GLOMERULAR HEMODYNAMICS IN CELL-FREE AND ERYTHROCYTE-PERFUSED ISOLATED RAT-KIDNEY

The cell-free isolated perfused kidney (IPK) is characterized by normal glomerular filtration rate (GFR) and very low filtration fraction (FF). Addition of erythrocytes to the perfusate (IEPK) increases FF while maintaining ''normal'' GFR levels. Micropuncture studies were performed in IPK and IEPK to establish the determinants of the glomerular ultrafiltration process responsible for low FF in IPK and to evaluate the impact of the addition of erythrocytes on these determinants. Nephron filtration rate was similar in IPK and IEPK (40 +/- 4 vs. 39 +/- 4 nl/min), whereas nephron perfusate flow was significantly higher in IPK (1,247 +/- 100 vs. 112 +/- 13 nl/min), leading to a superficial nephron FF of 3.4 +/- 0.2% in IPK and 36 +/- 2% in IEPK. Glomerular hydrostatic pressure (PG) and transcapillary hydrostatic pressure gradient (Delta P) were 53 +/- 2 and 33 +/- 1 mmHg, respectively, in IPK and 51 +/- 3 and 34 +/- 2 mmHg in IEPK, all normal values. Glomerular arteriolar resistances were significantly lower in IPK than in IEPK, and the glomerular ultrafiltration coefficient (L(p)A) was significantly lower in IPK (0.053 +/- 0.010 vs. 0.100 +/- 0.020 nl.s(-1).mmHg(-1)), but both values are within the normal in vivo range. These results demonstrate that low FF in IPK is not due to decreased Delta P or L(p)A values but to the high renal perfusion rate required to maintain normal Po and Delta P values. Addition of erythrocytes increases glomerular arteriolar resistances and restores glomerular hemodynamics to a pattern nearly identical to in vivo conditions. These data also suggest that IEPK constitutes an excellent tool to study glomerular function in vitro.