Altered regulation of the rostral ventrolateral medulla in hypertensive obese Zucker rats

Cardiovascular effects of angiotensin-(1-12) [ANG-(1-12)] were studied in the medial nucleus of the tractus solitarius (mNTS) in anesthetized, artificially ventilated, adult male Wistar rats. Microinjections (100 nl) of ANG-(1-12) (0.06 mM) into the mNTS elicited maximum decreases in mean arterial pressure (MAP; 34 ± 5.8 mmHg) and heart rate (HR; 39 ± 3.7 beats/min). Bilateral vagotomy abolished ANG-(1-12)-induced bradycardia. Efferent greater splanchnic nerve activity was decreased by microinjections of ANG-(1-12) into the mNTS. Blockade of ANG type 1 receptors (AT(1)Rs; using ZD-7155 or L-158,809), but not ANG type 2 receptors (AT(2)Rs; using PD-123319), significantly attenuated ANG-(1-12)-induced cardiovascular responses. Simultaneous inhibition of both angiotensin-converting enzyme (ACE; using captopril) and chymase (using chymostatin) completely blocked the effects of ANG-(1-12). Microinjections of A-779 [ANG-(1-7) antagonist] did not attenuate ANG-(1-12)-induced responses. Pressure ejection of ANG-(1-12) (0.06 mM, 2 nl) caused excitation of barosensitive mNTS neurons, which was blocked by prior application of the AT(1)R antagonist. ANG-(1-12)-induced excitation of mNTS neurons was also blocked by prior sequential applications of captopril and chymostatin. These results indicate that 1) microinjections of ANG-(1-12) into the mNTS elicited depressor and bradycardic responses by exciting barosensitive mNTS neurons; 2) the decreases in MAP and HR were mediated via sympathetic and vagus nerves, respectively; 3) AT(1)Rs, but not AT(2)Rs, mediated these actions of ANG-(1-12); 4) the responses were mediated via the conversion of ANG-(1-12) to ANG II and both ACE and chymase were involved in this conversion; and 5) ANG-(1-7) was not one of the metabolites of ANG-(1-12) in the mNTS.     

Effect of Barodenervation on Cardiovascular Responses Elicited from the Hypothalamic Arcuate Nucleus of the Rat

We have previously reported that chemical stimulation of the hypothalamic arcuate nucleus (ARCN) in the rat elicited increases as well as decreases in blood pressure (BP) and sympathetic nerve activity (SNA). The type of response elicited from the ARCN (i.e., increase or decrease in BP and SNA) depended on the level of baroreceptor activity which, in turn, was determined by baseline BP in rats with intact baroreceptors. Based on this information, it was hypothesized that baroreceptor unloading may play a role in the type of response elicited from the ARCN. Therefore, the effect of barodenervation on the ARCN-induced cardiovascular and sympathetic responses and the neurotransmitters in the hypothalamic paraventricular nucleus (PVN) mediating the excitatory responses elicited from the ARCN were investigated in urethane-anesthetized adult male Wistar rats. Bilateral barodenervation converted decreases in mean arterial pressure (MAP) and greater splanchnic nerve activity (GSNA) elicited by chemical stimulation of the ARCN with microinjections of N-methyl-D-aspartic acid to increases in MAP and GSNA and exaggerated the increases in heart rate (HR). Combined microinjections of NBQX and D-AP7 (ionotropic glutamate receptor antagonists) into the PVN in barodenervated rats converted increases in MAP and GSNA elicited by the ARCN stimulation to decreases in MAP and GSNA and attenuated increases in HR. Microinjections of SHU9119 (a melanocortin 3/4 receptor antagonist) into the PVN in barodenervated rats attenuated increases in MAP, GSNA and HR elicited by the ARCN stimulation. ARCN neurons projecting to the PVN were immunoreactive for proopiomelanocortin, alpha-melanocyte stimulating hormone (alpha-MSH) and adrenocorticotropic hormone (ACTH). It was concluded that increases in MAP and GSNA and exaggeration of tachycardia elicited by the ARCN stimulation in barodenervated rats may be mediated via release of alpha-MSH and/or ACTH and glutamate from the ARCN neurons projecting to the PVN.     

