Relationships between blood pressure, age, plasma renin activity and electrolyte excretion in normotensive subjects

Plasma renin activity (PRA), supine and sitting blood pressure (BP) and urinary sodium and potassium excretion were studied in 120 normotensive subjects aged 22-78 years, during strictly standardized conditions. PRA was measured after 1 h supine rest (basal PRA) after ambulation for 3-4 h (upright PRA) and after stimulation with 80 mg frusemide orally (stimulated PRA). PRA was determined with a new, simple, accurate and sensitive radioimmunoassay method. Supine and sitting systolic BP increased with age but no such correlation was found for the diastolic BP. No correlation could be shown between the BP levels and either the sodium or the potassium excretions. PRA levels increased about two-fold from basal to upright levels and about four-fold from basal levels after stimulation. No differences in mean PRA-levels were seen between males and females. We could not demonstrate any correlations between PRA-levels and 24 h sodium or potassium excretions, nor was there any relationship between age and PRA. Reference ranges are given for basal, upright and stimulated PRA in normotensive subjects. From these reference values a rational and clinically useful subdivision into low, normal and high renin groups can be made for hypertensive patients.