Effect of mannitol on glomerular ultrafiltration in the hydropenic rat.

The effect of mannitol upon glomerular ultrafiltration was examined in hydropenic Munich-Wistar rats. Superficial nephron filtration rate (sngfr) rose from 32.0+/-0.9 nl/min/g kidney wt to 42.0+/-1.6 (P < 0.001) in eight rats. Hydrostatic pressure gradients acting across the glomerular capillary (DeltaP) were measured in glomerular capillaries and Bowman's space with a servo-nulling device, systemic (piA) and efferent arteriolar oncotic pressures (piE) were determined by microprotein analysis. These data were applied to a computer-based mathematical model of glomerular ultrafiltration to determine the profile of effective filtration pressure (EFP = DeltaP - pi) and total glomerular permeability (L(p)A) in both states. Filtration equilibrium obtained in hydropenia (L(p)A >/= 0.099+/-0.006 nl/s/g kidney wt/mm Hg) and sngfr rose because EFP increased from a maximum value of 4.2+/-1.1 to 12.8+/-0.5 mm Hg after mannitol (P <0.01). This increase was due to both increased nephron plasma flow and decreased piA. Computer analysis of these data revealed that more than half (>58%) of this increase was due to decreased piA, consequent to dilution of protein. Since EFP was disequilibrated after mannitol, L(p)A could be calculated accurately (0.065 +/- 0.003 nl/s/g kidney wt/mm Hg) and was significantly lower than the minimum estimate in hydropenia.Therefore, sngfr does increase with mannitol and this increase is not wholly dependent upon an increase in nephron plasma flow since the major factor increasing EFP was decreased piA.