Renal angiotensin II receptors, hyperinsulinemia, and obesity

Angiotensin II, via activation of AT1 receptors in the kidney regulates sodium/fluid homeostasis and blood pressure. An exaggerated action of angiotensin II mediated via activation of AT1 receptors has been implicated in the increased renal sodium retention and the resetting of the pressure natriuresis in obesity related hypertension. Treatment of obese Zucker rats with AT1 receptor blockers reduces blood pressure to a greater extent and produces greater natriuresis. Also, there is an increased membranal AT1 receptor numbers and angiotensin II produces greater activation of sodium transporters in the isolated tubules from obese Zucker rats. Interestingly, AT2 receptors, which are believed to be beneficial to the renal and cardiovascular function in terms of their action on kidney and blood vessels, are greatly increased in proximal tubular membranes of obese Zucker rats. Whole animal and in vitro studies indicate that higher plasma insulin level, generally associated with obesity, is responsible for the up-regulation of both AT1 and AT2 receptors in the kidney. Determining the consequence of selective blocking of AT1 receptors and/or activation of the AT2 receptors on renal and cardiovascular function, and the effect of lowering insulin on these receptors present an important area of further investigation in obesity.     

Enhanced Angiotensin II-Induced Activation of Na+, K+-ATPase in the Proximal Tubules of Obese Zucker Rats

Renal angiotensin II (AII) is suggested to play a role in the enhanced sodium reabsorption that causes a shift in pressure natriuresis in obesity related hypertension; however, the mechanism is not known. Therefore, to assess the influence of AII on tubular sodium transport, we determined the effect of AII on the Na⁺, K⁺-ATPase activity (NKA), an active transporter regulated by the AT₁ receptor activity, in the isolated proximal tubules of lean and obese Zucker rats. Also, we determined the levels of the tubular AT₁ receptor and associated signal transducing G proteins, as the initial signaling components that mediate the effects of AII on Na⁺, K⁺-ATPase activity. In the isolated proximal tubules, AII produced greater stimulation of the NKA activity in obese compared with lean rats. Determination of the AT₁ receptors by Scatchard analysis of the [¹²⁵I] Sar-Ang II binding and Western blot analysis in the basolateral (BLM) and brush border membrane (BBM) revealed a modest but significant increase (23%) in the AT₁ receptor number mainly in the BLM of obese compared with lean rats. The AII affinity for AT₁ receptors, as determined by IC₅₀ values of AII to displace [¹²⁵I] Sar-Ang II binding in BLM and BBM were similar in lean and obese rats. Western blot analysis revealed significant increases in Giα₁, Giα₂, Giα₃, and Gq/₁₁α in BLM and Giα₁, Giα₃, and Gq/₁₁α in BBM of obese as compared with lean rats. The increase in the levels of the AT₁ receptor and G proteins, mainly in the BLM, may be contributing to the enhanced AII-induced activation of NKA in the proximal tubules of obese rats. This phenomenon, in part, may be responsible for the increased sodium reabsorption and the development of hypertension in obese Zucker rats.     

Increased Renal Angiotensin II AT1 Receptor Function in Obese Zucker Rat

Angiotensin II (Ang II) via the activation of AT₁ receptors and subsequent stimulation of the tubular sodium transporters increases sodium and water reabsorption in the proximal tubule. An enhanced tubular action of Ang II is implicated in obesity related hypertension; however, the mechanism of such a phenomenon is unknown. Present study was designed to determine the AT₁ receptor numbers and function in the proximal tubule of obese and lean Zucker rats. Obese Zucker rats were hypertensive and hyperinsulinemic. The plasma renin activity was similar in the lean and obese rats. Angiotensin II stimulated the Na,H‐exchanger (NHE) activity in the proximal tubule, but the stimulatory response was markedly greater in obese than in lean rats. Similarly, Ang II caused greater inhibition in cAMP accumulation in the proximal tubule of obese compared to lean rats. The [¹²⁵I]sar‐Ang II binding revealed a 100% increase in the AT₁ receptor number in the brush border membrane (BBM) of obese compared to lean rats. The Western blot analysis revealed a 36–51% increase in the Giα₁ and Giα₃ in the BBM of obese compared to lean rats. We conclude that increases in the AT₁ receptor number and abundance of the Giα on BBM may be responsible for the enhanced signaling and subsequent greater stimulation of NHE by Ang II in proximal tubules of obese rats. The greater stimulation of NHE by Ang II may contribute to the increased tubular sodium reabsorption and to the hypertension in obese Zucker rats.     

