INCREASE IN PLASMA SODIUM ENHANCES NATRIURESIS IN RESPONSE TO A SODIUM LOAD UNABLE TO CHANGE PLASMA ATRIAL PEPTIDE CONCENTRATION

The influence of plasma sodium concentration in the control of sodium excretion was investigated in conscious, water-diuretic dogs. NaCl was infused for 60 min as a hypertonic or isotonic solution at a rate of 60 μmol NaCl min-1 kg-1 body wt. Plasma sodium concentration rose only during hypertonic infusion (P < 0.05). Sodium excretion increased markedly with both infusions (hypertonic, from 2.4 ± 0.6 to 105 ± 27 μmol min-a1; isotonic, from 3.9 ± 1.3 to 58 ± 17 μmol min-1). Fractional sodium excretion increased more during hypertonic than during isotonic infusion. Hypertonic infusion decreased diuresis from 3.1 ± 0.5 to 1.3 ± 0.6 ml min-1, while isotonic infusion elicited an increase from 3.9 ± 0.5 to 7.2 ± 0.7 ml min-1. Plasma renin activity and plasma aldosterone decreased markedly in both series (P < 0.05), the relative changes in the two series being very similar. Central venous pressure increased (2.8 ± 0.7 to 4.5 ± 1.0 mmHg) during isotonic infusion but not significantly during hypertonic infusion. Arterial pressure, heart rate and plasma levels of atrial natriuretic peptide and catecholamines did not change measurably in either series. It is concluded that simultaneous increases in extracellular volume and sodium concentration cause a larger natriuretic response than a change in volume alone, and that a 40-fold increase in sodium excretion may occur without measurable changes in plasma atrial natriuretic peptide concentration.