1. Increased activity of the sympathetic nervous system has been implicated in the genesis of early hypertension in young people. Studies in pregnancy allow observations to be made on evolving, recently established and resolving hypertension in the human. We describe the results of two studies involving women who developed hypertension during pregnancy.

2. In the first study, plasma concentrations of noradrenaline and adrenaline were measured in 17 women with pregnancy induced hypertension (PIH) and 17 normotensive pregnant control subjects. Plasma noradrenaline (nmol/l) was lower in the PIH group compared with control patients in both semi-recumbent (1.11 ± 0.53 vs 1.98 ± 0.96, P < 0.001) and standing positions (1.31 ± 0.65 vs 2.57 ± 1.27, P < 0.005). Five days post partum, plasma noradrenaline had risen in the PIH group compared with pregnant values in semi-recumbent (1.65 ± 1.0 vs 1.11 ± 0.52, P < 0.05) and standing positions (2.46 ± 1.5 vs 1.31 ± 0.65, P < 0.05). In the normotensive patients plasma noradrenaline did not differ between post partum and pregnant values (1.51 ± 0.73 vs 1.98 ± 0.96 semi-recumbent; 2.00 ± 1.16 vs 2.57 ± 1.7 standing).

3. Logarithmic transformation of the noradrenaline concentration data resulted in a significant (P < 0.02) negative correlation with diastolic blood pressure in the pregnant patients but not post partum. Plasma adrenaline concentration was the same in both groups.

4. In the second study, plasma concentrations of noradrenaline and adrenaline were measured sequentially through pregnancy in five women who developed PIH and five control subjects who remained normotensive. There was no difference in catecholamine concentrations between groups at any stage, but noradrenaline concentration tended to fall as blood pressure rose in the PIH patients.

5. We conclude that the sympathetic nervous system does not contribute to the development of PIH but responds to the increased blood pressure with decreased activity.

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