The brain renin-angiotensin system and cardiovascular responses to stress: insights from transgenic rats with low brain angiotensinogen.

The renin-angiotensin system (RAS) has been identified as an attractive target for the treatment of stress-induced cardiovascular disorders. The effects of angiotensin (ANG) peptides during stress responses likely result from an integration of actions by circulating peptides and brain peptides derived from neuronal and glial sources. The present review focuses on the contribution of endogenous brain ANG peptides to pathways involved in cardiovascular responses to stressors. During a variety of forms of stress, neuronal pathways in forebrain areas containing ANG II or ANG-(1-7) are activated to stimulate descending angiotensinergic pathways that increase sympathetic outflow to increase blood pressure. We provide evidence that glia-derived ANG peptides influence brain AT(1) receptors. This appears to result in modulation of the responsiveness of the neuronal pathways activated during stressors that elevate circulating ANG peptides to activate brain pathways involving descending hypothalamic projections. It is well established that increased cardiovascular reactivity to stress is a significant predictor of hypertension and other cardiovascular diseases. This review highlights the importance of understanding the impact of RAS components from the circulation, neurons, and glia on the integration of cardiovascular responses to stressors.

Long-term systemic angiotensin II type 1 receptor blockade regulates mRNA expression of dorsomedial medulla renin-angiotensin system components.

In Fischer 344 (F344) rats, renin-angiotensin system (RAS) blockade for 1 yr with the angiotensin II type 1 (AT(1)) receptor blocker L-158,809 prevents age-related impairments in metabolic function, similar to transgenic rats with low glial angiotensinogen (Aogen). Brain RAS regulation may contribute to the benefits of long-term systemic AT(1) antagonism. We assessed the mRNA of RAS components in the dorsomedial medulla of F344 rats at 3 (young; n = 8) or 15 mo of age (old; n = 7) and in rats treated from 3 to 15 mo of age with 20 mg/l of the AT(1) receptor antagonist L-158,809 (Old+L; n = 6). Aogen and renin mRNA were lower in the young compared with old group. Angiotensin-converting enzyme (ACE) mRNA was lower in the old and Old+L compared with the young group. ACE2 and neprilysin expression were significantly higher in Old+L compared with young or old rats. AT(1b), AT(2), and Mas receptor mRNA were higher with treatment. Leptin receptor mRNA was lower in the old rats and this was prevented by L-158,809 treatment. Dual-specificity phosphatase 1 (DUSP1) mRNA was highest in the Old+L group. Aggregate correlate summation revealed a positive relationship for Mas receptor mRNA with food intake. The findings provide evidence for regulation of dorsomedial medullary renin and Aogen mRNA during aging. Long-term AT(1) receptor blockade increases the mRNA of the enzymes ACE2 and neprilysin and the MAS receptor, which could potentially shift the balance from ANG II to ANG-(1-7) and prevent age-related declines in the leptin receptor and its signaling pathway.

Blockade of the Renin-Angiotensin system improves insulin receptor signaling and insulin-stimulated skeletal muscle glucose transport in burn injury.

Burn injury is associated with a decline in glucose utilization and insulin sensitivity due to alterations in postreceptor insulin signaling pathways. We have reported that blockade of the renin-angiotensin system with losartan, an angiotensin II type 1 (AT1) receptor blocker, improves whole body insulin sensitivity and glucose metabolism after burn injury. This study examines whether losartan improves insulin signaling pathways and insulin-stimulated glucose transport in skeletal muscle in burn-injured rats. Rats were injured by a 30% full-skin-thickness scalding burn and treated with losartan or placebo for 3 days after burn. Insulin signaling pathways were investigated in rectus abdominus muscle taken before and 90 s after intraportal insulin injection (10 U·kg). Insulin-stimulated insulin receptor substrate 1-associated phosphatidylinositol 3-kinase and plasma membrane-associated GLUT4 transporter were substantially increased with losartan treatment in burn-injured animals (59% above sham). Serine phosphorylated AKT/PKB was decreased with burn injury, and this decrease was attenuated with losartan treatment. In a separate group of rats, the effect of insulin on 2-deoxyglucose transport was significantly impaired in burned as compared with sham soleus muscles, in vitro; however, treatment of burned rats with losartan completely abolished the reduction of insulin-stimulated 2-deoxyglucose transport. These findings demonstrate a cross talk between the AT1 and insulin receptor that negatively modulates insulin receptor signaling and suggest a potential role of renin-angiotensin system blockade as a therapeutic strategy for enhancing insulin sensitivity in skeletal muscle and improving whole-body glucose homeostasis in burn injury.

