Connexin Signaling in the Juxtaglomerular Apparatus (JGA) of Developing, Postnatal Healthy and Nephrotic Human Kidneys.

Our study analyzed the expression pattern of different connexins (Cxs) and renin positive cells in the juxtaglomerular apparatus (JGA) of developing, postnatal healthy human kidneys and in nephrotic syndrome of the Finnish type (CNF), by using double immunofluorescence, electron microscopy and statistical measuring. The JGA contained several cell types connected by Cxs, and consisting of macula densa, extraglomerular mesangium (EM) and juxtaglomerular cells (JC), which release renin involved in renin-angiotensin- aldosteron system (RAS) of arterial blood pressure control. During JGA development, strong Cx40 expression gradually decreased, while expression of Cx37, Cx43 and Cx45 increased, postnatally showing more equalized expression patterning. In parallel, initially dispersed renin cells localized to JGA, and greatly increased expression in postnatal kidneys. In CNF kidneys, increased levels of Cx43, Cx37 and Cx45 co-localized with accumulations of renin cells in JGA. Additionally, they reappeared in extraglomerular mesangial cells, indicating association between return to embryonic Cxs patterning and pathologically changed kidney tissue. Based on the described Cxs and renin expression patterning, we suggest involvement of Cx40 primarily in the formation of JGA in developing kidneys, while Cx37, Cx43 and Cx45 might participate in JGA signal transfer important for postnatal maintenance of kidney function and blood pressure control.

Connexins: key mediators of endocrine function.

The appearance of multicellular organisms imposed the development of several mechanisms for cell-to-cell communication, whereby different types of cells coordinate their function. Some of these mechanisms depend on the intercellular diffusion of signal molecules in the extracellular spaces, whereas others require cell-to-cell contact. Among the latter mechanisms, those provided by the proteins of the connexin family are widespread in most tissues. Connexin signaling is achieved via direct exchanges of cytosolic molecules between adjacent cells at gap junctions, for cell-to-cell coupling, and possibly also involves the formation of membrane "hemi-channels," for the extracellular release of cytosolic signals, direct interactions between connexins and other cell proteins, and coordinated influence on the expression of multiple genes. Connexin signaling appears to be an obligatory attribute of all multicellular exocrine and endocrine glands. Specifically, the experimental evidence we review here points to a direct participation of the Cx36 isoform in the function of the insulin-producing ß-cells of the endocrine pancreas, and of the Cx40 isoform in the function of the renin-producing juxtaglomerular epithelioid cells of the kidney cortex.

Plasticity of Renin Cells in the Kidney Vasculature.

During development, renin cells are precursors for arteriolar smooth muscle, mesangial cells, and interstitial pericytes. Those seemingly differentiated descendants retain the memory to re-express renin when there is a threat to homeostasis. Understanding how such molecular memory is constructed and regulated would be crucial to comprehend cell identity which is, in turn, intimately linked to homeostasis.

Renin release: sites, mechanisms, and control.

In the adult organism, systemically circulating renin almost exclusively originates from the juxtaglomerular cells in the afferent arterioles of the kidneys. These cells share similarities with pericytes and myofibro-blasts. They store renin in a vesicular network and granules and release it in a regulated fashion. The release mode of renin is not understood; in particular, the involvement of SNARE proteins is unknown. Renin release is acutely increased via the cAMP signaling pathway, which is triggered mainly by catecholamines and other G(s)-coupled agonists, and is inhibited by calcium-related pathways that are commonly activated by vasoconstrictors. Renin release from juxtaglomerular cells is directly modulated in an inverse fashion by the blood pressure inside the afferent arterioles and by the chloride content in the tubule fluid at the macula densa segment of the distal tubule. Renin release is stimulated by nitric oxide and by prostanoids released by neighboring endothelial and macula densa cells. Steady-state renin concentrations in the plasma are determined essentially by the number of renin-producing cells in the afferent arterioles, which changes in parallel with challenges to the renin-angiotensin-aldosterone system.

Iodinated contrast media cause direct tubular cell damage, leading to oxidative stress, low nitric oxide, and impairment of tubuloglomerular feedback.

