Renal Response to Chronic Intravenous Salt Loading in the Rat

The natriuresis of acute Ringer's loading is associated with a rise in the rate of delivery of fluid beyond the proximal tubule due both to a rise in glomerular filtration and a fall in absolute reabsorption, the latter being causally mediated, at least in part, by the accompanying fall in postglomerular vascular [protein]. To determine whether these factors also contribute to the renal response to chronic Ringer's loading, nine rats given continuous infusions, 30% body weight/day over 5-14 days, were studied using free-flow micro-puncture techniques. Results were compared with data from 10 chronic control rats given less than 1.5% body wt/day. Late proximal tubule fluid-to-plasma [inulin] ratios, (TF/P)(IN), single nephron glomerular filtration rate (SNGFR), absolute proximal reabsorption, and postglomerular vascular [protein] in chronic control rats and chronically loaded rats averaged 2.2+/-SE 0.1 (n = 35) and 1.5+/-0 (35), P<0.001; 37+/-2 (35) and 47+/-4 nl/min (35), P<0.05; 19+/-1 (35) and 16+/-2 nl/min (35), P>0.2; and 9.5+/-0.3 (8) and 8.6+/-0.3 g/100 ml (8), P>0.05, respectively. Thus the fall in (TF/P)(IN) and the rise in distal delivery during chronic Ringer's loading were due almost entirely to the rise in SNGFR, and not to any large fall in absolute reabsorption. Hence chronic and acute Ringer's loading increase delivery of proximal tubule fluid by different mechanisms, with chronic sodium homeostasis being governed overwhelmingly by adjustments in GFR. When, however, an acute Ringer's load was infused into chronically loaded rats, we observed significant and parallel reductions in absolute proximal reabsorption and postglomerular vascular [protein]. These findings suggest that the difference between the effects of chronic vs. acute Ringer's loading on absolute proximal reabsorption may have been due, at least in part, to the corresponding difference in the effects these two loading procedures have on postglomerular vascular [protein].