Role of the kidney in the plasma clearance of angiotensinogen in the rat: plasma clearance and tissue distribution of 125I-angiotensinogen

Perivascular Adipose Tissue in Vascular Function: Does Locally Synthesized Angiotensinogen Play a Role?

ABSTRACT

In recent years, perivascular adipose tissue (PVAT) research has gained special attention in an effort to understand its involvement in vascular function. PVAT is recognized as an important endocrine organ that secretes procontractile and anticontractile factors, including components of the renin-angiotensin-aldosterone system, particularly angiotensinogen (AGT). This review critically addresses the occurrence of AGT in PVAT, its release into the blood stream, and its contribution to the generation and effects of angiotensins (notably angiotensin-(1-7) and angiotensin II) in the vascular wall. It describes that the introduction of transgenic animals, expressing AGT at 0, 1, or more specific location(s), combined with the careful measurement of angiotensins, has revealed that the assumption that PVAT independently generates angiotensins from locally synthesized AGT is incorrect. Indeed, selective deletion of AGT from adipocytes did not lower circulating AGT, neither under a control diet nor under a high-fat diet, and only liver-specific AGT deletion resulted in the disappearance of AGT from blood plasma and adipose tissue. An entirely novel scenario therefore develops, supporting local angiotensin generation in PVAT that depends on the uptake of both AGT and renin from blood, in addition to the possibility that circulating angiotensins exert vascular effects. The review ends with a summary of where we stand now and recommendations for future research.

Resetting of renal tissular renin-angiotensin and bradykinin-kallikrein systems after unilateral kidney denervation in rats

The renal tissular renin-angiotensin and bradykinin-kallikrein systems control kidney function together with the renal sympathetic innervation but their interaction is still unclear. To further elucidate this relationship, we investigated these systems in rats 6 days after left kidney denervation (DNX, n = 8) compared to sham-operated controls (CTR, n = 8). Plasma renin concentration was unchanged in DNX vs. CTR (p = NS). Kidney bradykinin (BK) and angiotensin (Ang) I and II concentrations decreased bilaterally in DNX vs. CTR rats (~20 to 40%, p < 0.05) together with Ang IV and V concentrations that were extremely low (p = NS). Renin, Ang III and dopamine concentrations decreased by ~25 to 50% and norepinephrine concentrations by 99% in DNX kidneys (p < 0.05) but were unaltered in opposite kidneys. Ang II/I and KA were comparable in DNX, contralateral and CTR kidneys. Ang III/II increased in right vs. DNX or CTR kidneys (40-50%, p < 0.05). Ang II was mainly located in tubular epithelium by immunocytological staining and its cellular distribution was unaffected by DNX. Moreover, the angiotensinergic and catecholaminergic innervation of right kidneys was unchanged vs. CTR. We found an important dependency of tissular Ang and BK levels on the renal innervation that may contribute to the resetting of kidney function after DNX. The DNX-induced peptide changes were not readily explained by kidney KA, renin or plasma Ang I generation. However, tissular peptide metabolism and compartmentalization may have played a central role. The mechanisms behind the concentration changes remain unclear and deserve further clarification.

Uptake and metabolism of the novel peptide angiotensin-(1-12) by neonatal cardiac myocytes

Background

Angiotensin-(1–12) [Ang-(1–12)] functions as an endogenous substrate for the productions of Ang II and Ang-(1–7) by a non-renin dependent mechanism. This study evaluated whether Ang-(1–12) is incorporated by neonatal cardiac myocytes and the enzymatic pathways of ¹²⁵I-Ang-(1–12) metabolism in the cardiac myocyte medium from WKY and SHR rats.

Methodology/Principal Findings

The degradation of ¹²⁵I-Ang-(1–12) (1 nmol/L) in the cultured medium of these cardiac myocytes was evaluated in the presence and absence of inhibitors for angiotensin converting enzymes 1 and 2, neprilysin and chymase. In both strains uptake of ¹²⁵I-Ang-(1–12) by myocytes occurred in a time-dependent fashion. Uptake of intact Ang-(1–12) was significantly greater in cardiac myocytes of SHR as compared to WKY. In the absence of renin angiotensin system (RAS) enzymes inhibitors the hydrolysis of labeled Ang-(1–12) and the subsequent generation of smaller Ang peptides from Ang-(1–12) was significantly greater in SHR compared to WKY controls. ¹²⁵I-Ang-(1–12) degradation into smaller Ang peptides fragments was significantly inhibited (90% in WKY and 71% in SHR) in the presence of all RAS enzymes inhibitors. Further analysis of peptide fractions generated through the incubation of Ang-(1–12) in the myocyte medium demonstrated a predominant hydrolytic effect of angiotensin converting enzyme and neprilysin in WKY and an additional role for chymase in SHR.

