Kidney function during decreased perfusion pressure due to aortic clamping and hemorrhagic hypotension: a single nephron study in dog kidney.

Whole kidney and single nephron function was compared in dogs during a stepwise lowering of renal perfusion pressure (PP) induced by aortic clamping (C) or hemorrhagic hypotension (HH). In C, renal vascular resistance (RVR) decreased with decreasing PP, the decrease being due to a drop in afferent resistance (RA) only. HH was associated with a rise in RVR due in the first phase mainly to an increase in RE, later to both RA and RE. There was a decrease in total kidney blood flow (RBF), glomerular filtration rate (GFR), glomerular and peritubular capillary pressure, and proximal tubular pressure. In C, RBF and GFR showed no significant difference at PPs 17 and 13 kPa. However, single nephron pressures were slightly but significantly lowered. In both groups, glomerular capillary hydraulic pressure (GCP) was significantly higher when calculated as SFP + piA (stop flow pressure + systemic arterial plasma oncotic pressure) than when measured directly by puncturing glomerular capillaries after ablation of a small layer of superficial cortex. This difference disappeared at PP 8 kPa. Ultrafiltration coefficient (Kf) seemed to increase with decreasing PP.