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, despite 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 restriction. 2. Extremely low-dose infusions of arginine vasopressin were given to normal subjects after equilibration to high (260mmol/day) and low (20mmol/day) sodium intakes. 3. Salt restriction increased the antidiuretic effect of arginine vasopressin (2 fmol min -1 kg -1 arginine vasopressin increased urine osmolality from 67.8±2.6 to 196.3±35.7 mosmol/l in the high-salt study and from 268.3±49 mosmol/l in the low-salt study ; P<0.05 between sodium intakes). Glomerular filtration rate, estimated from inulin clearance, was unchanged during arginine vasopressin infusion irrespective of salt intake (high salt 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-aminohippurate clearance, fell further during infusion of 2 fmol min -1 kg -1 arginine vasopressin in the low-salt study than in the high-salt study (low salt, from 555.7±22.7 to 298.3± 27.6ml/min ; high salt, from 544.5±30.2 to 452.9± 28.9 ml/min ; P<0.05 between sodium intakes). 4. Plasma atrial natriuretic peptide concentration increased during infusion of 2 fmol min -1 kg -1 arginine vasopressin in the low-salt study (to 136.5%±19.9% of baseline, P<0.05), if anything falling in the high-salt study (to 90.5%±13.6% of baseline). Packed cell volume fell during arginine vasopressin infusion in the low-salt study (high salt, to 98.2%±0.4% of baseline ; low salt, to 95.7%± 0.4% of baseline, P< 0.05 for the low-salt study only). 5. Plasma 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 sensitivity to the hydro-osmotic and vascular effects of arginine vasopressin is enhanced by salt restriction.