Iodinated contrast media (CM) have adverse effects that may result in contrast-induced acute kidney injury. Oxidative stress is believed to play a role in CM-induced kidney injury. We test the hypothesis that oxidative stress and reduced nitric oxide in tubules are consequences of CM-induced direct cell damage and that increased local oxidative stress may increase tubuloglomerular feedback. Rat thick ascending limbs (TAL) were isolated and perfused. Superoxide and nitric oxide were quantified using fluorescence techniques. Cell death rate was estimated using propidium iodide and trypan blue. The function of macula densa and tubuloglomerular feedback responsiveness were measured in isolated, perfused juxtaglomerular apparatuses (JGA) of rabbits. The expression of genes related to oxidative stress and the activity of superoxide dismutase (SOD) were investigated in the renal medulla of rats that received CM. CM increased superoxide concentration and reduced nitric oxide bioavailability in TAL. Propidium iodide fluorescence and trypan blue uptake increased more in CM-perfused TAL than in controls, indicating increased rate of cell death. There were no marked acute changes in the expression of genes related to oxidative stress in medullary segments of Henle's loop. SOD activity did not differ between CM and control groups. The tubuloglomerular feedback in isolated JGA was increased by CM. Tubular cell damage and accompanying oxidative stress in our model are consequences of CM-induced direct cell damage, which also modifies the tubulovascular interaction at the macula densa, and may therefore contribute to disturbances of renal perfusion and filtration.

Regulation of renin secretion by renal juxtaglomerular cells.

A major rate-limiting step in the renin-angiotensin-aldosterone system is the release of active renin from endocrine cells (juxtaglomerular (JG) cells) in the media layer of the afferent glomerular arterioles. The number and distribution of JG cells vary with age and the physiological level of stimulation; fetal life and chronic stimulation by extracellular volume contraction is associated with recruitment of renin-producing cells. Upon stimulation of renin release, labeled renin granules "disappear;" the number of granules decrease; cell membrane surface area increases in single cells, and release is quantal. Together, this indicates exocytosis as the predominant mode of release. JG cells release few percent of total renin content by physiological stimulation, and recruitment of renin cells is preferred to recruitment of granules during prolonged stimulation. Several endocrine and paracrine agonists, neurotransmitters, and cell swelling converge on the stimulatory cyclic AMP (cAMP) pathway. Renin secretion is attenuated in mice deficient in beta-adrenoceptors, prostaglandin E(2)-EP4 receptors, Gsα protein, and adenylyl cyclases 5 and 6. Phosphodiesterases (PDE) 3 and 4 degrade cAMP in JG cells, and PDE3 is inhibited by cyclic GMP (cGMP) and couples the cGMP pathway to the cAMP pathway. Cyclic AMP enhances K(+)-current in JG cells and is permissive for secretion by stabilizing membrane potential far from threshold that activates L-type voltage-gated calcium channels. Intracellular calcium paradoxically inhibits renin secretion likely through attenuated formation and enhanced degradation of cAMP; by activation of chloride currents and interaction with calcineurin. Connexin 40 is necessary for localization of JG cells in the vascular wall and for pressure- and macula densa-dependent suppression of renin release.

Parallel regulation of renin and lysosomal integral membrane protein 2 in renin-producing cells: further evidence for a lysosomal nature of renin secretory vesicles.

