There is some experimental evidence that viable tubular epithelial cells released into the lumen during ischemia may adhere to other tubular cells and ECM and thereby cause intraluminal obstruction. This cellular adhesion during acute renal ischemia has been shown to involve integrin-mediated adhesion molecules via binding to Arg-Gly-Asp (RGD) sequences (47). In support of this possibility, synthetic cyclical RGD compounds administered during the reperfusion period have been shown to attenuate tubular obstruction and to reverse the related increase in proximal tubular pressure (48). Relief of tubular obstruction in experimental ARF, as assessed by nephron micropuncture, has also been shown with a solute diuresis induced by mannitol (49).
Tubuloglomerular balance and tubular fluid backleak in ischemic ARF. The decrease in proximal tubular sodium reabsorption that is associated with acute ischemic injury would increase sodium chloride delivery to the macula densa and thereby activate the tubuloglomerular feedback mechanism and decrease GFR (50). Micropuncture perfusion studies delivering increased sodium chloride to the macula densa have demonstrated a decrease in single-nephron GFR by as much as 50% (50). This degree of decline in GFR, however, could not explain the much greater decrease in GFR that is characteristic of clinical ARF. However, since the tone of the afferent arteriole modulates the tubuloglomerular feedback mechanism, the increased sensitivity of this glomerular arteriole to vasoconstriction, as discussed earlier (18), could enhance the sensitivity of the tubuloglomerular feedback response in patients with clinical ARF. Moreover, the combination of tubular cast formation and activation of the tubuloglomerular feedback mechanism during acute renal ischemia, both of which can be linked to the ARF-related decrease in proximal tubular sodium reabsorption, can provide an adequate explanation for the drastic fall in GFR observed in clinical ARF. With respect to the role of tubuloglomerular feedback in ischemic ARF, a potential beneficial effect should also be considered. Activation of the tubuloglomerular feedback mechanism and the resultant decrease in GFR during acute renal ischemia would decrease sodium chloride delivery to damaged tubules, thereby lessening the demand for ATP-dependent tubular reabsorption.
The loss of the tubular epithelial cell barrier and/or the tight junctions between viable cells (51) during acute renal ischemia could lead to a leak of glomerular filtrate back into the circulation. If this occurs and normally non-reabsorbable substances, such as inulin, leak back into the circulation, then a falsely low GFR will be measured as inulin clearance. It should be noted, however, that the degree of extensive tubular damage observed in experimental studies demonstrating tubular fluid backleak is rarely observed with clinical ARF in humans (52). Moreover, dextran sieving studies in patients with ARF demonstrated that, at best, only a 10% decrease in GFR could be explained by backleak of filtrate (53). Cadaveric transplanted kidneys with delayed graft function, however, may have severe tubular necrosis, and thus backleak of glomerular filtration may be more important in this setting. The various potential mechanisms whereby alterations in tubular factors can decrease GFR in ischemic renal injury are shown in Figure 9.