Dynamics of glomerular ultrafiltration. IX. Effects of plasma protein concentration

Experiments were carried out on 42 Munich Wistar rats with surface glomeruli accessible to micropuncture, to investigate the effects of reduction in systemic plasma protein concentration, C(A), and thus afferent oncotic pressure, II(A), on the determinants of glomerular ultrafiltration. In animals in which C(A) was reduced by a variety of maneuvers, observed values for single nephron GFR were lower than values predicted by the Starling relation, when the latter were calculated assuming that the observed increase in the net driving pressure for ultrafiltration (due to the reduction in II(A) was the only factor perturbed. In all experimental conditions where C(A) was reduced, rats were invariably observed to be at filtration pressure disequilibrium, permitting calculation of unique values of the ultrafiltration coefficient, K(f). In all low C(A) groups, mean values of K(f) were uniformly lower than values obtained in normoproteinemic control animals. The failure of SNGFR to rise to predicted values when C(A) is reduced is therefore due to the concomitant reduction in K(f). No morphological basis for this reduction in K(f) was discerned.