Ellen N. Tommasi

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.

The AT1 Receptor Antagonist, L-158,809, Prevents or Ameliorates Fractionated Whole-Brain Irradiation–Induced Cognitive Impairment

PURPOSE We hypothesized that administration of the angiotensin type 1 (AT1) receptor antagonist, L-158,809, to young adult male rats would prevent or ameliorate fractionated whole-brain irradiation (WBI)-induced cognitive impairment. MATERIALS AND METHODS Groups of 80 young adult male Fischer 344 x Brown Norway (F344xBN) rats, 12-14 weeks old, received either: (1) fractionated WBI; 40 Gy of gamma rays in 4 weeks, 2 fractions/week, (2) sham-irradiation; (3) WBI plus L-158,809 (20 mg/L drinking water) starting 3 days prior, during, and for 14, 28, or 54 weeks postirradiation; and (4) sham-irradiation plus L-158,809 for 14, 28, or 54 weeks postirradiation. An additional group of rats (n = 20) received L-158,809 before, during, and for 5 weeks postirradiation, after which they received normal drinking water up to 28 weeks postirradiation. RESULTS Administration of L-158,809 before, during, and for 28 or 54 weeks after fractionated WBI prevented or ameliorated the radiation-induced cognitive impairment observed 26 and 52 weeks postirradiation. Moreover, giving L-158,809 before, during, and for only 5 weeks postirradiation ameliorated the significant cognitive impairment observed 26 weeks postirradiation. These radiation-induced cognitive impairments occurred without any changes in brain metabolites or gross histologic changes assessed at 28 and 54 weeks postirradiation, respectively. CONCLUSIONS Administering L-158,809 before, during, and after fractionated WBI can prevent or ameliorate the chronic, progressive, cognitive impairment observed in rats at 26 and 52 weeks postirradiation. These findings offer the promise of improving the quality of life for brain tumor patients.

Injections of angiotensin‐converting enzyme 2 inhibitor MLN4760 into nucleus tractus solitarii reduce baroreceptor reflex sensitivity for heart rate control in rats

Injections of the angiotensin(1–7) [Ang(1–7)] antagonist [d‐Ala7]‐Ang(1–7) into the nucleus of the solitary tract (NTS) of Sprague–Dawley rats reduce baroreceptor reflex sensitivity (BRS) for control of heart rate by ∼40%, whereas injections of the angiotensin II (Ang II) type 1 receptor antagonist candesartan increase BRS by 40% when reflex bradycardia is assessed. The enzyme angiotensin‐converting enzyme 2 (ACE2) is known to convert Ang II to Ang(1–7). We report that ACE2 activity, as well as ACE and neprilysin activities, are present in plasma membrane fractions of the dorsomedial medulla of Sprague–Dawley rats. Moreover, we show that BRS for reflex bradycardia is attenuated (1.16 ± 0.29 ms mmHg−1 before versus 0.33 ± 0.11 ms mmHg−1 after; P < 0.05; n= 8) 30–60 min following injection of the selective ACE2 inhibitor MLN4760 (12 pmol in 120 nl) into the NTS. These findings support the concept that within the NTS, local synthesis of Ang(1–7) from Ang II is required for normal sensitivity for the baroreflex control of heart rate in response to increases in arterial pressure.

Angiotensin-(1-7) reduces renal angiotensin II receptors through a cyclooxygenase-dependent mechanism

