Abstract 513: Evaluation of Proinflammatory Effects of Low Dose Angiotensin II Infusion in Intact Female Mice and Adiponectin Deficient Mice

Circulating vasoactive peptide Angiotensin II (ANGII) has a well-known role in the development of hypertension and other cardiovascular diseases. In addition it has significant proinflammatory actions in the vascular wall inducing the production of reactive oxygen species, inflammatory cytokines and adhesion molecules. It has been previously shown that estrogen in female mice protects against ANGII mediated hypertension and against proinflammatory effects. It is not clear if the protective effects of estrogen extend to adiponectin deficient mice. Adiponectin is one of the few peptides secreted by fat to have anti-inflammatory properties. The present study evaluates the effect of chronic ANGII infusion on expression of cytokines in female C57BL/6J and adiponectin deficient mice (adipo-/-). Female mice (24- 28wks), were implanted with osmotic pump containing either ANG II (800 ng/Kg /min) or saline. Blood samples and tissue were collected at the end of 14 days. Plasma levels of TNFalpha and IL6 were measured using enzyme linked immunoabsorbent assay. Renal tissue expression of the cytokines were quantified using real-time PCR (Eppendorf Realplex 4 mastercycler) and SYBR Green ROX mastermix. Plasma TNFα levels were similar in saline infused C57Bl/6J(11±1 pg/ml) and adiponectin deficient mice(10± 1 pg/ml). ANGII did not significantly increase TNFα in the control(14±3 pg/ml) or adipo-/- mice(13+1 pg/ml). Plasma IL6 levels were also not significantly different in the two groups. A microarray for mRNA expression of markers of endothelial activation and adhesion molecules was also performed in the renal tissue. Preliminary data show that TNFalpha mRNA expression levels were not increased by ANG II infusion and IL6 expression was undetectable. ANGII also did not alter E-Selectin,VCAM1, Collagen1a1 and eNOS expression. In conclusion, ANGII infusion did not result in a proinflammatory milieu in both female C57BL/6J and Adipo -/- mice.

Estrogen receptor-alpha mediates estrogen protection from angiotensin II-induced hypertension in conscious female mice

It has been shown that the female sex hormones have a protective role in the development of angiotensin II (ANG II)-induced hypertension. The present study tested the hypotheses that 1) the estrogen receptor-alpha (ERalpha) is involved in the protective effects of estrogen against ANG II-induced hypertension and 2) central ERs are involved. Blood pressure (BP) was measured in female mice with the use of telemetry implants. ANG II (800 ng.kg(-1).min(-1)) was administered subcutaneously via an osmotic pump. Baseline BP in the intact, ovariectomized (OVX) wild-type (WT) and ERalpha knockout (ERalphaKO) mice was similar; however, the increase in BP induced by ANG II was greater in OVX WT (23.0 +/- 1.0 mmHg) and ERalphaKO mice (23.8 +/- 2.5 mmHg) than in intact WT mice (10.1 +/- 4.5 mmHg). In OVX WT mice, central infusion of 17beta-estradiol (E(2); 30 microg.kg(-1).day(-1)) attenuated the pressor effect of ANG II (7.0 +/- 0.4 mmHg), and this protective effect of E(2) was prevented by coadministration of ICI-182,780 (ICI; 1.5 microg.kg(-1).day(-1), 18.8 +/- 1.5 mmHg), a nonselective ER antagonist. Furthermore, central, but not peripheral, infusions of ICI augmented the pressor effects of ANG II in intact WT mice (17.8 +/- 4.2 mmHg). In contrast, the pressor effect of ANG II was unchanged in either central E(2)-treated OVX ERalphaKO mice (19.0 +/- 1.1 mmHg) or central ICI-treated intact ERalphaKO mice (19.6 +/- 1.6 mmHg). Lastly, ganglionic blockade on day 7 after ANG II infusions resulted in a greater reduction in BP in OVX WT, central ER antagonist-treated intact WT, central E(2) + ICI-treated OVX WT, ERalphaKO, and central E(2)- or ICI-treated ERalphaKO mice compared with that in intact WT mice given just ANG II. Together, these data indicate that ERalpha, especially central expression of the ER, mediates the protective effects of estrogen against ANG II-induced hypertension.

