Analysis of the pressure‐flow characteristics of isolated perfused rat kidneys with inhibited tubular reabsorption

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Abstract
The renal hemodynamics were studied in an isolated perfused rat kidney model modified for investigations of the glomerular permeability characteristics. The tubular reabsorptive activity was inhibited by perfusion at low temperature (8 degrees C) in the presence of furosemide and nitroprusside resulting in a dramatic increase in the filtered load of fluid and solute reaching the tubules and hence in tubular pressure. The glomerular filtration rate (GFR), arterial pressure (PA) and needle pressure (intrarenal tissue pressure, PiR) were continuously recorded and the glomerular hydrostatic pressure was estimated by an arterial occlusion technique. The pre- to postglomerular resistance ratio was calculated from the pressure vs. GFR relationships for two perfusates having differing oncotic pressures (pi = 5.5 and pi = 20 mmHg), from which estimations of glomerular hydrostatic pressures (PGC) were concomitantly made. Thus, increases in delta pi could be exactly counterbalanced by equally large increases in PGC for any given GFR, the needle and Bowman's capsule pressures being dependent on GFR but not on plasma colloid oncotic pressure. The experimental interventions resulted in a pronounced elevation of PiR as compared with in vivo conditions, while the PGC values were in a normal range, resulting in reduced glomerular filtration pressures. Furthermore, the clearance of albumin varied with the oncotic pressure in agreement with the notion of heteroporosity.