1. The relationship between plasma immunoreactive atrial natriuretic factor (Ir-ANF) and the urinary excretion of sodium, guanosine 3′:5′-cyclic monophosphate (cyclic GMP) and of tissue kallikrein was examined in seven healthy female volunteers.
2. Each volunteer attended on two occasions, a control and a saline infusion day. On the infusion day saline (2 litres, 0.9% NaCl) was administered over 60 min. Measurements of plasma Ir-ANF and urinary excretion of sodium, cyclic GMP and of tissue kallikrein were made at 30 min intervals during the infusion and for 3 h after the infusion.
3. Mean (± sem) urinary sodium excretion increased from a basal value (time 0) of 102 ±15 μmol/min to 222 ± 47 μmol/min 60–90 min from the start of the infusion and thereafter remained significantly elevated (P < 0.01) above sodium excretion on the control day.
4. In response to saline infusion there was a transient rise in mean (± sem) plasma Ir-ANF from 6.7 ± 0.8 pmol/l to a peak of 22.5 ±3.7 pmol/l at 75 min, falling to 12.7 ± 1.9 pmol/l at 135 min. The peak plasma Ir-ANF level on the infusion day was significantly elevated (P < 0.05) above the time-matched measurement on the control day.
5. Similarly, there was a transient rise in mean (± sem) urinary cyclic GMP excretion on the infusion day from 30.9 ±4.4 fmol/min to 64.6 ± 11.4 fmol/min during the 60–90 min collection period, returning to 43.7 ± 14.5 fmol/min at 210–240 min. Cyclic GMP excretion on the infusion day was significantly elevated (P < 0.05) above the time-matched measurement on the control day during the period 60–180 min.
6. Serum and urinary tissue kallikrein on the infusion day did not differ significantly from time-matched measurements on the control day.
7. Thus urinary cycle GMP but not tissue kallikrein follows the plasma Ir-ANF response to acute volume expansion with saline, and may be a useful marker of the activity of this peptide.