Frequency-dependent depression of exocytosis and the role of voltage-gated calcium channels

Synaptic vesicle exocytosis in primary cultures of baroreceptor neurons is reduced during high-frequency stimulation. Calcium influx through voltage-gated calcium channels (VGCC) is a key step in neurotransmitter release. With the help of FM2-10, a marker of synaptic vesicle recycling, the present study investigates the differential contribution of several VGCC subtypes to exocytosis in neuronal processes and how this contribution is altered at high frequencies. In control experiments, field stimulation at 0.5 Hz evoked about 66 +/- 5% destaining. Combined blockade of N- and P/Q-subtypes with Ctx-MVIIC was far more effective in reducing exocytosis (11 +/- 8%) than blocking N-type (49 +/- 5%, Ctx-GVIA) or P-type (46 +/- 1%, Agatoxin) alone. The effectiveness of the blockers also varied with the duration of stimulation: Ctx-GVIA attenuating exocytosis significantly in the first 60 s and Agatoxin affecting exocytosis only towards the end of 180 s stimulation period. Field stimulation at 10 Hz evoked exocytosis (36 +/- 18%) comparable to that evoked by 0.5 Hz in the presence of Ctx-GVIA. While blockade with Agatoxin had no effects, Ctx-GVIA, Ctx-MVIIC and L-type blocker Nifedepine had small but similar inhibitory effects on exocytosis at 10 Hz. The data suggest that N-type is the major contributor to exocytosis at 0.5 Hz, and this contribution is reduced during prolonged stimulation periods and at high frequencies.

Sex differences in the development of angiotensin II-induced hypertension in conscious mice

Sex has an important influence on blood pressure (BP) regulation. There is increasing evidence that sex hormones interfere with the renin-angiotensin system. Thus the purpose of this study was to determine whether there are sex differences in the development of ANG II-induced hypertension in conscious male and female mice. We used telemetry implants to measure aortic BP and heart rate (HR) in conscious, freely moving animals. ANG II (800 ng.kg(-1).min(-1)) was delivered via an osmotic pump implanted subcutaneously. Our results showed baseline BP in male and female mice to be similar. Chronic systemic infusion of ANG II induced a greater increase in BP in male (35.1 +/- 5.7 mmHg) than in female mice (7.2 +/- 2.0 mmHg). Gonadectomy attenuated ANG II-induced hypertension in male mice (15.2 +/- 2.4 mmHg) and augmented it in female mice (23.1 +/- 1.0 mmHg). Baseline HR was significantly higher in females relative to males (630.1 +/- 7.9 vs. 544.8 +/- 16.2 beats/min). In females, ANG II infusion significantly decreased HR. However, the increase in BP with ANG II did not result in the expected decrease in HR in either intact male or gonadectomized mice. Moreover, the slope of the baroreflex bradycardia to phenylephrine was blunted in males (-5.6 +/- 0.3 to -2.9 +/- 0.5) but not in females (-6.5 +/- 0.5 to -5.6 +/- 0.3) during infusion of ANG II, suggesting that, in male mice, infusion of ANG II results in a resetting of the baroreflex control of HR. Ganglionic blockade resulted in greater reduction in BP on day 7 after ANG II infusion in males compared with females (-61.0 +/- 8.9 vs. -36.6 +/- 6.6 mmHg), suggesting an increased contribution of sympathetic nerve activity in arterial BP maintenance in male mice. Together, these data indicate that there are sex differences in the development of chronic ANG II-induced hypertension in conscious mice and that females may be protected from the increases in BP induced by ANG II.

Frequency dependence of synaptic vesicle exocytosis in aortic baroreceptor neurons and the role of group III mGluRs