Renal angiotensin II type-2 receptors are upregulated and mediate the candesartan-induced natriuresis/diuresis in obese Zucker rats

Recently, there has been a growing interest in studying the role of angiotensin II type-2 (AT(2)) receptor in renal/cardiovascular function in pathological conditions. The present study was designed to determine the functional role of the AT(2) receptors on natriuresis/diuresis and compare the level of the tubular AT(2) receptor expression in obese and lean Zucker rats (12 weeks old). Under anesthesia, candesartan (angiotensin II type 1 [AT(1)]-specific antagonist; 100 microg/kg bolus) produced natriuresis/diuresis to a greater degree in obese than in lean rats. The specific AT(2) antagonist PD123319 (50 microg/kg per minute) after candesartan administration abolished the natriuretic/diuretic effects of candesartan in obese rats but not in lean rats. Infusion of AT(2) receptor agonist, CGP-42112A (1 microg/kg per minute), produced greater increase in sodium and urine excretion over basal in obese than in lean rats. The presence of the AT(2) receptor expression in the brush-border and basolateral membranes was confirmed by Western blotting using specific antibody and antigen-blocking peptide. Densitometric analysis of the bands revealed approximately 1.5- to 2.0-fold increase in the AT(2) receptor proteins in both membranes of obese compared with lean rats. Our results suggest upregulation of the AT(2) receptors, which play a role in mediating the natriuretic/diuretic effects of AT(1) receptor blockers in obese Zucker rats. We speculate that AT(2) receptors, by promoting sodium excretion, may protect obese Zucker rats against blood pressure increase associated with sodium and water retention.     

In hypertension, the kidney breaks your heart

The renin-angiotensin system (RAS) is a master regulator of blood pressure and fluid homeostasis. Because RAS components are expressed in several tissues that may influence blood pressure, studies using conventional gene targeting to globally interrupt the RAS have not determined the contributions of angiotensin II receptor type 1 (AT₁) receptors in specific tissue pools to blood pressure regulation and tissue injury. Recent experiments using kidney cross-transplantation and mice lacking the dominant murine AT₁ receptor isoform, AT 1A, have demonstrated that 1) AT₁ receptors inside and outside the kidney make equivalent contributions to normal blood pressure homeostasis, 2) activation of renal AT 1 receptors is required for the development of angiotensin II-dependent hypertension, and 3) this blood pressure elevation rather than activation of AT₁ receptors in the heart drives angiotensin II-induced cardiac hypertrophy. These findings, together with previous experiments, confirm the kidney’s critical role in the pathogenesis of hypertension and its complications.     

Novel actions of angiotensin II via its renal type-2 (AT2) receptor

The vast majority of the biologic effects of angiotensin II have been considered to be mediated by the subtype-1 (AT₁) receptor. The AT₂ receptor is expressed to a low degree in most adult cells and tissues, and its function has not been understood. Recent studies, however, have identified novel actions of angiotensin II mediated by the AT₂ receptor in the kidney. These AT₂ receptor actions have importance in the control of blood pressure and hypertension. The AT₂ receptor mediates a renal vasodilator cascade, including generation of bradykinin, nitric oxide, and cyclic GMP. This action of angiotensin II occurs when the renin-angiotensin system is activated, as in sodium depletion. The AT₂ receptor also appears to mediate prostaglandin (PG) F_α formation, probably by stimulating conversion of PGE2 to PGF_α. The AT₂ receptor plays a counter-regulatory vasodilator role opposing the vasoconstrictor actions of angiotensin II. The AT₁ and AT₂ receptors engage in inter-receptor “cross-talk.” In the absence of the AT₂ receptor, sustained angiotensin II pressor and antinatriuretic hypersensitivity occurs, mediated by a deficiency of bradykinin, nitric oxide, and cyclic GMP. The AT₂ receptor may play an important role in stimulating pressure natriuresis, but definitive studies are required to resolve this issue. The AT₂ receptor mediates several renal actions of angiotensin II, appears to be important in the physiologic regulation of blood pressure, and may be involved in the pathophysiology of hypertension.     

