Studies on the autonomic nervous system in borderline hypertension

Parameters of the autonomic nervous system were studied in normotensive subjects (NT; standing blood pressure (BP) less than or equal to 125/85 mmHg) and in subjects with borderline hypertension (BHT; 140/90 less than or equal to standing BP less than 160/100 mmHg). No differences in plasma noradrenaline and adrenaline levels were found between NT and BHT subjects, neither at rest nor during exercise at 75% of maximum work capacity. The dose of noradrenaline required to increase systolic BP by 10 mmHg was significantly higher in NT than in BHT subjects (5.13 +/- 0.42 vs 3.50 +/- 0.57 microgram . min -1). No difference between NT and BHT subjects was found in the dose of isoprenaline required to increase heart rate by 20 beats . min -1 (1.21 +/- 0.12 vs 1.09 +/- 0.11 microgram . min -1), suggesting decreased parasympathetic activity in the former group. The enhanced pressor effect of noradrenaline, together with the decreased parasympathetic activity, could explain the elevated blood pressure and heart rate in subjects with borderline hypertension.