Hemodilution mediates changes in renal hemodynamics after acute volume expansion in rats

The present study examined the factors responsible for triggering renal hemodynamic adjustments during acute volume expansion. The renal hemodynamic effects of graded volume expansion with 0.9% saline (Sal; 1, 2, and 4% of body wt), 7% BSA solution (0.35, 0.70, and 1.4% body wt), or whole blood from a donor rat (WBL; 0.35, 0.70, and 1.4% body wt) were compared in rats anesthetized with pentobarbital sodium. Neural influences on the kidney were eliminated by vagus nerves, baro/chemoreceptor afferents, and renal nerves section, and renal perfusion pressure was controlled at constant level (approximately 120 mmHg) throughout the experiments. In Sal- and BSA-expanded rats, renal blood flow (RBF) increased (Sal: 15, 40, 71%; BSA 17, 49, 107%) and renal vascular resistance (RVR) decreased in parallel with the degree of volume expansion (RVR: Sal 17, 31, 44%; and BSA: 15, 35, 54%). Renal hemodynamics remained unaltered after expansion with WBL. In rats expanded with Sal or BSA, correction of the fall of hematocrit restored RBF and RVR to control levels. Interference with tubuloglomerular feedback by uretheral obstruction had no effect on the decrease in RVR with Sal or BSA. Inhibition of the vascular tone by intrarenal papaverine infusion also did not alter the renal hemodynamic response to volume expansion with Sal or BSA. These findings suggest that the changes in renal hemodynamics after acute expansion are likely mediated by changes in rheologic properties of the blood rather than by changes in active vascular tone.