(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)
(a) This question required an explanation of the process of contraction in smooth muscle, as well as specific information pertaining to the smooth muscle of theuterus. 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).
(b) 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)