The factors determining glomerular ultrafiltration were measured directly in Munich Wistar rats with glomeruli on the kidney surface. The hydrostatic pressure gradient (ΔP) across the glomerular capillary was measured directly with a servo nulling device, nephron filtration rates with micropuncture techniques and afferent and efferent protein concentration and oncotic pressures (π) by microprotein methods. In hydropenic group 1 animals, the effective filtration pressure (EFP=ΔP-π) attained filtration equilibrium by the end of the glomerular capillary. In hydropenic group 2 rats, the EFP remained positive at the end of the capillary. The total glomerular permeability (LpA) could therefore be calculated uniquely in group 2 rats (0.048 nl/sec/g of kidney wt/mm Hg) and was significantly lower than the minimum estimate for group 1 (0.060 nl/sec/g of kidney wt/mm Hg). In group 2, infusion of hyperoncotic albumin increased systemic oncotic pressure (πA); nephron plasma flow, 185%; and nephron filtration rate, 71%. Mean integrated effective filtration pressure remained constant as determined by 2 computer based mathematical models of filtration. LpA could be uniquely determined in both states and rose from 0.048 to 0.082 nl/sec/g of kidney wt/mm Hg. This suggests that filtration rate can be altered by changes in LpA, and that in group 2, the infusion of hyperoncotic albumin increased total glomerular permeability.
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