The protease renin is the key enzyme in the renin-angiotensin system (RAS) that regulates extracellular volume and blood pressure. Renin is synthesized in renal juxtaglomerular cells (JG cells) as the inactive precursor prorenin. Activation of prorenin by cleavage of the prosegment occurs in renin storage vesicles that have lysosomal properties. To characterize the renin storage vesicles more precisely, the expression and functional relevance of the major lysosomal membrane proteins lysosomal-associated membrane protein 1 (LAMP-1), LAMP-2, and lysosomal integral membrane protein 2 (LIMP-2) were determined in JG cells. Immunostaining experiments revealed strong coexpression of renin with the LIMP-2 (SCARB2), while faint staining of LAMP-1 and LAMP-2 was detected in some JG cells only. Stimulation of the renin system (ACE inhibitor, renal hypoperfusion) resulted in the recruitment of renin-producing cells in the afferent arterioles and parallel upregulation of LIMP-2, but not LAMP-1 or LAMP-2. Despite the coregulation of renin and LIMP-2, LIMP-2-deficient mice had normal renal renin mRNA levels, renal renin and prorenin contents, and plasma renin and prorenin concentrations under control conditions and in response to stimulation with a low salt diet (with or without angiotensin-converting enzyme (ACE) inhibition). No differences in the size or number of renin vesicles were detected using electron microscopy. Acute stimulation of renin release by isoproterenol exerted similar responses in both genotypes in vivo and in isolated perfused kidneys. Renin and the major lysosomal protein LIMP-2 are colocalized and coregulated in renal JG cells, further corroborating the lysosomal nature of renin storage vesicles. LIMP-2 does not appear to play an obvious role in the regulation of renin synthesis or release.

Role of blood pressure in mediating the influence of salt intake on renin expression in the kidney.

The salt intake of an organism controls the number of renin-producing cells in the kidney by yet undefined mechanisms. This study aimed to assess a possible mediator role of preglomerular blood pressure in the control of renin expression by oral salt intake. We used wild-type (WT) mice and mice lacking angiotensin II type 1a receptors (AT(1a)-/-) displaying an enhanced salt sensitivity to renin expression. In WT kidneys, we found renin-expressing cells at the ends of all afferent arterioles. A low-salt diet (0.02%) led to a moderate twofold increase in renin-expressing cells along afferent arterioles. In AT(1a)-/- mice, lowering of salt content led to a 12-fold increase in renin expression. Here, the renin-expressing cells were distributed along the preglomerular vascular tree in a typical distal-to-proximal distribution gradient which was most prominent at high salt intake and was obliterated at low salt intake by the appearance of renin-expressing cells in proximal parts of the preglomerular vasculature. While lowering of salt intake produced only a small drop in blood pressure in WT mice, the marked reduction of systolic blood pressure in AT(1a)-/- mice was accompanied by the disappearance of the distribution gradient from afferent arterioles to arcuate arteries. Unilateral renal artery stenosis in AT(1a)-/- mice on a normal salt intake produced a similar distribution pattern of renin-expressing cells as did low salt intake. Conversely, increasing blood pressure by administration of the NOS inhibitor N-nitro-l-arginine methyl ester or of the adrenergic agonist phenylephrine in AT(1a)-/- mice kept on low salt intake produced a similar distribution pattern of renin-producing cells as did normal salt intake alone. These findings suggest that changes in preglomerular blood pressure may be an important mediator of the influence of salt intake on the number and distribution of renin-producing cells in the kidney.

Connecting tubule glomerular feedback mediates acute tubuloglomerular feedback resetting.

Tubuloglomerular feedback (TGF) and connecting tubule glomerular feedback (CTGF) are mechanisms that control afferent arteriole (Af-Art) tone. TGF, initiated by increased NaCl at the macula densa, causes Af-Art constriction. Prolonged activation of TGF leads to an attenuation or "resetting" of its constrictor effect. The mechanisms of TGF resetting remain incompletely understood. CTGF is initiated by increased NaCl in the connecting tubule and Na(+) entry via epithelial sodium channels (ENaC). Contrary to TGF, CTGF dilates the Af-Art. Here, we hypothesize that CTGF, in part, mediates TGF resetting. We performed micropuncture of individual rat nephrons while measuring stop-flow pressure (P(SF)), an index of glomerular filtration pressure and Af-Art tone. Increases in Af-Art tone cause P(SF) to decrease. TGF responses, measured as the decrease in P(SF) induced by switching late proximal tubule perfusion from 5 to 40 nl/min, were elicited before and after a 30-min period of sustained perfusion of the late proximal tubule at a rate of 40 nl/min designed to induce TGF resetting. TGF responses were 7.3 ± 0.3 and 4.9 ± 0.2 mmHg before and after resetting was induced (P < 0.001, n = 6). When CTGF was inhibited with the ENaC blocker benzamil (1 µM), TGF responses were 9.5 ± 0.3 and 8.8 ± 0.6 mmHg (NS, n = 6), thus resetting was abolished. In the presence of the carbonic anhydrase inhibitor acetazolamide (10 mM), TGF responses were 8.8 ± 0.6 and 3.3 ± 0.4 mmHg before and after resetting (P < 0.001, n = 6). With both acetazolamide and benzamil, TGF responses were 10.4 ± 0.2 and 8.4 ± 0.5 mmHg (P < 0.01, n = 6), thus resetting was attenuated. We conclude that CTGF, in part, mediates acutely induced TGF resetting.