At1 Receptor Density Changes During Development of Hypertension in Hyperinsulinemic Rats

In a previous study we showed that the renin-angiotensin system (RAS) plays a role in the etiology of fructose-induced hypertension. To our knowledge, no previous study has evaluated changes in angiotensin II (Ang II) type I receptor (AT₁) density in fructose-fed rats that are insulin resistant and hypertensive. The purpose of this study was to determine the changes in plasma Ang II and AT₁ density associated with the elevation of blood pressure in fructose-treated rats. Male Sprague-Dawley rats were divided into two groups and were fed either normal rat chow or a 60% fructose-enriched diet for four weeks. Plasma Ang II and serum insulin levels of the fructose-treated rats were significantly elevated (p<0.01) by the end of the second week of fructose treatment. Plasma Ang II levels of the fructose-fed rats returned to basal levels by the end of the fourth week of dietary treatment, whereas the serum insulin levels consistently remained elevated. Blood pressure was significantly elevated in the fructose-fed rats within two weeks of fructose treatment. Elevation of blood pressure was associated with left ventricular hypertrophy. Furthermore, there was a significant increase in AT₁ receptor density in the ventricles and a significant decrease in AT₁ receptor density in the aortas of fructose-fed rats at the end of fourth week. There were no significant changes in receptor density in the hypothalami or adrenal glands of fructose-treated rats. These results suggest that chronic fructose treatment activates the renin-angiotensin system, which is manifested by an increase in plasma Ang II, elevation of blood pressure, cardiac hypertrophy, and changes in AT₁ receptor density.     

The significance of brain aminopeptidases in the regulation of the actions of angiotensin peptides in the brain

From the outset, the concept of a brain renin-angiotensin system (RAS) has been controversial and this controversy continues to this day. In addition to the unresolved questions as to the means by which, and location(s) where brain Ang II is synthesized, and the uncertainties regarding the functionality of the different subtypes of Ang II receptors in the brain, a new controversy has arisen with respect to the identity of the angiotensin peptide(s) that activate brain AT₁ receptors. While it has been known for some time that Ang III can activate Ang II receptors with equivalent or near-equivalent efficacy to Ang II, it has been proposed that in the brain, only Ang III is active. This proposal, which we have named “The Angiotensin III Hypothesis” states that Ang II must be converted to Ang III in order to activate brain AT₁ receptors. This review examines several aspects of the controversies regarding the brain RAS with a special focus on brain aminopeptidases, studies that either support or refute The Angiotensin III Hypothesis, and the implications of The Angiotensin III Hypothesis for the activity of the brain RAS. It also addresses the need for further research that can test The Angiotensin III Hypothesis and definitively identify the angiotensin peptide(s) that activate brain AT₁ receptor-mediated effects.     

Enhanced angiotensin II-induced activation of Na+, K+-ATPase in the proximal tubules of obese Zucker rats

