The increasing complexity of the intratubular Renin-Angiotensin system.

The presence of receptors for angiotensin II (AII) on the luminal membranes of various nephron segments has been well established for many decades. Originally their function remained unclear because tubular fluid AII concentrations were thought to be quite low due to the presence of various degrading enzymes on the brush border of proximal tubular cells. However, a series of reports in the 1990s demonstrated that the proximal tubular concentrations of AI and AII are in the nanomolar range and much higher than can be explained by tubular fluid reabsorption or equilibration with the circulating levels. These findings led to a paradigm shift in our concepts regarding the role of luminal AII receptors in various nephron segments, and it is now well accepted that intraluminal AII and other angiotensin peptides exert various actions on transport systems in essentially all nephron segments predominantly through activation of AT1 receptors. In further studies, proximal tubular fluid samples were incubatedwith excess renin to determine substrate availability. The resultant AI concentrations demonstrated that the proximal tubular fluid angiotensinogen (AGT) concentrations are also very high and greater than the circulating concentrations, indicating that it is unlikely the tubular AGT concentrations are derived from filtered AGT, in particular considering the limited permeability of the AGT because of its large size. Using in situ perfusion of proximal tubules with artificial tubular fluid with the delivery of filtrate blocked, Braam et al. collected the tubular fluid from downstream segments and found these also had elevated AII concentrations in the nanomolar range, supporting tubular secretion of AII or its precursors. Studies on isolated perfused tubules from S2 segments indicated that AII is produced intracellularly and secreted preformed into the tubular lumen, supporting the presence of intact AGT in isolated proximal tubule segments. These findings, along with clear evidence for the presence of AGTmRNA and protein in proximal tubular cells, provided the foundation for the concept that the intratubular AII concentrations are derived primarily from locally synthesized substrate. The local production of AGT in the proximal tubule was supported by Terada et al., who demonstrated the presence of a large signal for AGTmRNA inmicrodissected proximal convoluted and straight tubules. In vitro studies have also consistently demonstrated mRNA encoding AGT in proximal tubular cell lines extracted from proximal tubule segments. However, Pohl et al. recently reported predominant localization of mRNA encoding AGT to S3 segments. The findings of kidney tubular mRNA encoding AGT notwithstanding, the kidney’s ability to produce AGT is dwarfed by that of the liver, the organ primarily responsible for the maintenance of circulating AGT concentrations. In addition, it is well recognized that filtered AGT can be taken up by proximal tubule scavenger receptors such as megalin and cubilin, or by a putative-specific AGT receptor on proximal tubular epithelial cells. Thus, various studies demonstrated that AGT is internalized by these transporters, which is consistent with observations that megalin binds and internalizes AII. The important question of the quantitative contribution of either source to total renal AGT content remained unresolved because of the lack of definitive tools needed to provide the answer. The paper by Matsusaka et al. in this issue of JASN provides critical data obtained in mice that had either liver AGTor the kidney AGTmRNA expression deleted genetically. Their results show that kidney-specific AGT-null mice have kidney AGT protein and AII levels similar to those in control mice, whereas the liver-specific AGT-null mice have markedly lower or undetectable levels of AGT in the proximal tubules and kidney AII. Further studies confirmed that AGTuptake by proximal tubular cells is mediated by megalin and that disruption of the filtration barrier markedly increases tubular and urinary AGT protein. The results thus indicate that, in normal nonhypertensive mice, most of the AGT protein present in proximal tubules is of liver origin. This finding is not unexpected, as the kidney levels of AGT protein in normal rats and mice are rather low, especially in comparison to the enhanced levels that develop in response to chronic AII infusions. In addition, AGT synthesis and secretion is a constitutive process, so cells expressing mRNA encoding AGT probably do not store the protein, but secrete it into the tubular fluid. To the extent that the proximal tubule AII concentrations in mice are as high as have been found in rat proximal tubular fluid, the results presented by Matsusaka et al. pose the interesting dilemma regarding the cellular pathways by which the increased intratubular AII levels are derived from the AGT taken up from the circulation by the proximal tubular cells. AII may be formed intracellularly from AGTreabsorbed from either the renal interstitium or tubular fluid, or the internalized AGT may be resecreted into the tubular lumen where Published online ahead of print. Publication date available at www.jasn.org.