EFFECT OF DIETARY SALT RESTRICTION ON RENAL SENSITIVITY TO VASOPRESSIN IN MAN

1. We have previously described a progressive antidiuresis in response to low-dose vasopressin infusion during salt restriction in man, desp ite stable or even declining plasma vasopressin concentration. In the present study we examine the hypothesis that renal sensitivity to the antidiuretic effect of arginine vasopressin may be enhanced by salt re striction, 2. Extremely low-dose infusions of arginine vasopressin wer e given to normal subjects after equilibration to high (260 mmol/day) and low (20 mmol/day) sodium intakes, 3. Salt restriction increased th e antidiuretic effect of arginine vasopressin (2 fmol min(-1) kg(-1) a rginine vasopressin increased urine osmolality from 67.8 +/- 2.6 to 19 6.3 +/- 35.7 mosmol/l in the high-salt study and from 268.3 +/- 49 mos mol/l in the low-salt study; P < 0.05 between sodium intakes), Glomeru lar filtration rate, estimated from inulin clearance, was unchanged du ring arginine vasopressin infusion irrespective of salt intake (high s alt 116.5 +/- 9.4 to 118.9 +/- 6.4 ml/min; low salt, 135.1 +/- 9.2 to 111.2 +/- 12.4 ml/min). Renal plasma flow, estimated from para-aminohi ppurate clearance, fell further during infusion of 2 fmol min(-1) kg(- 1) arginine vasopressin in the low-salt study than in the high-salt st udy (low salt, from 555.7 +/- 22.7 to 298.3 +/- 27.6 ml/min; high salt , from 544.5 +/- 30.2 to 452.9 +/- 28.9 ml/min; P < 0.05 between sodiu m intakes), 4. Plasma atrial natriuretic peptide concentration increas ed during infusion of 2 fmol min(-1) kg(-1) arginine vasopressin in th e low-salt study (to 136.5% +/- 19.9% of baseline, P < 0.05), if anyth ing falling in the high-salt study (to 90.5% +/- 13.6% of baseline), P acked cell volume fell during arginine vasopressin infusion in the low -salt study (high salt, to 98.2% +/- 0.4%, of baseline; low salt, to 9 5.7% +/- 0.4% of baseline, P < 0.05 for the low-salt study only), 5. P lasma arginine vasopressin concentration was constant throughout each study (high salt 0.48 +/- 0.12 to 0.48 +/- 0.1 pmol/l; low salt, 0.38 +/- 0.05 to 0.4 +/- 0.04 pmol/l), 6. Our findings suggest that renal s ensitivity to the hydro-osmotic and vascular effects of arginine vasop ressin is enhanced by salt restriction.