Renal angiotensin II (AII) is suggested to play a role in the enhanced sodium reabsorption that causes a shift in pressure natriuresis in obesity related hypertension; however, the mechanism is not known. Therefore, to assess the influence of AII on tubular sodium transport, we determined the effect of AII on the Na+, K+-ATPase activity (NKA), an active transporter regulated by the AT1 receptor activity, in the isolated proximal tubules of lean and obese Zucker rats. Also, we determined the levels of the tubular AT1 receptor and associated signal transducing G proteins, as the initial signaling components that mediate the effects of AII on Na+, K+-ATPase activity. In the isolated proximal tubules, AII produced greater stimulation of the NKA activity in obese compared with lean rats. Determination of the AT1 receptors by Scatchard analysis of the [125I] Sar-Ang II binding and Western blot analysis in the basolateral (BLM) and brush border membrane (BBM) revealed a modest but significant increase (23%) in the AT1 receptor number mainly in the BLM of obese compared with lean rats. The AII affinity for AT1 receptors, as determined by IC50 values of AII to displace [125I] Sar-Ang II binding in BLM and BBM were similar in lean and obese rats. Western blot analysis revealed significant increases in Gialpha1, Gialpha2, Gialpha3, and Gq/11alpha in BLM and Gialpha1, Gialpha3, and Gq/11alpha in BBM of obese as compared with lean rats. The increase in the levels of the AT1 receptor and G proteins, mainly in the BLM, may be contributing to the enhanced AII-induced activation of NKA in the proximal tubules of obese rats. This phenomenon, in part, may be responsible for the increased sodium reabsorption and the development of hypertension in obese Zucker rats.     

Evidence for a local angiotensin-generating system and dose-dependent inhibition of glucose-stimulated insulin release by angiotensin II in isolated pancreatic islets

Aims/hypothesis A local angiotensin-generating system has been found in the exocrine pancreas. This study aimed, primarily, to investigate the existence of a local angiotensin-generating system in the pancreatic islets and, secondly, to elucidate its role in regulating insulin secretion.Methods Real-time RT-PCR and western blot were used to investigate if angiotensin-generating components are present in the mouse pancreatic islets, which are subject to regulation by islet transplantation. The localisation of AT₁-receptors in islets was investigated by immunohistochemistry. Batch-type incubations of isolated islets were applied for studying the influence of angiotensin II on the glucose-stimulated insulin release, glucose oxidation and (pro)insulin, and total protein biosynthesis.Results Major components, namely angiotensinogen, ACE, AT ₁ - and AT ₂ -receptors, were expressed in endogenous islets. AT₁-receptors were localised to pancreatic beta cells. Exposure of the isolated islets to angiotensin II induced a dose-dependent inhibition of glucose-stimulated insulin release and inhibited (pro)insulin biosynthesis. This inhibitory action was fully preventable by pretreatment of the islets with losartan, an AT₁-receptor antagonist. We also investigated if the expression of these components was changed after islet transplantation. Notably, a markedly increased expression of mRNA for the AT ₁ -receptor was observed in islets retrieved from 4-week-old syngeneic islet transplants, a finding that was confirmed at the protein level.Conclusion/interpretation These data indicate the existence of an islet angiotensin-generating system of potential importance in the physiological regulation of glucose-induced insulin secretion, thus diabetes mellitus. The increased expression of the AT₁-receptor in islet transplants could have relevance to islet-graft function.Abbreviations Ang II Angiotensin II - AT₁ angiotensin II receptor type 1 - AT₂ angiotensin II receptor type 2 - Ao angiotensinogen - RAS Renin-angiotensin system - KRBB Krebs-Ringer bicarbonate buffer     

OBESE FEMALE SHHF/Mcc-facp RATS RESIST ANTIHYPERTENSIVE EFFECTS OF RENIN-ANGIOTENSIN SYSTEM INHIBITION

Gender and obesity may influence response to pharmacological modulation of the renin-angiotensin system. We used SHHF/Mcc-fa^cp rats to study effect of obesity and gender on the ability of an AT₁ receptor antagonist to decrease blood pressure. After 2 weeks treatment with irbesartan (50 mg/kg), only lean and obese males showed significant decreases in blood pressure, while obese females were completely resistant. Lean females showed a trend toward lowering of pressure (p = 0.06). However, irbesartan similarly shifted angiotensin II dose response curves to the right in all groups. Twelve weeks of irbesartan also failed to decrease blood pressure, but did significantly reduce heart weight in obese females. In untreated rats, obese females had lower plasma renin activity and serum angiotensin converting enzyme activity compared to lean males, while lean and obese females had increased urinary endothelin excretion. Despite an otherwise similar genetic background contributing to hypertension and heart failure, obese females have different patterns of humoral activation compared to lean males, which may contribute to their resistance to the depressor effects of irbesartan.     

