Autoregulation of intravascular pressure in preglomerular juxtamedullary vessels.

To quantify the functional significance of autoregulatory responses in preglomerular juxtamedullary (JM) vessels, intravascular pressures (P chi) were measured with a servonull instrument in blood-perfused arcuate (ArcA) and interlobular arteries (ILA), afferent arterioles (AA), and glomerular capillaries (GC) in vitro. P chi was determined at perfusion pressures (Pp) between approximately 60 and 150 mmHg, and the slope of the relationship between Px and Pp was estimated by linear regression. From the regression, Px was 98 +/- 1, 96 +/- 2, 64 +/- 9, and 48 +/- 2 mmHg for ArcA, ILA, AA, and GC, respectively, at the reference perfusion pressure of 100 mmHg. The results show good autoregulation of GC pressure (Pg), with a response slope of 0.10 +/- 0.07 mmHg per mmHg change in Pp, corresponding to an autoregulation index of 0.20 +/- 0.15. The slopes of the Px vs. Pp relationships in ArcA, ILA, and AA were 0.96 +/- 0.02, 0.79 +/- 0.08, and 0.32 +/- 0.11, respectively. To determine whether these observed relationships in preglomerular vessels reflect significant upstream resistance changes, we derived a new autoregulation index for Px measured at an arbitrary preglomerular location. This analysis takes into account the extent to which outflow pressure, Pg, is autoregulated by the ensemble action of the entire preglomerular vasculature. The analysis indicates that 20% autoregulatory compensation occurs upstream from the late ILA, 65% upstream from the late AA, and 80% for the entire preglomerular vascular tree. Hence, in the JM preglomerular circulation, the AA is the major site of autoregulatory resistances adjustment, with a smaller but significant contribution by the ILA.