Effect of volume expansion on papillary blood flow and sodium excretion

The present study examined whether changes in plasma oncotic pressure or hematocrit play a role in the redistribution of renal blood flow and the natriuretic response to extracellular fluid volume (ECFV) expansion with saline. Intravenous infusion of saline produced a 46% increase in the flow of red blood cells (RBCs) in the papilla of Inactin-anesthetized euvolemic Munich-Wistar rats (n = 6). This was primarily due to an increase in the number of functional capillaries perfused with moving RBCs, as indicated both by laser-Doppler flowmetry and videomicroscopy. The velocity of RBCs in ascending or descending vasa recta was not significantly altered by the infusion of saline. Plasma volume expansion with a 6% solution of albumin (n = 6) did not increase papillary RBC flow, whereas volume expansion with whole blood produced a 17% increase in the flow of RBCs in the papilla. Sodium excretion after ECFV expansion with saline (n = 6) was greater than that seen after plasma volume expansion with a 6% solution of albumin (n = 5). The results indicate that the rise in papillary RBC flow after ECFV expansion with saline is due to an increase in the number of perfused vasa recta capillaries. The failure of plasma volume expansion to alter papillary RBC flow suggests that changes in plasma oncotic pressure and/or renal interstitial pressure may signal the rise in papillary RBC flow after intravenous infusion of saline. The present study also indicates that laser-Doppler flowmetry is a useful technique to monitor changes in the flow, velocity, and concentration of moving RBCs in tissue.