Expression and localization of the AT1 and AT2 angiotensin II receptors and α1A and α1D adrenergic receptors in aorta of hypertensive and diabetic rats

Hypertension and diabetes are multifactorial diseases that frequently coexist and exacerbate each another. During the development of diabetes, the impairment of noradrenergic and renin-angiotensin systems has been reported in the response mediated by α₁-AR and AT₁ receptors. Although their participation in the development of cardiovascular complications is still controversial, some studies have found increased or diminished response to the vasoconstrictive effect of noradrenaline or angiotensin II in a time-dependent manner of diabetes. Thus, the aim of this work was to investigate the possible changes in the expression or localization of α₁-AR (α_1A and α_1D) and angiotensin II receptors (AT₁ and AT₂) in aorta of rats after 4 weeks of the onset of diabetes. In order to be able to examine the expression of these receptors, immunofluorescence procedure was performed in tunica intima and tunica media of histological sections of aorta. Fluorescence was detected by a confocal microscopy. Our results showed that the receptors are expressed in both tunics, where adrenergic receptors have a higher density in tunica intima and tunica media of SHR compared with WKY; meanwhile, the expression of angiotensin II receptors is not modified in both groups of rats. On the other hand, the results showed that diabetes produced an increase or a decrease in the expression of receptors that is not associated to a specific type of receptor, vascular region, or strain of rat. In conclusion, diabetes and hypertension modify the expression of the receptors in tunica intima and tunica media of aorta in a different way.     

Defective dopamine receptor function in proximal tubules of obese zucker rats

Some of the pathophysiological consequences of obesity include insulin resistance, increased renal sodium reabsorption, and the development of hypertension. Dopamine promotes renal sodium excretion via activation of D(1)-like receptors present on the proximal tubules. Reduced dopamine-induced natriuresis and a defect in D(1)-like receptor function have been reported in the proximal tubules of hypertensive animals. The present study investigated D(1)-like dopamine receptors and associated G proteins as the initial signaling components in the proximal tubular basolateral membranes of obese Zucker and control lean Zucker rats. We found that the obese rats were hyperinsulinemic, hyperglycemic, and hypertensive compared with the lean rats. Dopamine produced concentration-dependent inhibition of Na,K-ATPase activity in the proximal tubules of lean rats, whereas the inhibitory effect of dopamine was reduced in obese rats. The D(1)-like receptors measured by [(3)H]SCH 23390 binding revealed an approximately 45% decrease in B(max) without a change in K(d) in the basolateral membranes of obese rats compared with lean rats. Although we found an increase in G(q)/11alpha and no change in G(s)alpha in the basolateral membranes of obese rats, dopamine and SKF 38393 failed to stimulate G proteins as measured by [(35)S]GTPgammaS binding in obese rats, suggesting a receptor-G protein coupling defect. We conclude that decrease in D(1)-like dopamine receptor binding sites and diminished activation of G proteins, resulting perhaps from defective coupling, led to the reduced inhibition by dopamine of Na,K-ATPase activity in the proximal tubules of obese Zucker rats. Such a defect in renal dopamine receptor function may contribute to sodium retention and development of hypertension in obese rats.     

Renal sympathetic denervation lowers arterial pressure in canines with obesity-induced hypertension by regulating GAD65 and AT1R expression in rostral ventrolateral medulla