Renoprotective effect of pioglitazone by the prevention of glomerular hyperfiltration through the possible restoration of altered macula densa signaling in rats with type 2 diabetic nephropathy.

BACKGROUND/AIMS: Pioglitazone (PGZ), one of the thiazolidinediones, has been known to show renoprotective effects. In this study, we focused on the effect of PGZ on glomerular hyperfiltration (GHF), resultant glomerular injury and altered macula densa signaling as a cause of sustained GHF through modified tubuloglomerular feedback in rats with diabetic nephropathy. METHODS: Kidneys from 24-week-old male OLETF rats and LET rats, nondiabetic controls, were used for the experiment. PGZ was administered (10 mg/kg/day, p.o.) for 2 weeks from 22 to 24 weeks of age in some of the OLETF rats (OLETF+PGZ). RESULTS: Parameters relating GHF, kidney weight, creatinine clearance, urine albumin/creatinine ratio and glomerular surface were all increased in OLETF rats and partially restored in OLETF+PGZ rats. Expressions of desmin and TGF-ß were also increased in OLETF rats and restored in OLETF+PGZ rats. The changes in TGF-ß expression were confirmed to be independent of podocyte number. Finally, the immunoreactivity of neuronal nitric oxide synthase (nNOS) and cyclooxygenase 2 (COX-2) in the macula densa was assessed for the evaluation of macula densa signaling. Altered intensities of nNOS and COX-2 in OLETF rats were restored in OLETF+PGZ rats, which agreed with the gene expression analysis (nNOS: 100.2 ± 2.9% in LET, 64.2 ± 2.7% in OLETF, 87.4 ± 12.1% in OLETF+PGZ; COX-2: 100.8 ± 7.4% in LET, 249.2 ± 19.4% in OLETF, 179.9 ± 13.5% in OLETF+PGZ; n = 5) and the semiquantitative analysis of nNOS/COX-2-positive cells. CONCLUSION: PGZ effectively attenuated the GHF and hyperfiltration-associated glomerular injury in diabetic nephropathy. The restoration of altered macula densa signaling might be involved in the renoprotective effect of PGZ.

Macula densa sensing and signaling mechanisms of renin release.

Macula densa cells in the distal nephron, according to the classic paradigm, are salt sensors that generate paracrine chemical signals in the juxtaglomerular apparatus to control vital kidney functions, including renal blood flow, glomerular filtration, and renin release. Renin is the rate-limiting step in the activation of the renin-angiotensin system, a key modulator of body fluid homeostasis. Here, we discuss recent advances in understanding macula densa sensing and suggest these cells, in addition to salt, also sense various chemical and metabolic signals in the tubular environment that directly trigger renin release.

Atrap deficiency increases arterial blood pressure and plasma volume.

The angiotensin receptor-associated protein (Atrap) interacts with angiotensin II (AngII) type 1 (AT1) receptors and facilitates their internalization in vitro, but little is known about the function of Atrap in vivo. Here, we detected Atrap expression in several organs of wild-type mice; the highest expression was in the kidney where it localized to the proximal tubule, particularly the brush border. There was no Atrap expression in the renal vasculature or juxtaglomerular cells. We generated Atrap-deficient (Atrap-/-) mice, which were viable and seemed grossly normal. Mean systolic BP was significantly higher in Atrap-/- mice compared with wild-type mice. Dose-response relationships of arterial BP after acute AngII infusion were similar in both genotypes. Plasma volume was significantly higher and plasma renin concentration was markedly lower in Atrap-/- mice compared with wild-type mice. (125)I-AngII binding showed enhanced surface expression of AT1 receptors in the renal cortex of Atrap-/- mice, accompanied by increased carboanhydrase-sensitive proximal tubular function. In summary, Atrap-/- mice have increased arterial pressure and plasma volume. Atrap seems to modulate volume status by acting as a negative regulator of AT1 receptors in the renal tubules.