Long-term AT1 receptor blockade improves metabolic function and provides renoprotection in Fischer-344 rats.

Fischer-344 (F344) rats exhibit proteinuria and insulin resistance in the absence of hypertension as they age. We determined the effects of long-term (1 yr) treatment with the angiotensin (ANG) II type 1 (AT(1)) receptor blocker L-158,809 on plasma and urinary ANG peptide levels, systolic blood pressure (SBP), and indexes of glucose metabolism in 15-mo-old male F344 rats. Young rats at 3 mo of age (n = 8) were compared with two separate groups of older rats: one control group (n = 7) and one group treated with L-158,809 (n = 6) orally (20 mg/l) for 1 yr. SBP was not different between control and treated rats but was higher in young rats. Serum leptin, insulin, and glucose levels were comparable between treated and young rats, whereas controls had higher glucose and leptin with a similar trend for insulin. Plasma ANG I and ANG II were higher in treated than untreated young or older rats, as evidence of effective AT(1) receptor blockade. Urinary ANG II and ANG-(1-7) were higher in controls compared with young animals, and treated rats failed to show age-related increases. Protein excretion was markedly lower in treated and young rats compared with control rats (young: 8 +/- 2 mg/day vs. control: 129 +/- 51 mg/day vs. treated: 9 +/- 3 mg/day, P < 0.05). Long-term AT(1) receptor blockade improves metabolic parameters and provides renoprotection. Differential regulation of systemic and intrarenal (urinary) ANG systems occurs during blockade, and suppression of the intrarenal system may contribute to reduced proteinuria. Thus, insulin resistance, renal injury, and activation of the intrarenal ANG system during early aging in normotensive animals can be averted by renin-ANG system blockade.

Baroreceptor reflex regulation in anesthetized transgenic rats with low glia-derived angiotensinogen.

Endogenous angiotensin (ANG) II and ANG-(1-7) act at the nucleus tractus solitarius (NTS) to differentially modulate neural control of the circulation. The role of these peptides endogenous to NTS on cardiovascular reflex function was investigated in transgenic rats with low brain angiotensinogen (Aogen) due to glial overexpression of an antisense to Aogen (ASrAOGEN) and in Sprague-Dawley (SD) rats. Arterial baroreceptor reflex sensitivity (BRS) for control of heart rate (HR) in response to increases in mean arterial pressure (MAP) was tested before and after bilateral microinjection of the angiotensin type 1 (AT(1)) receptor blocker candesartan or the ANG-(1-7) receptor blocker (d-Ala(7))-ANG-(1-7) into the NTS of urethane-chloralose-anesthetized ASrAOGEN and SD rats. Baseline MAP was higher in ASrAOGEN than in SD rats under anesthesia (P < 0.01). Injection of candesartan or (d-Ala(7))-ANG-(1-7) decreased MAP (P < 0.01) and HR (P < 0.05) in ASrAOGEN, but not SD, rats. The BRS at baseline was similar in ASrAOGEN and SD rats. Candesartan increased BRS by 41% in SD rats (P < 0.01) but was without effect in ASrAOGEN rats. In contrast, the reduction in BRS after (d-Ala(7))-ANG-(1-7) administration was comparable in SD (31%) and ASrAOGEN rats (34%). These findings indicate that the absence of glia-derived Aogen is associated with 1) an increase in MAP under anesthesia mediated via AT(1) and ANG-(1-7) receptors within the NTS, 2) the absence of an endogenous ANG II contribution to tonic inhibition of BRS, and 3) a continued contribution of endogenous ANG-(1-7) to tonic enhancement of BRS.

Blockade of the renin-angiotensin system improves insulin sensitivity in thermal injury.