To explore the roles of glutamate acid decarboxylase 65 (GAD65) and angiotensin II type 1 receptor (AT₁R) in the action of renal sympathetic denervation (RSD) on obesity-induced hypertension in canines. Thirty-two beagles were randomly divided into a hypertensive model (n = 22) and control (n = 10) groups. A hypertensive canine model was established by feeding a high-fat diet. Twenty hypertensive beagles were randomized equally to a sham-surgery and RSD-treated group receiving catheter-based radiofrequency RSD. Compared with the control group, the sham-surgery group exhibited significant increases in blood pressure, serum angiotensin II level, rostral ventrolateral medulla (RVLM) glutamate level, and AT₁R mRNA and protein expression and decreases in γ-amino acid butyric acid (γ-GABA) level and GAD65 mRNA and protein expression in the RVLM (all P < 0.05). Treatment with RSD significantly attenuated the above abnormal alterations (all P < 0.05). Linear correlation analysis revealed that angiotensin II level was positively correlated with glutamate level (r = 0.804) and inversely correlated with γ-GABA level (r = ?0.765). GAD65 protein expression was positively correlated with γ-GABA level (r = 0.782). Catheter-based radiofrequency RSD can decrease blood pressure in obesity-induced hypertensive canines. The antihypertensive mechanism might be linked to upregulation of GAD65 and downregulation of AT₁R in the RVLM.     

Angiotensin AT1 receptor inhibition, angiotensin-converting enzyme inhibition, and combination therapy with developing heart failure: Cellular mechanisms of action

Background: Past studies have shown that angiotensin-converting enzyme inhibition (ACEI) alone, angiotensin AT₁ receptor blockade (AT₁ block) alone, and combined treatment have differential effects on left ventricular (LV) function and geometry with developing congestive heart failure (CHF). The purpose of this study was to more carefully examine the cellular basis for these differential effects by using a model of pacing CHF.Methods and Results: Pigs were randomly assigned to five groups: (1) rapid pacing (240 bpm) for 3 weeks (n = 9), (2) concomitant ACEI (benazeprilat, 0.187 mg/kg/day) and pacing (n = 9), (3) concomitant AT₁ block (valsartan, 3 mg/kg/day) and pacing (n = 9), (4) concomitant ACEI and AT₁ receptor blockade (benazeprilat/valsartan, 0.05/3 mg/kg/day, respectively) and pacing (n = 9), and (5) sham controls (n = 10). The dosage protocol was based on obtaining a 50% reduction in angiotensin I and angiotensin II pressor response with no significant effects on mean basal arterial pressure. In the pacing group, LV fractional shortening (LVFS) fell compared with control group (13.4 ± 1.4 v 39.1 ± 1.0%, P < .05). With AT₁ block, LVFS was unchanged from pacing only. ACEI and combined treatment increased LVFS from pacing values (25.2 ± 0.9 v 20.9 ± 1.9%, respectively, P < .05). LV myocyte shortening velocity was reduced with chronic pacing compared with control group (27.2 ± 0.6 v 58.6 ± 1.2 μm/s, P < .05) and remained reduced with AT₁ block (28.0 ± 0.5 μm/s, P < .05). Myocyte shortening velocity increased with ACEI or combination treatment (36.9 ± 0.7 ± 0.7 v 42.3 ± 0.8 μm/s, respectively, P < .05). Concomitant treatment with either ACEI or AT₁ blockade normalized myocyte action potential duration. In the combined ACEI and AT₁ blockade group, all parameters of the myocyte action potential were unchanged from control values.Conclusions: This study showed that combined ACEI and AT₁ receptor blockade produced beneficial effects on myocyte contractility and electrophysiology when compared with either monotherapy alone and therefore may provide unique benefits with CHF.     

Angiotensin II AT1 Receptor/Signaling Mechanisms in The Biphasic Effect of The Peptide on Proximal Tubular Na+,K+-ATPase

