Pharmacology of parturition

Uterine contractions in labour are influenced by endogenous substances such as oestrogens, progesterone, cortisol, oxytocin, prostaglandins, relaxin, adrenergic and cholinergic secretions, cyclic nucleotides and calcium ions. Effects of progesterone and oestrogens are complimentary as well as antagonistic to each other. They regulate formation of gap junctions, influx of calcium ions, synthesis of oxytocin, adrenergic receptors and of prostaglandins and cyclic nucleotides. Cortisol shares a role in a more complex endocrine trigger but is ineffective alone in the initiation of human labour. Adrenaline inhibits and noradrenaline promotes uterine contractions. Cholinergic stimulation increases cyclic GMP promoting uterine contractions. Calcium ions play a key role in uterine contractility. Oxytocin, prostaglandins E and F are powerful stimulants of uterine contractions. Prostaglandins stimulate pregnant uterus from early gestation unlike oxytocin which has little effect in the first and second trimester. They are extensively used for initiating labour and to arrest intractable atonic postpartum haemorrhage. In experiments and in vivo, their effects are modulated by other hormones and substances. With discovery of new drugs, knowledge of how they act on the uterus becomes important. The pharmacology of parturition that may help to understand the interaction of various agents on the pregnant uterus has been discussed.