Insulin resistance after burn is associated with alterations in postreceptor insulin signaling and abnormal glucose homeostasis. The renin-angiotensin system (RAS) exerts a largely inhibitory role on insulin action and is activated after burn injury. We hypothesized that upregulation of RAS is involved in the development of insulin resistance in burned rats. We examined the possibility that an angiotensin II type 1 (AT1) receptor blocker, losartan, enhances insulin sensitivity and thereby increases glucose tolerance in thermally injured rats. A 30% body surface area burn was induced by immersion of the dorsum into water with a temperature level of 95 degrees C for 15 s. Sham-burned rats were immersed in water with a temperature level of 23 degrees C. Losartan (30 mg/kg per day) or placebo (water) was given by gavage immediately after the burn injury and daily for 3 days postburn injury, resulting in sham-burned, burn placebo, and burn losartan groups. Plasma angiotensin II levels between burn placebo and sham-burned groups were not different 3 days after burn injury. However, losartan significantly increased plasma angiotensin II levels (P < 0.05), suggesting blockade of the AT1 receptor. An oral glucose tolerance test was performed 3 days postburn injury. There was an increase in the area under the curve for insulin and the glucose insulin index in burn placebo group as compared with sham-burned group, indicating insulin resistance. Losartan treatment abolished the insulin resistance in burn as evidenced by an area under the curve for insulin and glucose insulin index lower than that in the burn placebo group and similar to that in the sham-burned group. This suggests that insulin resistance and glucose intolerance associated with burn injury is, in part, caused by RAS.

Rats with low brain angiotensinogen do not exhibit insulin resistance during early aging.

During aging increases in body weight, insulin resistance, and elevated systolic pressure contribute to the development of metabolic syndrome. Long-term systemic blockade of the renin-angiotensin system (RAS) with either an angiotensin (Ang) II type 1 (AT1) receptor antagonist or angiotensin converting enzyme inhibitor improves insulin sensitivity and decreases risk of new onset (type II) diabetes. However, the role of the brain RAS in mediating development of insulin insensitivity during aging is not known. Therefore, we compared responses to an oral glucose load in transgenic rats with selective antisense suppression of brain angiotensinogen (ASrAogen); (mRen2)27 rats with high brain angiotensin II; and control Hannover Sprague-Dawley (SD) rats, at wk 16 and 68 of age. ASrAogen animals had lower body weight than either SD or (mRen2)27 rats at both ages (p < 0.001). The oral glucose tolerance test at 16 wk in (mRen2)27 animals revealed a higher glucose-insulin index (154,421 +/- 11,231 units; p < 0.05) and a lower glucose-insulin index in ASrAogen rats (41,580 +/- 10,923 units, p < 0.05) compared to SD rats (97,134 +/- 19,822 units), suggesting insulin resistance in the (mRen2)27 and enhanced insulin sensitivity in the ASrAogen relative to SD rats. At 68 wk, the glucose-insulin index remained low in the ASrAogen rats as evidence of maintained insulin sensitivity during aging compared with either SD or (mRen2)27 (p < 0.05). SD animals do not differ from (mRen2)27 rats at 68 wk indicating the development of a state of relative insulin resistance with increased age in the SD rats. Moreover, there was a positive correlation (r = 0.44; p < 0.05) between body weight and the glucose-insulin index in SD, but not ASrAogen or (mRen2)27 rats. The relationships between insulin and leptin, insulin and glucose, and leptin and body weight observed in SD rats were absent in ASrAogen and (mRen2)27 rats. We conclude that the glial RAS plays a role in development of insulin resistance as well as influencing weight gain associated with early aging.

Growth, metabolism, and blood pressure disturbances during aging in transgenic rats with altered brain renin-angiotensin systems.

Transgenic rats with targeted decreased glial expression of angiotensinogen (ASrAogen rats) did not show an increase in systolic pressure compared with Sprague-Dawley (SD) rats during aging (15-69 wk of age). ASrAogen animals had lower body weights throughout the study, similar to reports for animals with systemic knockout of angiotensinogen or treated long term with renin-angiotensin system (RAS) blockers. Further characterization of indexes of growth and metabolism in ASrAogen rats compared with (mRen2)27 and SD rats, which express elevated versus normal brain and tissue angiotensin II levels, respectively, revealed that serum leptin was 100-200% higher in SD and (mRen2)27 rats at 46 wk and 69 wk of age. Consistent with low serum leptin, ASrAogen rats had higher food intake (73%) compared with SD or (mRen2)27 rats. (mRen2)27 rats had higher resting insulin levels than ASrAogen rats at all ages. Insulin levels were constant during aging in ASrAogen rats, whereas an increase occurred in SD rats, leading to higher insulin levels at 46 and 69 wk of age compared with ASrAogen rats. IGF-1 was comparable among strains at all ages, but (mRen2)27 rats had longer and ASrAogen rats had shorter tail lengths versus SD rats at 15 wk of age. In conclusion, reduced expression of glial angiotensinogen blunts the age-dependent rise in insulin levels and weight gain, findings that mimic the effects of long-term systemic blockade of the RAS or systemic knockout of angiotensinogen. These data implicate glial angiotensinogen in the regulation of body metabolism as well as hormonal mechanisms regulating blood pressure.