The present study was designed to determine the cellular signaling mechanisms responsible for mediating the effects of angiotensin II on proximal tubular Na⁺,K⁺-ATPase activity. Angiotensin II produced a biphasic effect on Na⁺,K⁺-ATPase activity: stimulation at 10^-13-10^-10M followed by inhibition at 10^-7-10^-5M of angiotensin II. The stimulatory and inhibitory effects of angiotensin II were antagonized by losartan (1nM) suggesting the involvement of AT, receptor. Angiotensin IT produced inhibition of forskolin-stimulated CAMP accumulation at 10^-13-10^-10M followed by a stimulation in basal CAMP levels at 10^-7-10^-5M. Pretreatment of proximal tubules with losartan (1nM) antagonized both the stimulatory and inhibitory effects of angiotensin II on CAMP accumulation. Pretreatment of the proximal tubules with pertussis toxin (PTx) abolished the stimulation of Na⁺,K⁺-ATPase activity but did not affect the inhibition of Na⁺,K⁺-ATPase activity produced by angiotensin II. Pretreatment of the tubules with cholera toxin did not alter the biphasic effect of angiotensin II on Na⁺,K⁺-ATPase activity. Mepacrine (l0μM), a phospholipase A2 (PLA2) inhibitor, reduced only the inhibitory effect of angiotensin II on Na⁺,K⁺-ATPase activity. These results suggest that the activation of AT₁angiotensin II receptors stimulates Na⁺,K⁺-ATPase activity via a PTx-sensitive G protein-linked inhibition of adenylyl cyclase pathway, whereas the inhibition of Na⁺,K⁺-ATPase activity following AT₁receptor activation involves multiple signaling pathways which may include stimulation of adenylyl cyclase and PLA2     

Diminished Natriuretic Response to Dopamine D1 Receptor Agonist,SKF‐38393 in Obese Zucker Rats

Dopamine causes natriuresis and diuresis via activation of D₁ receptors located on the renal proximal tubules and subsequent inhibition of the sodium transporters, Na‐H exchanger and Na⁺/K⁺ ATPase. We have reported that dopamine fails to inhibit the activities of these two transporters in the obese Zucker rats (OZR). The present study was designed to examine the functional consequence of this phenomenon by determining the natriuretic and diuretic response to D₁ receptor activation in lean Zucker rats (LZR) and OZR. In 11–12 week‐old OZR and LZR, natriuretic and diuretic responses to intravenously administered D₁ receptor agonist, SKF 38393 (3 µg/kg/min for 30 min) were measured under Inactin^® anesthesia. Plasma insulin and glucose levels were significantly higher in the obese rats as compared to the lean rats. Intravenous infusion of SKF 38393 caused significant increases in urine flow, urinary sodium excretion (U_NaV), fractional excretion of sodium (FE_Na), and glomerular filtration rate (GFR) in the lean rats. However, the natriuretic and diuretic response to SKF 38393 was markedly blunted in OZR. Infusion of SKF 38393 did not cause significant changes in the mean blood pressure and heart rate in either of the two groups. We suggest that the diminished natriuretic response to D₁ receptor activation in OZR is the consequence of the previously reported defect in the D₁ receptor–G‐protein coupling and the failure of dopamine to inhibit the sodium transporters in these animals.     

Long-term low salt diet increases blood pressure by activation of the renin-angiotensin and sympathetic nervous systems

ABSTRACT

Background The aim of this study was to investigate the effect of long-term low salt diet on blood pressure and its underlying mechanisms.

Methods Male Sprague-Dawley (SD) rats were divided into normal salt diet group (0.4%) and low salt diet group (0.04%). Blood pressure was measured with the non-invasive tail-cuff method. The contractile response of isolated mesenteric arteries was measured using a small vessel myograph. The effects on renal function of the intrarenal arterial infusion of candesartan (10 μg/kg/min), an angiotensin II receptor type 1 (AT₁R) antagonist, were also measured. The expressions of renal AT₁R and mesenteric arterial α_1A, α_1B, and α_1D adrenergic receptors were quantified by immunoblotting. Plasma levels of angiotensin II were also measured.

Results Systolic blood pressure was significantly increased after 8 weeks of low salt diet. There were no obvious differences in the renal structure between the low and normal salt diet groups. However, the plasma angiotensin II levels and renal AT₁R expression were higher in low than normal salt diet group. The intrarenal arterial infusion of candesartan increased urine flow and sodium excretion to a greater extent in the low than normal salt diet group. The expressions of α_1A and α_1D, but not α_1B, adrenergic receptors, and phenylephrine-induced contraction were increased in mesenteric arteries from the low salt, relative to the normal salt diet group.

Conclusion Activation of the renin-angiotensin and sympathetic nervous systems may be involved in the pathogenesis of long-term low salt diet-induced hypertension.     