Synaptic transmission between baroreceptor afferents and the nucleus tractus solitarius (NTS) is essential for reflex regulation of blood pressure. High frequency stimulation of the afferents in vivo leads to a decrease in synaptic strength and is generally attributed to reduction in presynaptic neurotransmitter release. It has been hypothesized that during high frequency stimulation glutamate a major neurotransmitter at the baroreceptor afferent terminals inhibits its own release via presynaptic group III metabotropic glutamate receptors (mGluRs). A key player in modulation of presynaptic release is vesicle exocytosis. The present study utilized cultured aortic baroreceptor neurons and the styryl dye FM2-10 to characterize (1) the dependence of exocytosis at these afferent nerve terminals on the frequency of neuronal activation, (2) the effect of duration of stimulation on the rate of exocytosis and (3) the role of mGluRs in the frequency-dependent modulation of exocytosis. Destaining in the FM2-10 loaded boutons during 3 min of stimulation, a measure of exocytosis, progressively decreased with increasing frequency (0.5, 1.0 and 10 Hz). Blockade of group III mGluRs with 300 microM (RS)-cyclopropyl-4-phosphonophenylglycine (CPPG) facilitated exocytosis evoked by 10 Hz stimulation but not at 0.5 Hz. The data suggest that aortic baroreceptor terminals exhibit frequency-dependent depression of exocytosis and support a role for group III mGluRs in the frequency-dependent modulation of exocytosis.

Effects of Salt Rich Diet in the Obese Zucker Rats: Studies on Renal Function During Isotonic Volume Expansion

Obese Zucker rats (OZR) are hyperinsulenemic, hyperglycemic and dyslipidemic and develop salt dependent hypertension. Since salt sensitivity is considered to be due to impaired handling of renal sodium excretion, these studies were conducted in the obese and lean Zucker rats (LZR) anesthetized with Inactin to evaluate renal function under basal conditions and during acute isotonic fluid volume expansion (VE). Mean Arterial blood pressure (MBP), heart rate (HR), renal blood flow(RBF) and glomerular filtration rate (GFR) were not significantly different between the lean Zucker rats fed normal diet or that fed salt rich diet(8% NaCI). However, basal UV and UNaV were significantly greater in the LZR fed high salt. During VE essentially identical increases occurred in GFR, UV and UNaV in both the lean groups. In the OZR fed salt rich diet also, there were no significant changes in the heart rate, RBF and GFR. However, arterial blood pressure of the OZR fed salt rich diet was significantly greater than that of the OZR on the normal diet as well as that of both the lean groups. Also, as in the LZR, basal UV and UNaV were significantly greater in the salt fed obese rats. During volume expansion there were no impairments in the ability of the obese groups fed normal or salt rich diet to eliminate sodium and water during volume load. In fact, the net sodium and water excretions during and 60 min after VE in both the obese groups were significantly greater than that of corresponding lean groups. Furthermore, these values in the OZR fed salt rich diet were significantly greater than that of the obese rats on normal salt diet perhaps due to the contribution of pressure natriuretic mechanisms'. These data demonstrate that although OZR are salt sensitive, the renal mechanisms that would collectively respond to acute isotonic VE were fully functional. An unexpected and a novel finding in these studies is that the salt rich diet, in addition to increasing arterial blood pressure also significantly lowered plasma of insulin levels and enhanced glucose and cholesterol levels in the obese Zucker rats.

17 beta-Estradiol inhibits angiotensin II activation of area postrema neurons

It is well established that the area postrema, as a circumventricular organ, is susceptible to modulation by circulating hormones and peptides. Furthermore, activation of the area postrema has been shown to modulate central neurons involved in the regulation of cardiovascular function and blood pressure. In particular, the vasoactive peptide angiotensin II (ANG II) has been shown to inhibit baroreflex regulation of heart rate and increase sympathetic outflow and blood pressure via activation of area postrema neurons. Estrogen is thought to protect against hypertension in both humans and animal models and has been shown in a number of systems to alter the effects of ANG II. The purpose of the present study was to determine the effects of estrogen on ANG II activation of area postrema neurons. In this study, the effects of ANG II and KCl on fura 2-measured cytosolic Ca2+ concentration ([Ca2+]i) responses in cultured area postrema neurons in the presence and absence of 12-h exposure to 100 nM 17 beta-estradiol (E2) were evaluated. In neurons incubated in control vehicle media, 50 nM ANG II increased [Ca2+]i by 92 +/- 12%. In neurons preincubated with 100 nM E2, ANG II increased [Ca2+]i by only 68 +/- 11%, for a total inhibition of the ANG II-evoked response of 24%. Coapplication of the estrogen receptor antagonist ICI-182,780 did not inhibit the effects of E2. In the same cells in which the effects of E2 on ANG II-evoked responses were tested, the effects of incubation in E on the depolarization-induced increased [Ca2+2]i due to 60 mM KCl were also tested. Incubation of the cells with 100 nM E increased the KCl-evoked [Ca2+2]i response, and this response was blocked by ICI-182,780. These results suggest that in the area postrema, estrogen may utilize multiple pathways to modulate neural activity and responses to ANG II.

