Autoradiographic localization of active renin in the juxtaglomerular apparatus of the dog kidney: effects of sodium intake

Recent Updates on the Proximal Tubule Renin-Angiotensin System in Angiotensin II-Dependent Hypertension

It is well recognized that the renin-angiotensin system (RAS) exists not only as circulating, paracrine (cell to cell), but also intracrine (intracellular) system. In the kidney, however, it is difficult to dissect the respective contributions of circulating RAS versus intrarenal RAS to the physiological regulation of proximal tubular Na(+) reabsorption and hypertension. Here, we review recent studies to provide an update in this research field with a focus on the proximal tubular RAS in angiotensin II (ANG II)-induced hypertension. Careful analysis of available evidence supports the hypothesis that both local synthesis or formation and AT1 (AT1a) receptor- and/or megalin-mediated uptake of angiotensinogen (AGT), ANG I and ANG II contribute to high levels of ANG II in the proximal tubules of the kidney. Under physiological conditions, nearly all major components of the RAS including AGT, prorenin, renin, ANG I, and ANG II would be filtered by the glomerulus and taken up by the proximal tubules. In ANG II-dependent hypertension, the expression of AGT, prorenin, and (pro)renin receptors, and angiotensin-converting enzyme (ACE) is upregulated rather than downregulated in the kidney. Furthermore, hypertension damages the glomerular filtration barrier, which augments the filtration of circulating AGT, prorenin, renin, ANG I, and ANG II and their uptake in the proximal tubules. Together, increased local ANG II formation and augmented uptake of circulating ANG II in the proximal tubules, via activation of AT1 (AT1a) receptors and Na(+)/H(+) exchanger 3, may provide a powerful feedforward mechanism for promoting Na(+) retention and the development of ANG II-induced hypertension.

New insights and perspectives on intrarenal renin-angiotensin system: focus on intracrine/intracellular angiotensin II

Although renin, the rate-limiting enzyme of the renin-angiotensin system (RAS), was first discovered by Robert Tigerstedt and Bergman more than a century ago, the research on the RAS still remains stronger than ever. The RAS, once considered to be an endocrine system, is now widely recognized as dual (circulating and local/tissue) or multiple hormonal systems (endocrine, paracrine and intracrine). In addition to the classical renin/angiotensin I-converting enzyme (ACE)/angiotensin II (Ang II)/Ang II receptor (AT₁/AT₂) axis, the prorenin/(Pro)renin receptor (PRR)/MAP kinase axis, the ACE2/Ang (1-7)/Mas receptor axis, and the Ang IV/AT₄/insulin-regulated aminopeptidase (IRAP) axis have recently been discovered. Furthermore, the roles of the evolving RAS have been extended far beyond blood pressure control, aldosterone synthesis, and body fluid and electrolyte homeostasis. Indeed, novel actions and underlying signaling mechanisms for each member of the RAS in physiology and diseases are continuously uncovered. However, many challenges still remain in the RAS research field despite of more than one century's research effort. It is expected that the research on the expanded RAS will continue to play a prominent role in cardiovascular, renal and hypertension research. The purpose of this article is to review the progress recently being made in the RAS research, with special emphasis on the local RAS in the kidney and the newly discovered prorenin/PRR/MAP kinase axis, the ACE2/Ang (1-7)/Mas receptor axis, the Ang IV/AT₄/IRAP axis, and intracrine/intracellular Ang II. The improved knowledge of the expanded RAS will help us better understand how the classical renin/ACE/Ang II/AT₁ receptor axis, extracellular and/or intracellular origin, interacts with other novel RAS axes to regulate blood pressure and cardiovascular and kidney function in both physiological and diseased states.

Genetic deletion of AT1a receptors attenuates intracellular accumulation of ANG II in the kidney of AT1a receptor-deficient mice

We and others have previously shown that high levels of ANG II are accumulated in the rat kidney via a type 1 (AT(1)) receptor-mediated mechanism, but it is not known which AT(1) receptor is involved in this process in rodents. We tested the hypothesis that AT(1a) receptor-deficient mice (Agtr1a-/-) are unable to accumulate ANG II intracellularly in the kidney because of the absence of AT(1a) receptor-mediated endocytosis. Adult male wild-type (Agtr1a+/+), heterozygous (Agtr1a+/-), and Agtr1a-/- were treated with vehicle, ANG II (40 ng/min ip via osmotic minipump), or ANG II plus the AT(1) antagonist losartan (10 mg.kg(-1).day(-1) po) for 2 wk. In wild-type mice, ANG II induced hypertension (168 +/- 4 vs. 113 +/- 3 mmHg, P < 0.001), increased kidney-to-body weight ratio (P < 0.01), caused pressure natriuresis (P < 0.05), and elevated plasma and whole kidney ANG II levels (P < 0.001). Concurrent administration of ANG II with losartan attenuated these responses to ANG II. In contrast, Agtr1a-/- mice had lower basal systolic pressures (P < 0.001), smaller kidneys with much fewer AT(1b) receptors (P < 0.001), higher basal 24-h urinary sodium excretion (P < 0.01), as well as basal plasma and whole kidney ANG II levels (P < 0.01). However, intracellular ANG II levels in the kidney were lower in Agtr1a-/- mice. In Agtr1a-/- mice, ANG II slightly increased systolic pressure (P < 0.05) but had no effect on the kidney weight, urinary sodium excretion, and whole kidney ANG II levels. Losartan restored systolic pressure to basal levels and decreased whole kidney ANG II levels by approximately 20% (P < 0.05). These results demonstrate a predominant role of AT(1a) receptors in blood pressure regulation and in the renal responses to long-term ANG II administration, that AT(1b) receptors may play a limited role in blood pressure control and mediating intrarenal ANG II accumulation in the absence of AT(1a) receptors.

Lack of connexin 40 causes displacement of renin-producing cells from afferent arterioles to the extraglomerular mesangium

In the adult kidney, renin-producing cells are typically located in the walls of afferent arterioles at the transition into the glomerular capillary network. The mechanisms that are responsible for restricting renin expression to the juxtaglomerular position are largely unknown. This study showed that in mice that lack connexin 40 (Cx40), the predominant connexin of renin-producing cells, renin-positive cells are absent in the vessel walls and instead are found in cells of the extraglomerular mesangium, glomerular tuft, and periglomerular interstitium. Blocking macula densa transport function by acute administration of loop diuretics strongly enhances renin secretion in vivo and in isolated perfused kidneys of wild-type mice. This effect of loop diuretics is markedly attenuated in vivo and even blunted in vitro in Cx40-deficient mice. Even after prolonged stimulation of renin secretion by severe sodium depletion, renin expression is not seen in juxtaglomerular cells or in cells of more proximal parts of the arterial vessel wall as occurs normally. Instead, renin remains restricted to the extra-/periglomerular interstitium in Cx40-deficient mice. In contrast to the striking displacement of renin-expressing cells in the adult kidney, renin expression in the vessels of the developing kidney was found to be normal. This is the first evidence to indicate that cell-to-cell communication via gap junctions is essential for the correct juxtaglomerular positioning and recruitment of renin-producing cells. Moreover, these findings support the notion that gap junctions are relevant for the macula densa signaling to renin-producing cells.