Quantitative Importance of Changes in Postglomerular Colloid Osmotic Pressure in Mediating Glomerulotubular Balance in the Rat

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Abstract

In recent studies in this laboratory employing normal hydropenic rats we have demonstrated that the reduction in absolute proximal reabsorption that attends the experimental reduction of single nephron glomerular filtration rate (SNGFR) (glomerulotubular balance) is mediated, at least in part, by the accompanying decline in postglomerular vascular protein concentration, and therefore, postglomerular colloid osmotic pressure (πEA). The present study was undertaken to define the quantitative contribution of these changes in πEA to the changes in absolute proximal reabsorption measured under these conditions. A protocol was employed which enabled us to examine the effects on absolute proximal reabsorption of reductions in filtered load brought about under conditions in which πEA remained essentially unchanged. Thus, after partial aortic constriction in 16 plasma-loaded rats, near constancy of πEA was observed in 10 (a change in efferent arteriolar protein concentration of 0.4 g/100 ml or less) and in these, uniform reductions in SNGFR averaging 16.7 nl/min were attended by reductions in absolute proximal reabsorption averaging only 1.7 nl/min, or 7% of preconstriction values. These findings, taken together with previous observations from this laboratory, suggest that the proximal reabsorptive adjustment that characterizes glomerulotubular balance in the rat is markedly blunted when changes in πEA are prevented. In the remaining six rats, a mean reduction in filtered load comparable to that observed in the above group was attended by slightly to moderately greater reductions in efferent arteriolar protein concentration, thereby fulfilling less well the stated aim of this study. Nevertheless, in accord with the above conclusion, these relatively greater reductions in πEA were accompanied by correspondingly greater reductions in absolute proximal reabsorption.

Authors

Barry M. Brenner, Julia L. Troy, Terrance M. Daugharty, Robert M. MacInnes