Effects of angiotensins on day-night fluctuations and stress-induced changes in blood pressure

In this study we evaluated by telemetry the effects of ANG II and ANG-(1-7) infusion on the circadian rhythms of blood pressure (BP) and heart rate (HR) and on the cardiovascular adjustment resulting from restraint stress in rats. ANG II or ANG-(1-7) or vehicle were infused subcutaneously for 7 days. Restraint stress was carried out before, during, and after infusion at 7-day intervals. Parallel with an increase in MAP, ANG II infusion produced an inversion of MAP circadian rhythm with a significant MAP acrophase inversion. It also produced bradycardia during the first 3 days of infusion. Thereafter, HR progressively increased, reaching values similar to or above those of the control period at the end of the infusion period. HR circadian variation was not changed by ANG II infusion. Strikingly, ANG II significantly attenuated the increase in MAP induced by restraint stress without altering the HR response. ANG-(1-7) infusion produced a slight but significant decrease in MAP restricted to the daytime period. No significant changes in the MAP acrophase were observed. In addition, ANG-(1-7) infusion produced a small but significant sustained bradycardia. ANG-(1-7) did not change cardiovascular responses to restraint stress. These data indicate that ANG II can influence the activity of brain areas involved in the determination of stress-induced or circadian-dependent variations of blood pressure without changing HR fluctuations. A significant modulatory influence of ANG-(1-7) on basal MAP and HR is also suggested.     

Baroreflex modulation by angiotensins at the rat rostral and caudal ventrolateral medulla

We determined the effect of microinjection of ANG-(1-7) and ANG II into two key regions of the medulla that control the circulation [rostral and caudal ventrolateral medulla (RVLM and CVLM, respectively)] on baroreflex control of heart rate (HR) in anesthetized rats. Reflex bradycardia and tachycardia were induced by increases and decreases in mean arterial pressure produced by intravenous phenylephrine and sodium nitroprusside, respectively. The pressor effects of ANG-(1-7) and ANG II (25 pmol) after RVLM microinjection (11 +/- 0.8 and 10 +/- 2 mmHg, respectively) were not accompanied by consistent changes in HR. In addition, RVLM microinjection of these angiotensin peptides did not alter the bradycardic or tachycardic component of the baroreflex. CVLM microinjections of ANG-(1-7) and ANG II produced hypotension (-11 +/- 1.5 and -11 +/- 1.9 mmHg, respectively) that was similarly not accompanied by significant changes in HR. However, CVLM microinjections of angiotensins induced differential changes in the baroreflex control of HR. ANG-(1-7) attenuated the baroreflex bradycardia (0.26 +/- 0.06 ms/mmHg vs. 0.42 +/- 0.08 ms/mmHg before treatment) and facilitated the baroreflex tachycardia (0.86 +/- 0.19 ms/mmHg vs. 0.42 +/- 0.10 ms/mmHg before treatment); ANG II produced the opposite effect, attenuating baroreflex tachycardia (0.09 +/- 0.06 ms/mmHg vs. 0.31 +/- 0.07 ms/mmHg before treatment) and facilitating the baroreflex bradycardia (0.67 +/- 0.16 ms/mmHg vs. 0.41 +/- 0.05 ms/mmHg before treatment). The modulatory effect of ANG II and ANG-(1-7) on baroreflex sensitivity was completely abolished by peripheral administration of methylatropine. These results suggest that ANG II and ANG-(1-7) at the CVLM produce a differential modulation of the baroreflex control of HR, probably through distinct effects on the parasympathetic drive to the heart.     

