. Higher marks in this section were awarded for specific unwanted effects relevant to use as a tocolytic e.g., foetal tachycardia for salbutamol as result of crossing the placenta. Marks were awarded to doses for these drugs when used as a tocolytic, as opposed to for other indications (i.e., salbutamol infusion rather than nebulised).

V1i / 24A10: Explain the excitation contraction mechanism as it relates to the smooth muscle of the myometrium including a gravid uterus

Part A

Smooth muscle is a type of involuntary muscle, under the regulation of the ANS.

There are 2 types of smooth muscle  single- unit & multi-unit smooth muscle (located throughout the body within the walls of hollow organs and tubes)

Myometrium smooth muscle is non striated, mono nucleated and contractions provides the driving force for parturition, as well as minimizing haemorrhage in the immediate post partum period.

Uterine smooth muscle is single-unit, with action potentials rapidly propagating to neighbouring cells via gap junctions, allowing for synchronous contraction as a functional syncytium. Caveolae increase surface area to facilitate calcium entry.

Smooth muscle of the uterus/ myometrium is not innervated by the ANS (other single unit smooth muscle syncytia are under Parasympathetic action of ACh on muscarinic receptors, and limited sympathetic innervation via noradrenaline).

The unstable resting membrane potential of smooth muscle allows for easier depolarization in the presence of an excitatory stimulus (threshold potential of -35 mV). Inhibitory signals will instead hyperpolarize cell membrane, inhibiting smooth muscle contraction.

The resting membrane potential (RMP) of myometrium smooth muscle is approx -50 to -60mV, and action potentials will occur in single-unit smooth muscles either spontaneously or via the action of:

Hormones

  • Oxytocin, oestrogen, prostaglandins
  • This is mediated by flow of calcium ions into the cell.
  • No response to circulating neurotransmitters (eg cathecolamines, O2/ CO2) as seen with other single unit SM cells eg bronchial, enteric.

Stretch

Stretch of smooth muscle sheets opens VG Ca++ channels on the wall of smooth muscle, causing influx and compensatory contraction to oppose further stretch (even in absence of neural input).

  • (Known as “MYOGENIC RESPONSE” within arterial vascular system to autoregulate blood flow).

Excitation- contraction coupling of smooth muscle

Smooth muscle cells contain actin and myosin, and calmodulin as main regulatory protein (no troponin).

Due to poorly developed sarcoplasmic reticulum and storage potential, sources of (mostly extracellular) calcium to enter smooth muscle cell and initiate cross- bridge latching include the activation of:

  • Voltage- gated Ca++ channels (L- type threshold potential -30m mV)
  • Ligand gated Ca++ channels (hormones and neurotransmitters)
  • Stretch- responsive Ca++ channels
  • Calcium induced- calcium release via direct stimulation of SR.

Key steps

  1. Stimulus causes influx of calcium ions into smooth muscle cell, where can bind to calmodulin (a regulatory protein) and form a calcium- calmodulin complex (CCC).
  2. The calcium- calmodulin complex can then activate smooth muscle contractions via 3 mechanisms:
    • Myosin light- chain kinase (MLCK): MLCK is a sarcoplasmic enzyme activated by the CCC, causing phosphorylation of myosin light chains, and activating myosin heads to allow formation of cross bridges with actin filaments. Requires energy from ATP for phosphorylation to occur.
    • Caldesmon: similar role to Troponin- T in skeletal muscle, Caldesmon activation by the CCC causes its conformational change and movement of tropomyosin to uncover myosin heads → allows actomyosin crossbridge formation.
    • Calponin: protein which inhibits myosin ATPase. CCC and calcium can directly activate calponin, leading to inhibition of smooth muscle relaxation ie sustain contraction via crossbridge formation.
  3. Similarly to skeletal muscle, activated myosin head can bind to neighbouring actin molecule and form crossbridge; the energised myosin head then flexes and causes a “power stroke”, moving actin filaments closer to the centre ie shorten.
  4. Release of ADP and inorganic phosphate allows for new formation of ATP, which continues to energize myosin head. Process continues until intracellular calcium concentration decreases, reducing overall availability of CCC.
  5. Relaxation of smooth muscle; ATP consuming process involving myosin phosphatase dephosphorylation of MLCK, where phosphate detached from myosin head ie cross bridge cessation.