The role of calcium in the regulation of renin secretion.

Renin is the enzyme which is the rate-limiting step in the formation of the hormone angiotensin II. Therefore, the regulation of renin secretion is critical in understanding the control of the renin-angiotensin-aldosterone system and its many biological and pathological actions. Renin is synthesized, stored in, and released from the juxtaglomerular (JG) cells of the kidney. While renin secretion is positively regulated by the "second messenger" cAMP, unlike most secretory cells, renin secretion from the JG cell is inversely related to the extracellular and intracellular calcium concentrations. This novel relationship is referred to as the "calcium paradox." This review will address observations made over the past 30 years regarding calcium and the regulation of renin secretion, and focus on recent observations which address this scientific conundrum. These include 1) receptor-mediated pathways for changing intracellular calcium; 2) the discovery of a calcium-inhibitable isoform of adenylyl cyclase associated with renin in the JG cells; 3) calcium-sensing receptors in the JG cells; 4) calcium-calmodulin-mediated signals; 5) the role of phosphodiesterases; and 6) connexins, gap junctions, calcium waves, and the cortical extracellular calcium environment. While cAMP is the dominant second messenger for renin secretion, calcium appears to modulate the integrated activities of the enzymes, which balance cAMP synthesis and degradation. Thus this review concludes that calcium modifies the amplitude of cAMP-mediated renin-signaling pathways. While calcium does not directly control renin secretion, increased calcium inhibits and decreased calcium amplifies cAMP-stimulated renin secretion.

Direct demonstration of tubular fluid flow sensing by macula densa cells.

Macula densa (MD) cells in the cortical thick ascending limb (cTAL) detect variations in tubular fluid composition and transmit signals to the afferent arteriole (AA) that control glomerular filtration rate [tubuloglomerular feedback (TGF)]. Increases in tubular salt at the MD that normally parallel elevations in tubular fluid flow rate are well accepted as the trigger of TGF. The present study aimed to test whether MD cells can detect variations in tubular fluid flow rate per se. Calcium imaging of the in vitro microperfused isolated JGA-glomerulus complex dissected from mice was performed using fluo-4 and fluorescence microscopy. Increasing cTAL flow from 2 to 20 nl/min (80 mM [NaCl]) rapidly produced significant elevations in cytosolic Ca(2+) concentration ([Ca(2+)](i)) in AA smooth muscle cells [evidenced by changes in fluo-4 intensity (F); F/F(0) = 1.45 ± 0.11] and AA vasoconstriction. Complete removal of the cTAL around the MD plaque and application of laminar flow through a perfusion pipette directly to the MD apical surface essentially produced the same results even when low (10 mM) or zero NaCl solutions were used. Acetylated α-tubulin immunohistochemistry identified the presence of primary cilia in mouse MD cells. Under no flow conditions, bending MD cilia directly with a micropipette rapidly caused significant [Ca(2+)](i) elevations in AA smooth muscle cells (fluo-4 F/F(0): 1.60 ± 0.12) and vasoconstriction. P2 receptor blockade with suramin significantly reduced the flow-induced TGF, whereas scavenging superoxide with tempol did not. In conclusion, MD cells are equipped with a tubular flow-sensing mechanism that may contribute to MD cell function and TGF.

Acute activation of the calcium-sensing receptor inhibits plasma renin activity in vivo.