Cardiovascular Responses to Chemical Stimulation of the Hypothalamic Arcuate Nucleus in the Rat: Role of the Hypothalamic Paraventricular Nucleus

The mechanism of cardiovascular responses to chemical stimulation of the hypothalamic arcuate nucleus (ARCN) was studied in urethane-anesthetized adult male Wistar rats. At the baseline mean arterial pressure (BLMAP) close to normal, ARCN stimulation elicited decreases in MAP and sympathetic nerve activity (SNA). The decreases in MAP elicited by ARCN stimulation were attenuated by either gamma-aminobutyric acid (GABA), neuropeptide Y (NPY), or beta-endorphin receptor blockade in the ipsilateral hypothalamic paraventricular nucleus (PVN). Combined blockade of GABA-A, NPY1 and opioid receptors in the ipsilateral PVN converted the decreases in MAP and SNA to increases in these variables. Conversion of inhibitory effects on the MAP and SNA to excitatory effects following ARCN stimulation was also observed when the BLMAP was decreased to below normal levels by an infusion of sodium nitroprusside. The pressor and tachycardic responses to ARCN stimulation at below normal BLMAP were attenuated by blockade of melanocortin 3/4 (MC3/4) receptors in the ipsilateral PVN. Unilateral blockade of GABA-A receptors in the ARCN increased the BLMAP and heart rate (HR) revealing tonic inhibition of the excitatory neurons in the ARCN. ARCN stimulation elicited tachycardia regardless of the level of BLMAP. ARCN neurons projecting to the PVN were immunoreactive for glutamic acid decarboxylase 67 (GAD67), NPY, and beta-endorphin. These results indicated that: 1) at normal BLMAP, decreases in MAP and SNA induced by ARCN stimulation were mediated via GABA-A, NPY1 and opioid receptors in the PVN, 2) lowering of BLMAP converted decreases in MAP following ARCN stimulation to increases in MAP, and 3) at below normal BLMAP, increases in MAP and HR induced by ARCN stimulation were mediated via MC3/4 receptors in the PVN. These results provide a base for future studies to explore the role of ARCN in cardiovascular diseases.     

Cold pressor test in the rat: medullary and spinal pathways and neurotransmitters

This study was designed to delineate the medullary and spinal pathways mediating the cardiovascular responses to cold pressor test (CPT) and to identify neurotransmitters in these pathways. Experiments were done in barodenervated, urethane-anesthetized, male Wistar rats. The CPT was performed by immersing the limbs and ventral half of the body of the rat in ice-cold water (0.5 degrees C) for 2 min. CPT elicited an immediate increase in mean arterial pressure (MAP), heart rate (HR), and greater splanchnic nerve activity (GSNA). Bilateral blockade of ionotropic glutamate receptors (iGLURs) in the rostral ventrolateral medullary pressor area (RVLM) significantly attenuated the CPT-induced responses. Bilateral blockade of gamma-aminobutyric acid (GABA) receptors, but not iGLURs, in the nucleus ambiguus (nAmb) significantly reduced the CPT-induced increases in HR, but not MAP. Blockade of spinal iGLURs caused a significant reduction in CPT-induced increases in MAP and GSNA, whereas the increases in HR were reduced to a lesser extent. Combination of the blockade of spinal iGLURs and bilateral vagotomy or intravenous atropine almost completely blocked CPT-induced tachycardia. Midcollicular decerebration significantly reduced CPT-induced increases in MAP and HR. These results indicated that: 1) CPT-induced increases in MAP, HR, and GSNA were mediated by activation of iGLURs in the RVLM and spinal cord, 2) activation of GABA receptors in the nAmb also contributed to the CPT-induced tachycardic responses, and 3) brain areas rostral to the brain stem also participated in the CPT-induced pressor and tachycardic responses.     

