Physiology of uterine contractions, tocolysis and pharmacology

Question 1

Blood supply to the uterus in lat pregnancy?

Answer 1

500ml/minute

Question 2

What causes contraction and relaxation of the uterus?

Answer 2

Depolarisation and depolarisation of the smooth muscle

Needs to be coordinated to labour - gap junctions form between muscle cells, initially there are only a few but their frequency increase closer to term

Question 3

What factors cause depolarisation of the muscle cell? What drugs act on them?

Answer 3

1. Voltage dependent calcium channels: allow an influx of calcium to initiate a contraction. Both calcium channel blockers and MgSO4 work on these receptors.
2. https://www.ogmagazine.org.au/16/2-16/uterotonics-tocolytics/

Question 4

How does progesterone cause quiescence?

Answer 4

The expression of VDCCs on the myometrial cell surface is controlled by progesterone. Indeed, progesterone has a number of important roles in reducing activity of the myometrial cells. Progesterone suppresses gene expression of connexins and VDCCs, it reduces synthesis of prostaglandin and up-regulates the nitric oxide (NO) system. Withdrawal of progesterone effects appears to be the major mechanism leading to the onset of labour.