Unlike skeletal muscles, most smooth muscle contractions are prolonged and tonic in nature, providing an underlying baseline tone (up to 10 x slower and 30 x longer in smooth muscle vs skeletal muscle). This is due to: 

  • If myosin is dephosphorylated whilst still attached to actin, the crossbridge remains in situ and is known as “latch bridge formation”, with no additional ATP consumption. This allow for smooth muscle tension to remain high without further ATP, and remain contracted for longer periods.
  • Enzymatic phosphorylation (with MLCK) required before actin- myosin binding can occur (rate limiting step)

Slow removal of intracellular calcium from poorly developed SR; in a process which requires ATP.

Part B

Tocolytic Drugs

ie suppress or delay uterine smooth muscle contraction, clinically used In premature labour

Salbutamol

Nifedipine

Magnesium Sulphate

Class

Salbutamol

Beta- 2 adrenergic, synthetic sympathomimetic

Nifedipine

  • Calcium Channel Blocker
  • Dihydropyridines

Magnesium Sulphate

Electrolyte

Tocolysis Dose

Salbutamol

  • IV slow boluses of 50 mcg, max dose 250 mcg IV total
  • Ampoule: 5mg/ 5ml
  • Half life 4-6 hours

Nifedipine

  • PO/ SL 10-20mg Q4-6h (1ST Line)
  • Titrate dose to tocolytic effect;
  • Max 160mg/day

Magnesium Sulphate

  • IV 4g (over 20 mins)
  • Then IV 1- 4 g/hr (Max for 24 hrs if undelivered)
  • Aim serum conc Mg++ 1.7 – 3.5 mmol/L

Mechanism of action

Salbutamol

Beta- 2 stimulation at G (s) PCR’s on uterine smooth muscle activate adenyl cyclase → rise in cAMP → phosphorylation of MLCK via protein kinase A = SM relaxation

Nifedipine

Blocking of membrane L-type VG Ca++ channels leads to reduction in Ca++ entry inside smooth muscle cell = less  formation of calcium-calmodulin complex = reduced SM contraction

Magnesium Sulphate

Direct calcium antagonist at membrane VG Ca++ channels and Ca++ binding sites on SR = reduced Ca++ influx into cell

Ie uterine smooth muscle relaxation

Adverse effects

Salbutamol

IV administration can cross placenta –> fetal tachycardia

CVS:

  • Tachycardia (stop if HR >140), hypotension, red SVR, inc Myocardial O2 demand Arrhythmias – ventricular ectopics/ SVT/ VF

 

Resp

  • Bronchodilation
  • Hypoxia 2Y to increased oxygen demand +/- risk pulmonary oedema from cardiac SEs +/- reversal HPV

Metabolic:

  • Inc lactate
  •  Hyperglycaemia, incr gycogenolysis

Misc:

  • Tremor , hypokalaemia

 

C/I as IV bolus:

  • Cardiac disease
  • Acute hypertensive crises
  • Hyperthyroidisim.

Relative C/I: T2DM

Nifedipine

CVS:

  • Reduced tone in peripheral & coronary arteries = red SVR, red BP; may elicit rebound tachycardia with inc in CO (may worsen oxygen supply/ demand ratio).
  • Flushing, headaches, N&V
  • Risk AV block if used with magnesium
  • Do NOT use IV Salbutamol in combination with nifedipine

Magnesium Sulphate

Crosses placenta and elevates fetal plasma conc.

CVS:

  • Transient hypotension
  • CP, palpitations, pulmonary oedema

CNS:

Blurred vision

 

Vasodilatory SE’s – flushing, headache, N&V

 

Cardiac and respiratory arrest in toxicity.

References

Author: Ines Vaz