Alterations of the renin-angiotensin system at the RVLM of transgenic rats with low brain angiotensinogen

The transgenic rats TGR(ASrAOGEN) (TGR) with low levels of brain angiotensinogen were analyzed for cardiovascular reactivity to microinjections of ANG II and angiotensin receptor (AT(1)) antagonists [CV-11974, AT(1) specific; A-779, ANG-(1--7) selective; sarthran, nonspecific] into the rostral ventrolateral medulla (RVLM) of conscious rats. Microinjection of ANG II resulted in a significantly higher increase in the mean arterial pressure (MAP) of TGR than control [Sprague-Dawley (SD)] rats, suggesting an upregulation of ANG II receptors in TGR. CV-11974 produced an increase in MAP of SD but not in TGR rats. A-779 produced a depressor response in SD but not in TGR rats. Conversely, sarthran produced a similar decrease of MAP in both rat groups. The pressor effect of the AT(1) antagonist may indicate an inhibitory role of AT(1) receptors in the RVLM. On the other hand, ANG-(1--7) appears to have a tonic excitatory role in this region. The altered response to specific angiotensin antagonists in TGR further supports the functionally relevant decrease in angiotensins in the brains of TGR and corroborates the importance of the central renin-angiotensin system in cardiovascular homeostasis.     

Microinjections of urocortin1 into the nucleus ambiguus of the rat elicit bradycardia

Urocortins are members of the hypothalamic corticotropin-releasing factor (CRF) peptide family. Urocortin1 (UCN1) mRNA has been reported to be expressed in the brainstem neurons. The present investigation was carried out to test the hypothesis that microinjections of UCN1 into the nucleus ambiguus (nAmb) may elicit cardiac effects. Urethane-anesthetized, artificially ventilated, adult male Wistar rats, weighing between 300-350 g, were used. nAmb was identified by microinjections of l-glutamate (5 mM, 30 nl). Microinjections (30 nl) of different concentrations (0.062, 0.125, 0.25, and 0.5 mM) of UCN1 into the nAmb elicited bradycardic responses (26.5 ± 1, 30.1 ± 1.7, 46.9 ± 1.7, and 40.3 ± 2.6 beats/min, respectively). These heart rate responses were not accompanied by significant changes in mean arterial pressure. The bradycardic responses to maximally effective concentration of UCN1 (0.25 mM) were significantly (P < 0.05) attenuated by prior microinjections of a selective antagonist (NBI 27914, 1.5 mM) for CRF type 1 receptor (CRF1R). Prior microinjections of ionotropic glutamate receptor (iGLUR) antagonists [d-(-)-2-amino-7-phosphono-heptanoic acid and 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo-(f)quinoxaline-7-sulfonamide disodium] also attenuated the bradycardia elicited by UCN1 microinjections into the nAmb. Microinjections of NBI 27914 (1.5 mM) into the nAmb did not alter baroreflex responses. Bilateral vagotomy abolished the bradycardic responses to microinjections of UCN1 into the nAmb. These results indicated that 1) microinjections of UCN1 into the nAmb elicited bradycardia, 2) the bradycardia was vagally mediated, 3) activation of CRF1Rs in the nAmb was responsible for the actions of UCN1, and 4) activation of iGLURs in the nAmb also participated in the bradycardia elicited by UCN1.     

Does angiotensin II have a significant tonic action on cardiovascular neurons in the rostral and caudal VLM?

The peptidic ANG II receptor antagonists [Sar(1),Ile(8)]ANG II (sarile) or [Sar(1),Thr(8)]ANG II (sarthran) are known to decrease arterial pressure and sympathetic activity when injected into the rostral part of the ventrolateral medulla (VLM). In anesthetized rabbits and rats, the profound depressor and sympathoinhibitory response after bilateral microinjections of sarile or sarthran into the rostral VLM was unchanged after prior selective blockade of angiotensin type 1 (AT(1)) and ANG-(1---7) receptors, although this abolished the effects of exogenous ANG II. Unlike the neuroinhibitory compounds muscimol or lignocaine, microinjections of sarile in the rostral VLM did not affect respiratory activity. Sarile or sarthran in the caudal VLM resulted in a large pressor and sympathoexcitatory response, which was also unaffected by prior blockade of AT(1) and ANG-(1---7) receptors. The results indicate that the peptidic ANG receptor antagonists profoundly inhibit the tonic activity of cardiovascular but not respiratory neurons in the VLM and that these effects are independent of ANG II or ANG-(1---7) receptors.     

