Identification of a renin threshold and its relationship to salt intake in a patient with pure autonomic failure.

Animal studies have demonstrated a threshold below which renin release increases proportionally to a decrease in renal perfusion pressure. Demonstration of a similar mechanism in humans, however, has proved difficult, as any attempt to lower blood pressure below the putative renin threshold results in renin release mediated by reflex activation of the sympathetic nervous system. In this study, we report on our observations in a 71-year-old woman who presented with a 20-year history of faintness and syncope and was diagnosed as having pure autonomic failure. Graded head-up tilting resulted in a stepwise reduction in mean arterial blood pressure to a minimum of 54 mm Hg, with no signs of increased sympathetic activity. A fall in blood pressure below 80 mm Hg resulted in a distinct rise in plasma renin activity, and a similar threshold pressure was observed under both a 50- and a 100-mmol/d sodium chloride diet. Below the threshold, response to changes in perfusion pressure was proportionally greater under the 50-mmol/d diet than under a 100- or 200-mmol/d diet. These observations demonstrate that a pressure threshold for renin release at 10 to 15 mm Hg below ambient blood pressure, as described previously in animal studies, is also present in humans. The significance of this pressure-dependent mechanism of renin release for the long-term regulation of blood pressure and water and mineral balance in humans remains to be determined.