In vitro, the renin-secreting juxtaglomerular cells express the calcium-sensing receptor, and its activation with the calcimimetic cinacalcet inhibits renin release. To test whether the activation of calcium-sensing receptor similarly inhibits plasma renin activity (PRA) in vivo, we hypothesized that the calcium-sensing receptor is expressed in juxtaglomerular cells in vivo, and acutely administered cinacalcet would inhibit renin activity in anesthetized rats. Since cinacalcet inhibits parathyroid hormone, which may stimulate renin activity, we sought to determine whether cinacalcet inhibits renin activity by decreasing parathyroid hormone. Lastly, we hypothesized that chronically administered cinacalcet would inhibit basal and stimulated renin in conscious rats. Calcium-sensing receptors and renin were localized in the same juxtaglomerular cells using immunofluorescence in rat cortical slices fixed in vivo. Cinacalcet was administered acutely via intravenous bolus in anesthetized rats and chronically in conscious rats by oral gavage. Acute administration of cinacalcet decreased basal renin activity from 13.6 ± 2.4 to 6.1 ± 1.1 ng ANG I·ml(-1)·h(-1) (P < 0.001). Likewise, cinacalcet decreased furosemide-stimulated renin from 30.6 ± 2.3 to 21.3 ± 2.3 ng ANG I·ml(-1)·h(-1) (P < 0.001). In parathyroidectomized rats, cinacalcet decreased renin activity from 9.3 ± 1.3 to 5.2 ± 0.5 ng ANG I·ml(-1)·h(-1) (P < 0.05) similar to sham-operated controls (13.5 ± 2.2 to 6.6 ± 0.8 ng ANG I·ml(-1)·h(-1), P < 0.05). Chronic administration of cinacalcet over 7 days had no significant effect on PRA under basal or stimulated conditions. In conclusion, calcium-sensing receptors are expressed in juxtaglomerular cells in vivo, and acute activation of these receptors with cinacalcet inhibits PRA in anesthetized rats, independent of parathyroid hormone.

Novel mechanisms for the control of renin synthesis and release.

Renin is the key regulated step in the enzymatic cascade that leads to angiotensin generation and the control of blood pressure and fluid/electrolyte homeostasis. In the adult unstressed animal, renin is synthesized and released by renal juxtaglomerular cells. However, when homeostasis is threatened, the number of cells that express and release renin increases and extends beyond the juxtaglomerular area; the result is an increase in circulating renin and the reestablishment of homeostasis. The increase in the number of renin cells, a process termed recruitment, is achieved by dedifferentiation and re-expression of renin in cells derived from the renin lineage. The mechanisms that regulate the related processes of reacquisition of the renin phenotype, renin synthesis, and renin release are beginning to be understood. Numerous studies point to cAMP as a central common factor for the regulation of renin phenotype. In addition, we are seeing the emergence of gap junctions and microRNAs as new and promising avenues for a more complete understanding of the complex regulation of the renin cell.

Activation of the succinate receptor GPR91 in macula densa cells causes renin release.

Macula densa (MD) cells of the juxtaglomerular apparatus (JGA) are salt sensors and generate paracrine signals that control renal blood flow, glomerular filtration, and release of the prohypertensive hormone renin. We hypothesized that the recently identified succinate receptor GPR91 is present in MD cells and regulates renin release. Using immunohistochemistry, we identified GPR91 in the apical plasma membrane of MD cells. Treatment of MD cells with succinate activated mitogen-activated protein kinases (MAPKs; p38 and extracellular signal-regulated kinases 1/2) and cyclooxygenase 2 (COX-2) and induced the synthesis and release of prostaglandin E(2), a potent vasodilator and classic paracrine mediator of renin release. Using microperfused JGA and real-time confocal fluorescence imaging of quinacrine-labeled renin granules, we detected significant renin release in response to tubular succinate (EC(50) 350 microM). Genetic deletion of GPR91 (GPR91(-/-) mice) or pharmacologic inhibition of MAPK or COX-2 blocked succinate-induced renin release. Streptozotocin-induced diabetes caused GPR91-dependent upregulation of renal cortical phospho-p38, extracellular signal-regulated kinases 1/2, COX-2, and renin content. Salt depletion for 1 wk increased plasma renin activity seven-fold in wild-type mice but only 3.4-fold in GPR91(-/-) mice. In summary, MD cells can sense alterations in local tissue metabolism via accumulation of tubular succinate and GPR91 signaling, which involves the activation of MAPKs, COX-2, and the release of prostaglandin E(2). This mechanism may be integral in the regulation of renin release and activation of the renin-angiotensin system in health and disease.