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.     

Hypotensive effect of ANG II and ANG-(1-7) at the caudal ventrolateral medulla involves different mechanisms

The objective of the present study was to determine the contribution of the autonomic nervous system and nitric oxide to the depressor effect produced by unilateral microinjection of ANG-(1-7) and ANG II into the caudal ventrolateral medulla (CVLM). Unilateral microinjection of ANG-(1-7), ANG II (40 pmol), or saline (100 nl) was made into the CVLM of male Wistar rats anesthetized with urethane before and after intravenous injection of 1) methyl-atropine, 2.5 mg/kg; 2) prazosin, 25 microg/kg; 3) the nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), 5 mg/kg; or 4) the specific inhibitor of neuronal NOS, 7-nitroindazole (7-NI), 45 mg/kg. Arterial pressure and heart rate (HR) were continuously monitored. Microinjection of ANG-(1-7) or ANG II into the CVLM produced a significant decrease in mean arterial pressure (MAP; -11 +/- 1 mmHg, n = 12 and -10 +/- 1 mmHg, n = 10, respectively) that was not accompanied by consistent changes in HR or in cardiac output. The effect of ANG-(1-7) was abolished after treatment with methyl-atropine (-3 +/- 0.6 mmHg, n = 9) or L-NAME (-2.3 +/- 0.5 mmHg, n = 8) or 7-NI (-2.8 +/- 0.6 mmHg, n = 5). In contrast, these treatments did not significantly interfere with the ANG II effect (-10 +/- 2.6 mmHg, n = 8; -8 +/- 1.5 mmHg, n = 8; and -12 +/- 3.6 mmHg, n = 6; respectively). Peripheral treatment with prazosin abolished the hypotensive effect of ANG-(1-7) and ANG II. Microinjection of saline did not produce any significant change in MAP or in HR. These results suggest that the hypotensive effect produced by ANG II at the CVLM depends on changes in adrenergic vascular tonus and, more importantly, the hypotensive effect produced by ANG-(1-7) also involves a nitric oxide-related mechanism.     

Intracisternal galanin/angiotensin II interactions in central cardiovascular control

The aim of this work was to investigate the interactions between angiotensin II (Ang II) and galanin(1-29) [GAL(1-29)] or its N-terminal fragment galanin(1-15) [GAL(1-15)] on central cardiovascular control. The involvement of angiotensin type1 (AT1) receptor subtype was analyzed by the AT1 antagonist, DuP 753. Anesthesized male Sprague-Dawley rats received intracisternal microinjections of Ang II (3 nmol) with GAL(1-29) (3 nmol) or GAL(1-15) (0.1 nmol) alone or in combination. The changes in mean arterial pressure (MAP) and heart rate (HR) recorded from the femoral artery were analyzed. The injection of Ang II and GAL(1-15) alone did not produce any change in MAP. However, coinjections of both Ang II and GAL(1-15) elicited a significant vasopressor response. This response was blocked by DuP 753. Ang II and GAL(1-15) alone produced an increase in HR. The coinjections of Ang II with GAL(1-15) induced an increase in HR not significantly different from the tachycardia produced by each peptide. The presence of DuP 753 counteracted this response. GAL(1-29) alone elicited a transient vasopressor response that disappeared in the presence of Ang II. The coinjections of Ang II with GAL(1-29) and with DuP 753 restored the transient vasopressor effect produced by GAL(1-29). GAL(1-29) produced a slight but significant tachycardic effect that was not modified in the presence of Ang II. The presence of DuP 753 did not modify the tachycardic response produced by Ang II and GAL(1-29). These results give indications for the existence of a differential modulatory effect of Ang II with GAL(1-15) and GAL(1-29) on central blood pressure response that might be dependent on the activity of the angiotensin AT1 receptor subtype.     

