Restoration and maintenance of glomerular filtration by mannitol during hypoperfusion of the kidney.

Glomerular filtration (GF) during progressive reduction of renal perfusion pressure by aortic clamping was studied in hydropenic rats and in rats infused with isotonic saline, hypertonic saline, or mannitol. As judged by visual observation of Lissamine green movements in superficial nephrons. GF was absent in hydropenic or saline-loaded rats at 40 mm Hg aortic pressure, but continued in some nephrons of all rats infused with mannitol and of some rats infused with hypertonic saline. Urine flow persisted only in rats infused with mannitol. By use of the qualitative Hanssen technique, it was found that all glomeruli in superficial and deep portions of the cortex were perfused at 40 mm Hg in all groups of rats. By the same method. GF continued in 1% of nephrons in hydropenic rats, 12% of nephrons in isotonic saline-loaded rats, and 78% of nephrons in rats infused with mannitol. By means of a quantitative Hanssen technique, GF was 5.8 nl/min per nephron in mannitol-infused rats and not measurable (< 0.5 nl) in hydropenic rats. Superficial and deep nephrons were similar in both qualitative and quantitative studies. Although urine flow did not persist in rats infused with hypertonic saline, GF was detected in four of seven studies by the Hanssen method (mean, 9.1 nl/min per nephron). In additional experiments, mannitol infused after perfusion pressure had already been lowered to 40 mm Hg in hydropenic rats reestablished urine flow and GF (mean, 9.8 nl/min). Furosemide, isotonic and hypertonic saline did not restart urine flow; however, GF (Lissamine green) was restarted by hypertonic saline. We conclude that mannitol can maintain or reestablish by an extratubular mechanism GF which otherwise would not occur during renal hypoperfusion. Hypertonic saline has a similar effect on GF in some cases, but urine flow is not maintained, implying complete reabsorption of filtrate.