Influence of the level of hydration on the renal response to a protein meal

Recent studies have suggested that the renal effects of high protein intake could be mediated, at least in part, by vasopressin and/or an increase in the urinary concentrating activity. The present study investigated the influence of the level of hydration, and hence of the activity of the concentrating process, on the renal response to an acute oral protein load. Clearance studies were performed before (Control) and during three hours after a protein meal (1.5 g/kg body wt protein as cooked meat) in ten healthy volunteers. This study was performed twice at a two to three week interval under either constant low (LowH) or high (HighH) hydration. In spite of the marked difference in initial diuresis (3.1 +/- 0.3 in LowH vs. 13.9 +/- 0.7 ml/min in HighH) and urine osmolality (501 +/- 42 in LowH vs. 99 +/- 3 mOsm/kg H2O in HighH), a similar relative decrease in urine flow rate was observed following the meal in both conditions. TcH2O increased progressively by 70% in LowH whereas CH2O decreased by 40% in HighH. Plasma vasopressin showed a progressive increase with time in LowH (from 1.10 +/- 0.26 in control, to 1.98 +/- 0.35 pg/ml at the third hour after the PM, P < 0.05) but not in HighH (0.53 +/- 0.09 to 0.70 +/- 0.17 pg/ml). Glomerular filtration rate (inulin clearance) increased significantly on the second post-prandial hour under LowH but not under HighH. Excretions rates of Na, Cl, K, and urea increased after the meal, however, not to the same extent nor with the same time course in the two conditions. Significant positive correlations were observed between GFR and TcH2O, urine osmolality, or the ratio of urine-to-plasma urea concentrations in LowH. These results suggest that the protein-induced hyperfiltration is partially blunted by a high water intake, and hence is dependent, directly or indirectly, on the urine concentrating activity.