Attenuation of aortic baroreflex responses by microinjections of endomorphin-2 into the rostral ventrolateral medullary pressor area of the rat

The presence of mu-opioid receptors and endomorphins has been demonstrated in the general area encompassing the rostral ventrolateral medullary pressor area (RVLM). This investigation was carried out to test the hypothesis that endomorphins in the RVLM may have a modulatory role in regulating cardiovascular function. Blood pressure and heart rate (HR) were recorded in urethane-anesthetized male Wistar rats. Unilateral microinjections of endomorphin-2 (0.0125-0.5 mmol/l) into the RVLM elicited decreases in mean arterial pressure (16-30 mmHg) and HR (12-36 beats/min), which lasted for 2-4 min. Bradycardia was not vagally mediated. The effects of endomorphin-2 were mediated via mu-opioid receptors because prior microinjections of naloxonazine (1 mmol/l) abolished these responses; the blocking effect of naloxonazine lasted for 15-20 min. Unilateral stimulations of aortic nerve for 30 s (at frequencies of 5, 10, and 25 pulses/s; each pulse 0.5 V and 1-ms duration) elicited depressor and bradycardic responses. These responses were significantly attenuated by microinjections of endomorphin-2 (0.2 and 0.4 mmol/l). The inhibitory effect of endomorphin-2 on baroreflex responses was prevented by prior microinjections of naloxonazine. Microinjections of naloxonazine alone did not affect either baseline blood pressure and HR or baroreflex responses. These results indicate that endomorphin-2 elicits depressor and bradycardic responses and inhibits baroreflex function when injected into the RVLM. These effects are consistent with the known hyperpolarizing effect of opioid peptides on RVLM neurons.     

Attenuating effect of angiotensin-(1-7) on angiotensin II-mediated NAD(P)H oxidase activation in type 2 diabetic nephropathy of KK-A(y)/Ta mice

ANG-(1-7) is associated with vasodilation and nitric oxide synthase stimulation. However, the role of ANG-(1-7) in type 2 diabetes mellitus is unknown. In this study, we examined the hypothesis that ANG-(1-7) attenuates ANG II-induced reactive oxygen species stress (ROS)-mediated injury in type 2 diabetic nephropathy of KK-A(y)/Ta mice. KK-A(y)/Ta mice were divided into four groups: 1) a control group; 2) ANG II infusion group; 3) ANG II+ANG-(1-7) coinfusion group; and 4) ANG II+ANG-(1-7)+d-Ala(7)-ANG-(1-7) (A779) coinfusion group. In addition, primary mesangial cells were cultured and then stimulated with 25 mM glucose with or without ANG II, ANG-(1-7), and A779. The ANG II+ANG-(1-7) coinfusion group showed a lower urinary albumin/creatinine ratio increase than the ANG II group. ANG-(1-7) attenuated ANG II-mediated NAD(P)H oxidase activation and ROS production in diabetic glomeruli and mesangial cells. ANG II-induced NF-κB and MAPK signaling activation was also attenuated by ANG-(1-7) in the mesangial cells. These findings were related to improved mesangial expansion and to fibronectin and transforming growth factor-β1 production in response to ANG II and suggest that ANG-(1-7) may attenuate ANG II-stimulated ROS-mediated injury in type 2 diabetic nephropathy. The ACE2-ANG-(1-7)-Mas receptor axis should be investigated as a novel target for treatment of type 2 diabetic nephropathy.     