Conclusions/Significance

These studies demonstrate that neonatal myocytes sequester angiotensin-(1–12) and revealed the enzymes involved in the conversion of the dodecapeptide substrate to biologically active angiotensin peptides.

Molecular and Pathophysiological Features of Angiotensinogen: A Mini Review

The renin-angiotensin system is an essential regulatory system for blood pressure and fluid homeostasis. Angiotensinogen is the only known precursor of all the peptides generated in this system. While many of the basic understandings of angiotensinogen have come from research efforts to define its role in blood pressure regulation, novel pathophysiological functions of angiotensinogen have been discovered in the last two decades including kidney developmental abnormalities, atherosclerosis, and obesity. Despite the impressive advance in the understanding of angiotensinogen gene structure and protein functions, some fundamental questions remain unanswered. In this short review, we provide contemporary insights into the molecular characteristics of angiotensinogen and its pathophysiological features. In light of the recent progress, we emphasize some newly recognized functional features of angiotensinogen other than its regulation on blood pressure.

Specific receptor for angiotensinogen on human renal cells

BACKGROUND

We recently demonstrated the existence of an angiotensinogen (AGT) receptor on placental cells. It has been established that there is a tissue-specific renin-angiotensin system (RAS) in the human kidney. This study focused on whether human renal proximal tubule epithelial cells possessed an AGT receptor.

METHODS

Human renal proximal tubule epithelial cells were cultured in plastic wells. Binding assays were carried out by adding iodinated angiotensinogen ((125)I-AGT) to each culture well, with or without unlabeled AGT. The cells were washed, lysed, and the radioactivity was measured.

RESULTS

Human renal proximal tubule epithelial cells bound (125)I-AGT in a time-dependent manner. This binding was competitively and specifically inhibited by unlabeled AGT. Bound (125)I-AGT was competitively displaced by AGT. Acid washing removed 30% at 8 h, indicating that 70% bound AGT had been internalized. Scatchard plot binding analysis showed that the identified AGT receptor possessed a single class of high-affinity binding sites (K(d)=1.73 nmol, B(max)=23.39 pmol/10(6) cells).

CONCLUSION

The results of this study provide evidence for the presence of an AGT receptor on human renal proximal tubule epithelial cells. Our finding suggests that the AGT receptor may be an integral component of the renal RAS.

Release and degradation of angiotensin I and angiotensin II from angiotensinogen by neutrophil serine proteinases

Cathepsin G, elastase, and proteinase 3 are serine proteinases released by activated neutrophils. Cathepsin G can cleave angiotensinogen to release angiotensin II, but this activity has not been previously reported for elastase or proteinase 3. In this study we show that elastase and proteinase 3 can release angiotensin I from angiotensinogen and release angiotensin II from angiotensin I and angiotensinogen. The relative order of potency in releasing angiotensin II by the three proteinases at equivalent concentrations is cathepsin G > elastase > proteinase 3. When all three proteinases are used together, the release of angiotensin II is greater than the sum of the release when each proteinase is used individually. Cathepsin G and elastase can also degrade angiotensin II, reactions which might be important in regulating the activity of angiotensin II. The release and degradation of angiotensin II by the neutrophil proteinases are reactions which could play a role in the local inflammatory response and wound healing.

Regulation of hepatic angiotensinogen gene expression in nephrotic rats

Plasma angiotensinogen (Ao) concentration (PAC), urinary Ao excretion (UAE), hepatic levels of Ao mRNA and plasma renin concentration (PRC) were studied in control and nephrotic rats subjected to the following treatments: dexamethasone (DEX), ethinyl-estradiol (EE), tri-iodothyronine (T3), bilateral nephrectomy (NX), captopril (CAP) and adrenalectomy (ADX). In nephrotic rats PAC diminished, UAE and PRC augmented and Ao mRNA levels were not altered. In control rats, DEX, EE, T3 and NX increased PAC and Ao mRNA levels whereas CAP diminished PAC but not affected Ao mRNA. ADX diminished PAC and Ao mRNA levels. In nephrotic rats, these treatments produced the same effect than in control rats except in ADX which did not affect PAC. These data suggest that the decreased PAC is not related to alterations in hepatic Ao gene expression but to elevated PRC and UAE.