Intrarenal angiotensin III is the predominant agonist for proximal tubule angiotensin type 2 receptors

In angiotensin type 1 receptor-blocked rats, renal interstitial (RI) administration of des-aspartyl(1)-angiotensin II (Ang III) but not angiotensin II induces natriuresis via activation of angiotensin type 2 receptors. In the present study, renal function was documented during systemic angiotensin type 1 receptor blockade with candesartan in Sprague-Dawley rats receiving unilateral RI infusion of Ang III. Ang III increased urine sodium excretion, fractional sodium, and lithium excretion. RI coinfusion of specific angiotensin type 2 receptor antagonist PD-123319 abolished Ang III-induced natriuresis. The natriuretic response observed with RI Ang III was not reproducible with RI angiotensin (1-7) alone or together with angiotensin-converting enzyme inhibition. Similarly, neither RI angiotensin II alone or in the presence of aminopeptidase A inhibitor increased urine sodium excretion. In the absence of systemic angiotensin type 1 receptor blockade, Ang III alone did not increase urine sodium excretion, but natriuresis was enabled by the coinfusion of aminopeptidase N inhibitor and subsequently blocked by PD-123319. In angiotensin type 1 receptor-blocked rats, RI administration of aminopeptidase N inhibitor alone also induced natriuresis that was abolished by PD-123319. Ang III-induced natriuresis was accompanied by increased RI cGMP levels and was abolished by inhibition of soluble guanylyl cyclase. RI and renal tissue Ang III levels increased in response to Ang III infusion and were augmented by aminopeptidase N inhibition. These data demonstrate that endogenous intrarenal Ang III but not angiotensin II or angiotensin (1-7) induces natriuresis via activation of angiotensin type 2 receptors in the proximal tubule via a cGMP-dependent mechanism and suggest aminopeptidase N inhibition as a potential therapeutic target in hypertension.     

Angiotensin II type 2 receptor agonist directly inhibits proximal tubule sodium pump activity in obese but not in lean Zucker rats

We have reported recently that the renal angiotensin II type 2 (AT2) receptors are upregulated and involved in promoting natriuresis/diuresis in obese but not in lean Zucker rats. In the present study, we tested the hypothesis that there is an enhanced AT2 receptor signaling via NO/cGMP pathway leading to greater inhibition of the Na(+), K(+)-ATPase (NKA) activity in the proximal tubules (PT) of obese rather than lean Zucker rats. The AT2 agonist CGP42112 (0.1 to 100 nmol/L) inhibited (33% at 100 nmol/L) the NKA activity in the PTs of obese but not in lean Zucker rats. The AT2 antagonist PD123319 (1 micromol/L), not the angiotensin II type 1 antagonist losartan (1 micromol/L), significantly diminished the CGP42112-induced inhibition of the NKA activity in obese rats. The AT2 agonist (10 nmol/L)-induced NKA inhibition was abolished by the soluble guanylate cyclase inhibitor 1H-[1,2,4] oxadiazolo-[4,3-a] quinoxalin-1-one (10 micromol/L), the NO synthase inhibitor NG-nitro-L-arginine methyl ester (100 micromol/L), and the protein kinase G inhibitor K1388 (2 micromole/L). CGP42112 (10 nmol/L) caused an increase in serine phosphorylation of NKA alpha1-subunit in PT of obese rats. Measurement of cGMP and NO revealed that CGP42112 (0.1 to 100 nmol/L) increased cGMP and NO accumulation in the PTs of obese but not lean rats. The CGP42112-induced stimulation of NO and cGMP was blocked by PD123319 (1 micromol/L), NG-nitro-L-arginine methyl ester (100 micromol/L), and 1H-[1,2,4] oxadiazolo-[4,3-a] quinoxalin-1-one (10 micromol/L) but not by losartan (1 micromol/L). The data suggest that the AT2 receptor activation via stimulation of the NO/cGMP/protein kinase G pathway directly inhibits the tubular NKA activity that provides as a mechanism responsible for the AT2 receptor-mediated natriuresis in obese but not in lean Zucker rats.