In the kidney, angiotensin-(1–7) [Ang-(1–7)] exhibits diuretic and natriuretic properties associated with an increase in prostaglandin production. The prohypertensive effects of Ang II are attenuated in rats infused with Ang-(1–7), consistent with recent work showing that Ang-(1–7) downregulates AT1 receptors in Chinese hamster ovary–AT1A or vascular smooth muscle cells. To determine whether exposure to Ang-(1–7) reduces AT1 receptors in the kidney through an increase in prostaglandin production, kidney slices from Sprague-Dawley rats were incubated with 10 n M–1 &mgr;M Ang-(1–7) in the presence or absence of 5 &mgr;M meclofenamate, a cyclooxygenase inhibitor. Following these treatments, the kidney slices were retrieved, frozen, and sectioned for determination of [125I]-Ang II binding using in vitro receptor autoradiography. Greater than 90% of the specific binding was competed for by losartan, indicating that the majority of binding was to the AT1 receptor. Incubation of kidney slices with 1 &mgr;M Ang-(1–7) caused a 20% reduction in [125I]-Ang II binding (n = 8) in the cortical tubulointerstitium, which was prevented when Ang-(1–7)-treated slices were incubated in the presence of 5 &mgr;M meclofenamate (1 ± 2% increase, n = 8; p < 0.05). Incubation with 5 &mgr;M meclofenamate alone had no effect on [125I]-Ang II binding (−3 ± 3%). The decrease in [125I]-Ang II binding with Ang-(1–7) was also blocked by the Ang-(1–7) antagonist [d-Ala7]-Ang-(1–7). Treatment with 1 &mgr;M [d-Ala7]-Ang-(1–7) alone had no effect on [125I]-Ang II binding (−3 ± 6% of control). Pretreatment with 1 &mgr;M Ang II caused a similar reduction in [125I]-Ang II binding in the cortical tubulointerstitium. Neither Ang-(1–7) nor Ang II had any effect on [125I]-Ang II binding in the glomeruli and the area of the vasa recta of the kidney. These original findings suggest that prior exposure to Ang-(1–7) or Ang II causes a modest decrease in the number of AT1 receptors in the cortical tubulointerstitial area of the kidney. The reduction in Ang II binding by Ang-(1–7) was blocked by meclofenamate and [d-Ala7]-Ang-(1–7), suggesting that cyclooxygenase products released through activation of a novel receptor participate in this effect.

Chronic immunoneutralization of brain angiotensin-(1-12) lowers blood pressure in transgenic (mRen2)27 hypertensive rats.

Angiotensin-(1-12) [ANG-(1-12)] is a newly identified peptide detected in a variety of rat tissues, including the brain. To determine whether brain ANG-(1-12) participates in blood pressure regulation, we treated male adult (mRen2)27 hypertensive rats (24-28 wk of age) with Anti-ANG-(1-12) IgG or Preimmune IgG via an intracerebroventricular cannula for 14 days. Immunoneutralization of brain ANG-(1-12) lowered systolic blood pressure (-43 +/- 8 mmHg on day 3 and -26 +/- 7 mmHg on day 10 from baseline, P < 0.05). Water intake was lower on intracereroventricular day 6 in the Anti-ANG-(1-12) IgG group, accompanied by higher plasma osmolality on day 13, but there were no differences in urine volume, food intake, or body weight during the 2-wk treatment. In Preimmune IgG-treated animals, there were no significant changes in these variables over the 2-wk period. The antihypertensive effects produced by endogenous neutralization of brain ANG-(1-12) suggest that ANG-(1-12) is functionally active in brain pathways regulating blood pressure.

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.

Deletion of proton-sensing receptor GPR4 associates with lower blood pressure and lower binding of angiotensin II receptor in SFO

Diets rich in grains and meat and low in fruits and vegetables (acid-producing diets) associate with incident hypertension, whereas vegetarian diets associate with lower blood pressure (BP). However, the pathways that sense and mediate the effects of acid-producing diets on BP are unknown. Here, we examined the impact of the deletion of an acid sensor GPR4 on BP. GPR4 is a proton-sensing G protein-coupled receptor and an acid sensor in brain, kidney, and blood vessels. We found that GPR4 mRNA was higher in subfornical organ (SFO) than other brain regions. GPR4 protein was abundant in SFO and present in capillaries throughout the brain. Since SFO partakes in BP regulation through the renin-angiotensin system (RAS), we measured BP in GPR4-/- and GPR4+/+ mice and found that GPR4 deletion associated with lower systolic BP: 87 ± 1 mmHg in GPR4-/- (n = 35) vs. 99 ± 2 mmHg (n = 29) in GPR4+/+; P < 0.0001, irrespective of age and sex. Angiotensin II receptors detected by 125I-Sarthran binding were lower in GPR4-/- than GPR4+/+ mice in SFO and in paraventricular nucleus of hypothalamus. Circulating angiotensin peptides were comparable in GPR4-/- and GPR4+/+ mice, as were water intake and excretion, serum and urine osmolality, and fractional excretion of sodium, potassium, or chloride. A mild metabolic acidosis present in GPR4-/- mice did not associate with elevated BP, implying that deficiency of GPR4 may preclude the effect of chronic acidosis on BP. Collectively, these results posit the acid sensor GPR4 as a novel component of central BP control through interactions with the RAS.