1. The present study was designed to investigate the relationships between circulating atrial natriuretic peptide, plasma and urinary cyclic GMP and sodium excretion under basal conditions and in response to changes in dietary sodium intake.
2. Measurements of plasma atrial natriuretic peptide and plasma and urinary (24 h collections) cyclic GMP, sodium and creatinine were made in (i) 30 normotensive subjects on their normal sodium intake and (ii) 12 subjects on the 5th day of a low and on the 5th day of a high sodium intake.
3. Plasma cyclic GMP, urinary cyclic GMP and fractional excretion of cyclic GMP in 30 normotensive subjects on their normal sodium intake were (means ± SEM) 5.4 ± 0.5 pmol/ml, 434.5 ± 31.8 pmol/min and 86.9 ± 8.6%, respectively. There were significant correlations between urinary cyclic GMP and its corresponding filtered load (r = 0.55) and between the renal clearance of cyclic GMP and that of creatinine (r = 0.44), but there were no significant associations between circulating atrial natriuretic peptide and plasma cyclic GMP or the fractional excretion of cyclic GMP or between urinary sodium and the fractional excretion of cyclic GMP.
5. Plasma atrial natriuretic peptide was significantly raised on the 5th day of the high sodium intake compared with the low sodium intake (10.6 ± 1.6 versus 4.2 ± 0.9 pg/ml; P <0.05). Similarly, there were increases in urinary cyclic GMP excretion (692.3 ± 43.4 versus 427.4 ± 41.9 pmol/min, P <0.05), but there were no significant differences in the fractional excretion of cyclic GMP.
6. As neither plasma nor urinary cyclic GMP was strongly associated with circulating levels of atrial natriuretic peptide, the present study suggests that other factors may be more important than circulating atrial natriuretic peptide as determinants of extracellular cyclic GMP.
