ACE inhibitors and AT1 receptor antagonists—beyond the haemodynamic effect

The diversity of its causes, the unpredictability of its clinical course, and our expanding knowledge of the conditions that may exacerbate or retard its progression suggest that glomerular sclerosis cannot be attributed to a single aberration in glomerular physiology. Nonetheless, the welter of clinical and experimental observations is beginning to yield a pattern. Agents or conditions injurious to glomerular epithelium tend to cause glomerular sclerosis. Agents or conditions that induce short-term or long-term activation of mesangial cells may lead to glomerular sclerosis. Indeed, one contribution of the healthy epithelium may be to serve as a tonic inhibitor of the intraglomerular processes arising from mesangial-cell activation. Long-term activation of the mesangium is associated with the proliferation and infiltration of cells and with the expansion of the mesangial matrix--the antecedents of sclerosis. We anticipate that different diseases associated with glomerular sclerosis will be found to depend to varying extents on these two potential mechanisms of sclerosis. Beyond a certain threshold of glomerular injury, glomerular diseases share an additional factor: the capacity of both intrinsic cells and infiltrating cells to alter the microenvironment of the glomerulus so that sclerosis progresses inexorably long after the disappearance of the initiating insult. Several potential risk factors may contribute to the progression of chronic renal disease. These factors include systemic hypertension, proteinuria, hyperlipidemia, high protein intake, and probably conditions that lead to glomerular hypertrophy. Interventions designed to minimize the potential contribution of these factors to the progression of renal insufficiency may halt or slow the loss of function of the kidney. Clinical trials designed to examine the effects of these factors on the progressive course of renal insufficiency will help to establish their role and relative importance in humans.

Unilateral ureteral obstruction (UUO) results in tubulointerstitial fibrosis of the obstructed kidney (OBK). In this study we report that a specific angiotensin II (Ang II) receptor antagonists, SC-51316, ameliorates the expansion of the renal cortical interstitium in the OBK of the rat at five days of UUO. This is similar to the effect of an angiotensin converting enzyme (ACE) inhibitor, enalapril. SC-51316 (20 mg/liter in the drinking water) or enalapril (200 mg/liter in the drinking water) was administered beginning 24 hours before UUO and continued through five days after UUO. The relative volume of the tubulointerstitium (Vv) was measured by a point-counting method, and monocyte/macrophage infiltration, alpha smooth muscle actin (alpha SMA), proliferating cell nuclear antigen (PCNA), and collagen type IV (collagen IV) protein deposition were examined histologically using specific antibodies. We also examined the mRNA levels of transforming growth factor beta 1 (TGF-beta 1) and collagen IV by reverse transcription polymerase chain reaction. In untreated rats with UUO, Vv was remarkably expanded; collagen IV and alpha SMA protein deposition in the interstitium and PCNA labeling of nuclei were increased. These changes were significantly ameliorated by administration of an ACE inhibitor or an Ang II receptor antagonist. A monocyte/macrophage infiltration was evident in the OBK of untreated or Ang II receptor antagonist treated rats but was greatly reduced in the OBK of rats given enalapril. Increased expression of TGF-beta 1 mRNA and collagen IV mRNA was blunted (40 to 75%) by the administration of Ang II receptor antagonist or enalapril. The Ang II receptor antagonist or the ACE inhibitor did not affect the contralateral kidney of rats with UUO or the control kidney of normal rats. This study indicates that the renin-angiotensin system has a major role in the pathogenesis of the tubulointerstitial fibrosis of obstructive nephropathy. The tubulointerstitial fibrosis of obstructive nephropathy is most likely mediated by an increased level of Ang II in renal tissue.

The renin-angiotensin system seems to play an important role in the pathogenesis of renal interstitial fibrosis. However, the potential direct effects of angiotensin II (Ang II) on cultured renal fibroblasts have been little studied. We have observed that rat renal interstitial fibroblasts (NRK 49F cell line) possess AT1 receptors coupled to intracellular calcium mobilization. Exposure of these cells to Ang II induced several short and long growth-related metabolic events mediated by the AT1 receptor, including c-fos gene expression, changes in cell cycle and cell proliferation. Activation of interstitial fibroblasts by Ang II could also contribute to extracellular matrix accumulation. Stimulation with Ang II increased mRNA expression of TGF-beta 1, fibronectin and type I collagen. In fact, Ang II enhanced fibronectin production via AT1 receptors by a process depending on autocrine TGF-beta secretion. The mechanism of some Ang II actions (calcium mobilization and fibronectin production) depended on protein kinase C and tyrosine kinase activation. We further investigated whether renal fibroblasts could express some components of the renin-angiotensin system. These cells constitutively expressed the angiotensinogen gene that was up-regulated by Ang II. Collectively, these results indicate that in renal interstitial fibroblasts Ang II causes hyperplasia and extracellular matrix production via the AT1 receptor. Ang II may initiate a positive feedback regulation of fibroblasts growth, inducing the expression of TGF-beta 1 and angiotensinogen genes. Ang II, acting directly on interstitial fibroblasts, may be implicated in the pathogenesis of renal fibrosis.