ANG II in median preoptic nucleus and pressor responses to CSF sodium and high sodium intake in SHR

Pressor responses to increases in cerebrospinal fluid (CSF) sodium in Wistar rats and to high salt intake in spontaneously hypertensive rats (SHR) involve both brain ouabainlike activity ("ouabain") and the brain renin-angiotensin system (RAS). Because some of the effects of "ouabain" are mediated by the median preoptic nucleus (MnPO) and this nucleus contains all elements of the RAS, the present study assessed possible interactions of "ouabain" and ANG II in this nucleus. In conscious Wistar rats, injection of ANG II into the MnPO significantly increased mean arterial pressure (MAP) and heart rate (HR). This response was not affected by pretreatment with a subpressor dose of ouabain. MAP and HR increases by ouabain in the MnPO were significantly attenuated by MnPO pretreatment with losartan. In Wistar rats, losartan in the MnPO also abolished pressor and HR responses to intracerebroventricular 0.3 M NaCl and attenuated MAP and HR responses to intracerebroventricular ouabain. Five weeks of a high-salt diet in SHRs resulted in exacerbation of hypertension and increased responses to air-jet stress and intracerebroventricular guanabenz. Losartan injected into the MnPO reversed the salt-sensitive component of the hypertension and normalized the depressor response to guanabenz but did not change responses to air-jet stress. We conclude that in the MnPO, ANG II via AT(1) receptors mediates cardiovascular responses to an acute increase in CSF sodium as well as the chronic pressor responses to high sodium intake in SHR.     

Androgens augment renal vascular responses to ANG II in New Zealand genetically hypertensive rats

Males develop higher blood pressure than do females. This study tested the hypothesis that androgens enhance responsiveness to ANG II during the development of hypertension in New Zealand genetically hypertensive (NZGH) rats. Male NZGH rats were obtained at 5 wk of age and subjected to sham operation (Sham) or castration (Cas) then studied at three age groups: 6-7, 11-12, and 16-17 wk. Mean arterial blood pressure (MAP), heart rate (HR), and renal blood flow (RBF) measurements were recorded under Inactin anesthesia. These variables were measured after enalapril (1 mg/kg) treatment and during intravenous ANG II infusion (20, 40, and 80 ng/kg/min). Plasma testosterone was measured by ELISA. Angiotensin type 1 (AT1) receptor expression was assessed by Western blot analysis and RT-PCR. ANG II-induced MAP responses were significantly attenuated in Cas NZGH rats. At the highest ANG II dose, MAP increased by 40+/-4% in Sham vs. 22+/-1% in Cas NZGH rats of 16-17 wk of age. Similarly, renal vascular resistance (RVR) responses to ANG II were reduced by castration (209+/-20% in Sham vs. 168+/-10% in Cas NZGH rats at 16-17 wk of age). Castration also reduced MAP recorded in conscious NZGH rats of this age group. Testosterone replacement restored baseline MAP and the pressor and RVR responses to ANG II. Castration reduced testosterone concentrations markedly. Testosterone treatment restored these concentrations. Neither castration nor castration+testosterone treatment affected AT1 receptor mRNA or protein expression. Collectively, these data suggest that androgens modulate renal and systemic vascular responsiveness to ANG II, which may contribute to androgen-induced facilitation of NZGH rat hypertension.     

Central effects of angiotensin II in conscious hamsters: drinking,pressor response,and release of vasopressin

Summary The effects of intracerebroventricular (icv) injections of angiotensin II (ANG II) on water intake, blood pressure, heart rate, and plasma arginine-vasopressin (AVP) concentration were studied in chronically instrumented adult male Syrian golden hamsters (Mesocricetus auratus). Furthermore, the effects of pharmacological ganglionic blockade, and of vascular AVP receptor blockade, on central ANG II-induced cardiovascular responses were investigated. ANG II (1, 10, and 100 ng, icv) elicited dose-dependent increases in water intake and arterial blood pressure. Heart rate showed a biphasic response with a short initial non dose-dependent tachycardic and a subsequent longer lasting bradycardic phase. Plasma AVP concentration was increased two and a half fold with 100 ng ANG II icv. Both ganglionic blockade and vascular AVP receptor blockade significantly attenuated the central ANG II-induced pressor response. The tachycardic phase of the heart rate response was abolished by ganglionic blockade and the bradycardic phase was significantly diminished by AVP receptor blockade. The results support the hypothesis that brain ANG II may participate in the central control of body fluid volume and in central cardiovascular regulation in conscious hamsters.     