Impaired heart rate baroreflex in older rats: role of endogenous angiotensin-(1-7) at the nucleus tractus solitarii.

Age-related baroreflex reductions in function may originate from central neural dysregulation as well as vascular structural/functional changes. We determined the role of 2 angiotensin (Ang) peptides at the nucleus tractus solitarii in age-related baroreflex impairment. Baroreflex sensitivity control of heart rate in response to increases in blood pressure was tested in younger (3 to 5 months) and older (16 to 20 months) anesthetized male Sprague-Dawley rats before and after bilateral solitary tract injections of the Ang II type 1 (AT1) receptor antagonist candesartan (24 pmol) or the Ang-(1-7) antagonist (D-Ala7)-Ang-(1-7) (144 fmol or 24 pmol). Basal reflex sensitivity of older rats was significantly lower than younger rats. In younger rats, the reflex was facilitated by bilateral candesartan injections and attenuated by bilateral (D-Ala7)-Ang-(1-7) injections. In older rats, the reflex was facilitated by AT1 blockade; however, (D-Ala7)-Ang-(1-7) injected into the solitary tract nucleus had no effect. Neprilysin mRNA in the medulla was lower in older rats compared with younger rats, whereas angiotensin-converting enzyme (ACE), ACE2, and mas receptor mRNA levels of older rats did not differ from values of younger rats. Thus, opposing actions of endogenous Ang II and Ang-(1-7) in the solitary tract nucleus contribute to baroreflex function in response to increases in mean arterial pressure of younger rats. The attenuated counterbalancing effect of Ang-(1-7) on baroreflex function is lost in older rats, which may be attributable to diminished production of the peptide from neprilysin.

Divergent regulation of circulating and intrarenal renin-angiotensin systems in response to long-term blockade.

BACKGROUND/AIMS: Long-term treatment with angiotensin-converting enzyme (ACE) inhibitors or angiotensin (Ang) II type I (AT(1)) receptor blockers can improve kidney function and attenuate the progressive decline in kidney function associated with age. In this study in Wistar rats medicated for 22 months, we determined the effects of enalapril (10 mg/kg/day) and losartan (30 mg/kg/day) treatment, in comparison with vehicle (tap water), on renal AngII receptor density and circulating and urinary components of the renin-angiotensin system (RAS). METHODS: Kidney sections were incubated with [(125)I-sarcosine(1)-threonine(8)]AngII (0.6 nM) for Ang receptor density, and Ang peptides were determined using radioimmunoassays. RESULTS: Receptor density was approximately 50% higher in vasa recta, glomeruli, and tubulointerstitium in enalapril-treated rats and lower in vasa recta and glomeruli in losartan-treated relative to vehicle-treated rats. Losartan and enalapril treatment elevated plasma levels of AngI and Ang-(1-7) while AngII increased only in losartan-treated rats. In contrast, both treatments were associated with a reduction in urinary excretion of all three Ang peptides as compared with control rats. CONCLUSION: The reduction in urinary Ang peptides with losartan and enalapril treatment suggests that blockade of intrarenal AngII may be an important mechanism underlying the renoprotection seen with such treatments.

Central depletion of angiotensinogen is associated with elevated AT1 receptors in the SFO and PVN.

The brain renin-angiotensin system (RAS) is important in fluid balance and blood pressure regulation. In this study, we compared angiotensin (Ang) receptor density in the subfornical organ (SFO) and paraventricular nucleus (PVN) of a) brain angiotensinogen deficient rats (ASrAogen); b) those with high levels of brain Ang II [(mRen2)27]; c) Hannover Sprague Dawley (SD) rats at 48 and 68 wks of age. Since there was no difference between the two ages in any of the three strains, the data from the 48 and 68 wk time points were combined. There was a significantly higher level of AT1 receptors in the SFO and PVN of ASrAogen animals compared to both the SD and (mRen2)27 rats. This suggests that the brain RAS is important in regulating receptor density and that the differences may be explained by lower levels of the peptide locally. These higher levels of receptors suggest that the ASrAogen animals in adulthood and early aging would be more sensitive to either circulating or endogenous brain Ang II than the SD animals of similar age. In contrast, the similar receptor density in the (mRen2)27 and SD rats suggest that previous reports of reduced responses in the (mRen2)27 rats may result from differences in post receptor mechanisms such as intracellular signaling. Moreover, our data reveal that functional assessments are necessary in addition to receptor density levels to understand the consequences of long-term alterations in brain tissue peptides.