Response to β-blockers in maternal and fetal rat hearts in vitro

The effects of β-blockers on maternal and fetal heart rates have been assessed by comparing isoprenaline concentration-heart rate relationships of hearts isolated from pregnant rats. The normal and maximal heart rates obtained for the maternal and fetal hearts were similar to published data. A slightly but significantly higher concentration of isoprenaline was required to produce 50% of the maximal response of fetal hearts than maternal hearts, suggesting that fetal hearts were less sensitive to isoprenaline than the maternal hearts. The β-blockers used (propranolol, labetalol, metoprolol and atenolol) all showed a lower affinity to the β-receptors of fetal hearts than those of maternal hearts, as indicated by significant differences in the pA2 values. Given the similar effects of the β-blockers in the maternal and fetal hearts it is concluded that pharmacokinetic considerations and β-blocker selectivity should be used as the basis of choice when treating maternal hypertension during pregnancy.