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

The cell-free isolated perfused kidney (IPK) is characterized by norma l glomerular filtration rate (GFR) and very low filtration fraction (F F). Addition of erythrocytes to the perfusate (IEPK) increases FF whil e maintaining ''normal'' GFR levels. Micropuncture studies were perfor med in IPK and IEPK to establish the determinants of the glomerular ul trafiltration process responsible for low FF in IPK and to evaluate th e impact of the addition of erythrocytes on these determinants. Nephro n filtration rate was similar in IPK and IEPK (40 +/- 4 vs. 39 +/- 4 n l/min), whereas nephron perfusate flow was significantly higher in IPK (1,247 +/- 100 vs. 112 +/- 13 nl/min), leading to a superficial nephr on FF of 3.4 +/- 0.2% in IPK and 36 +/- 2% in IEPK. Glomerular hydrost atic pressure (PG) and transcapillary hydrostatic pressure gradient (D elta P) were 53 +/- 2 and 33 +/- 1 mmHg, respectively, in IPK and 51 /- 3 and 34 +/- 2 mmHg in IEPK, all normal values. Glomerular arteriol ar resistances were significantly lower in IPK than in IEPK, and the g lomerular 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 bo th values are within the normal in vivo range. These results demonstra te 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 an d Delta P values. Addition of erythrocytes increases glomerular arteri olar resistances and restores glomerular hemodynamics to a pattern nea rly identical to in vivo conditions. These data also suggest that IEPK constitutes an excellent tool to study glomerular function in vitro.