Hemodynamic influence of altered distal salt delivery on glomerular microcirculation.

The current notion that tubuloglomerular feedback-mediated regulation of GFR involves selective changes in afferent arteriolar vasomotor tone stems from the familiar finding that manipulation of distal salt delivery alters glomerular capillary hydraulic pressure (PGC). Stop-flow technique, which has been used for this PGC measurement, however, has recently been shown to fail to provide a reliable estimate of true PGC. In the present study, glomerular dynamics were examined by direct glomerular puncture during loop perfusion with Ringer solution at rates of 0 and 40 nl/min. With the increased loop flow, single nephron GFR (SNGFR) decreased by approximately 35%. Because directly measured PGC remained unchanged, this reduction in SNGFR was due to the falls in glomerular plasma flow rate and ultrafiltration coefficient, the former a consequence of increases in both afferent and efferent arteriolar resistances. By contrast, PGC values estimated by the stop-flow technique declined substantially with increased loop flow. These results indicate that tubuloglomerular feedback-induced regulation of GFR is mediated by changes in the vasomotor tone of preglomerular, glomerular, and postglomerular vessels. In view of the close anatomical contact between mesangial cells and these vessels, mesangial contractility may play a role as the single effector mechanism regulating the vasomotor tone of these vessels.