A computational model of the circulating renin-angiotensin system and blood pressure regulation.

UNLABELLED: The renin-angiotensin system (RAS) is critical in sodium and blood pressure (BP) regulation, and in cardiovascular-renal (CVR) diseases and therapeutics. As a contribution to SAPHIR project, we present a realistic computer model of renin production and circulating RAS, integrated into Guyton's circulatory model (GCM). Juxtaglomerular apparatus, JGA, and Plasma modules were implemented in C ++/M2SL (Multi-formalism Multi-resolution Simulation Library) for fusion with GCM. Matlab optimization toolboxes were used for parameter identification. In JGA, renin production and granular cells recruitment (GCR) are controlled by perfusion pressure (PP), macula densa (MD), angiotensin II (Ang II), and renal sympathetic activity (RSNA). In Plasma, renin and ACE (angiotensin-converting enzyme) activities are integrated to yield Ang I and II. Model vs. data deviation is given as normalized root mean squared error (nRMSE; n points). IDENTIFICATION: JGA and Plasma parameters were identified against selected experimental data. After fusion with GCM: (1) GCR parameters were identified against Laragh's PRA-natriuresis nomogram; (2) Renin production parameters were identified against two sets of data ([renin] transients vs. ACE or renin inhibition). Finally, GCR parameters were re-identified vs. Laragh's nomogram (nRMSE 8%, n = 9). VALIDATION: (1) model BP, PRA and [Ang II] are within reported ranges, and respond physiologically to sodium intake; (2) short-term Ang II infusion induces reported rise in BP and PRA. The modeled circulating RAS, in interaction with an integrated CVR, exhibits a realistic response to BP control maneuvers. This construction will allow for modelling hypertensive and CVR patients, including salt-sensitivity, polymorphisms, and pharmacotherapeutics.

Who and where is the renal baroreceptor?: the connexin hypothesis.

Gap junctions are emerging as a fundamental mechanism for the control of renin synthesis and release. Connexin40 is prominent in juxtaglomerular cells. When missing, it results in hyperreninemia and hypertension. Schweda et al. offer exciting data demonstrating that connexin45, a connexin with different biophysical properties, can replace connexin40 functions related to the control of renin.

The renin-angiotensin system in nonmammalian vertebrates.

The most primitive components of the RAS appeared early in the phylogenetic history of vertebrate animals. It is probable that renin granules were present in the kidneys of ancestral chordates before divergence in the evolution of actinopterygian fish and tetrapods occurred. Granulated juxtaglomerular cells similar to the renin-containing cells of the mammalian nephron are found in most extant vertebrate species although not in agnathan and elasmobranch fish. A macula densa occurs in amphibians, birds and mammals; and an extraglomerular mesangium, only in birds and mammals. Renin-like activity and angiotensin-like pressor material have been demonstrated in all classes of vertebrates. The amino acid sequences of native ANG I have been determined for representative species of teleost fish, amphibian, reptile and bird. These peptides differ from mammalian angiotensins at positions 1, 5 and 9. The RAS appears to be involved in osmoregulation, ionoregulation and the control of blood circulation. Prolonged hypovolemic hypotension or sodium depletion increases renin levels. Angiotensins elicit drinking and stimulate transepithelial ion transport. However, direct steroidogenic and antidiuretic hormone-releasing activities, which would promote mineral and fluid conservation, have not been demonstrated unambiguously in nonmammalian vertebrates. ANG II raises blood pressure by direct vasoconstrictor action on arteriolar muscles in some animals, but perhaps more generally by acting on the nervous system and adrenal paraneurons. In birds the hormone also has a hypotensive effect. ANG II stimulates the SNS in agnathans, elasmobranchs, teleosts, amphibians, reptiles, birds and mammals. Thus, modulation of sympathetic activity may be one of the most primitive and conservative functions of the RAS. For this reason, nonmammalian vertebrates are valuable models for studying the neurogenic actions of angiotensin II relevant to hypertensive disease.