Immunoreactive Substance P in Human Plasma: Response to Changes in Posture and Sodium Balance

Abstract

1. In healthy volunteers plasma concentrations of immunoreactive substance P were measured in response to changes in posture and dietary salt intake. 2. In 14 subjects plasma immunoreactive substance P was 168 ± 31 pmol/l when subjects were supine and 401 ± 51 pmol/l (P < 0.001) when they were ambulant. 3. Measurement of supine plasma immunoreactive substance P at 6 h intervals gave a mean value of 240 ± 39 pmol/l at 14.00 hours and a lowest value of 76 ± 9 pmol/l at 02.00 hours. 4. In eight healthy subjects plasma immunoreactive substance P rose only slightly from 169 ± 41 pmol/l, on a sodium intake ad lib., to 244 ± 45 pmol/l by day 4 of dietary sodium restriction (35 mmol/day) and significantly fell to 51 ± 20 pmol/l (P < 0.001) by day 4 of high sodium intake (350 mmol/day). 5. Although exogenous substance P was shown to be natriuretic in dog and rat, the present results do not favour a role of endogenous substance P as a circulating natriuretic factor in man.