Angiotensin-(1 7) stimulates the phosphorylation of JAK2, IRS-1 and Akt in rat heart in vivo: role of the AT1 and Mas receptors

Angiotensin (ANG) II exerts a negative modulation on insulin signal transduction that might be involved in the pathogenesis of hypertension and insulin resistance. ANG-(1-7), an endogenous heptapeptide hormone formed by cleavage of ANG I and ANG II, counteracts many actions of ANG II. In the current study, we have explored the role of ANG-(1-7) in the signaling crosstalk that exists between ANG II and insulin. We demonstrated that ANG-(1-7) stimulates the phosphorylation of Janus kinase 2 (JAK2) and insulin receptor substrate (IRS)-1 in rat heart in vivo. This stimulating effect was blocked by administration of the selective ANG type 1 (AT(1)) receptor blocker losartan. In contrast to ANG II, ANG-(1-7) stimulated cardiac Akt phosphorylation, and this stimulation was blunted in presence of the receptor Mas antagonist A-779 or the phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin. The specific JAK2 inhibitor AG-490 blocked ANG-(1-7)-induced JAK2 and IRS-1 phosphorylation but had no effect on ANG-(1-7)-induced phosphorylation of Akt, indicating that activation of cardiac Akt by ANG-(1-7) appears not to involve the recruitment of JAK2 but proceeds through the receptor Mas and involves PI3K. Acute in vivo insulin-induced cardiac Akt phosphorylation was inhibited by ANG II. Interestingly, coadministration of insulin with an equimolar mixture of ANG II and ANG-(1-7) reverted this inhibitory effect. On the basis of our present results, we postulate that ANG-(1-7) could be a positive physiological contributor to the actions of insulin in heart and that the balance between ANG II and ANG-(1-7) could be relevant for the association among insulin resistance, hypertension, and cardiovascular disease.     

Angiotensin-(1-7) attenuates the chronotropic response to angiotensin II via stimulation of PTEN in the spontaneously hypertensive rat neurons

Several studies have focused on the beneficial effects of peripheral angiotensin-(1-7) [Ang-(1-7)] in the regulation of cardiovascular function, showing its counterregulatory effect against the actions of angiotensin II (ANG II). However, its actions in the central nervous system are not completely understood. In the present study, we investigated the intracellular mechanisms underlying the action of ANG-(1-7) using the patch-clamp technique in neurons cultured from the hypothalamus of neonatal spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. Superfusion of neurons with ANG II (100 nM) significantly increased neuronal firing in both strains of rats, and this chronotropic effect of ANG II was significantly enhanced in prehypertensive SHR neurons compared with WKY rat neurons. The enhanced chronotropic effect of ANG II was attenuated by a phosphatidylinositol 3-kinase (PI3-kinase) inhibitor, LY 294002 (10 μM). Superfusion of neurons with ANG-(1-7) (100 nM) did not alter the neuronal firing rate in either SHR or WKY neurons; however, it significantly attenuated the chronotropic action of ANG II exclusively in prehypertensive SHR neurons. This counterregulatory effect of ANG-(1-7) on ANG II action in prehypertensive SHR neurons was attenuated by cotreatment with either A-779, a Mas receptor antagonist, or bisperoxovanadium, a phosphatase and tensin homologue deleted on chromosome ten (PTEN) inhibitor. In addition, incubation of WKY and prehypertensive SHR neurons with ANG-(1-7) significantly increased PTEN activity. The data demonstrate that ANG-(1-7) counterregulates the chronotropic action of ANG II via a PTEN-dependent signaling pathway in prehypertensive SHR neurons.