PERFUSION-PRESSURE DEPENDENCY OF IN-VIVO RENAL TUBULAR DYNAMICS

To examine whether changes in renal perfusion pressure (RPP) within th e range of autoregulation induce detectable changes in tubular dynamic s in an entire nephron population of the intact kidney, we measured, u sing electron beam computed tomography (EBCT), transit times (TT, s) a nd intratubular concentration (%) of filterable contrast media in vari ous nephron segments simultaneously with renal regional perfusion. In seven dogs (group A) this was performed at the upper and lower limits of autoregulation (RPP = 130 and 95 mmHg, respectively) while group B (n = 5) served as control. In group A alone, a decrease in RPP led to an increase in TT by 40%, 68%, and 32% in the proximal tubules, ascend ing limb of Henle's loop, and distal tubules, respectively, in associa tion with an increase in intratubular concentration (+50%, 80%, and 42 %, respectively). Papillary perfusion decreased, whereas perfusion of the adjacent, outlying inner medulla increased. The decrease in papill ary perfusion correlated positively with the concurrent change in sodi um excretion (R = 0.81). This study demonstrates that changes in RPP w ithin the autoregulatory range elicit changes of tubular sodium reabso rption mainly in proximal, distal, and ascending tubules, in which mos t of the nephrons participate. These tubular changes are associated wi th an alteration of perfusion circumscribed to two areas of the inner renal medulla.