Estrogen modulation of baroreflex function in conscious mice

It has been suggested that estrogen modulates baroreflex regulation of autonomic function. The present study evaluated the effects of estrogen on baroreflex regulation of heart rate in response to changes in blood pressure with phenylephrine (PE), ANG II, and sodium nitroprusside (SNP) in a conscious mouse model. Males and ovariectomized females with (OvxE+) and without (OvxE-) estradiol replacement chronically implanted with arterial and venous catheters were used in these studies. The slope of the baroreflex bradycardic responses to PE was significantly facilitated in OvxE+ females (-7.65 +/- 1.37) compared with OvxE- females (-4.5 +/- 0.4). Likewise, the slope of the baroreflex bradycardic responses to ANG II was significantly facilitated in OvxE+ females (-7.97 +/- 1.06) compared with OvxE- females (-4.8 +/- 1.6). Reflex tachycardic responses to SNP were comparable in all the groups. Finally, in male mice, the slope of ANG II-induced baroreflex bradycardia (-5.17 +/- 0.95) was significantly less than that induced by PE (-8.50 +/- 0.92), but this ANG II-mediated attenuation of reflex bradycardia was not observed in the female mice. These data support the hypothesis that estrogen facilitates baroreflex function in female mice and suggest that ANG II-mediated acute blunting of baroreflex regulation of heart rate may be sex dependent.

Role of the area postrema in angiotensin II modulation of baroreflex control of heart rate in conscious mice

This study reports the effects of angiotensin II (ANG II), arginine vasopression (AVP), phenylephrine (PE), and sodium nitroprusside (SNP) on baroreflex control of heart rate in the presence and absence of the area postrema (AP) in conscious mice. In intact, sham-lesioned mice, baroreflex-induced decreases in heart rate due to increases in arterial pressure with intravenous infusions of ANG II were significantly less than those observed with similar increases in arterial pressure with PE (slope: -3.0 +/- 0.9 vs. -8.1 +/- 1.5 beats x min(-1) x mmHg(-1)). Baroreflex-induced decreases in heart rate due to increases in arterial pressure with intravenous infusions of AVP were the same as those observed with PE in sham animals (slope: -5.8 +/- 0.7 vs. -8.1 +/- 1.5 beats x min(-1) x mmHg(-1)). After the AP was lesioned, the slope of baroreflex inhibition of heart rate was the same whether pressure was increased with ANG II, AVP, or PE. The slope of the baroreflex-induced increases in heart rate due to decreases in arterial blood pressure with SNP were the same in sham- and AP-lesioned animals. These results indicate that, similar to other species, in mice the ability of ANG II to acutely reset baroreflex control of heart rate is dependent on an intact AP.

Expression of metabotropic glutamate receptor 8 in autonomic cell groups of the medulla oblongata of the rat

Metabotropic glutamate receptors (mGluRs) in the medulla oblongata have been suggested to have a functional role in the regulation of cardiovascular baroreflexes. The present study examines the localization of mGluR8 autonomic nuclei of the medulla of the rat. mGluR8 immunoreactivity was observed in the cell bodies and/or processes of the dorsolateral, interstitial, medial, intermediate, ventral, ventrolateral, subpostremal, commissural, parvicellular and gelatinosus subnuclei of the nucleus tractus solitarius (NTS). The intensity of mGluR8 staining was highest in the commissural and interstitial subnuclei at the level of the area postrema. Commissural NTS is involved in regulation of baro-, and chemo-reflexes whereas the interstitial nucleus mediates respiratory reflexes. In the area postrema, diffuse staining was observed in the cell bodies, dendrites and fibers of the dorsal and central regions. In vagal outflow nuclei, mGluR8 immunoreactivity was observed in: (1). the cell bodies and processes of the dorsal motor nucleus of the vagus (DMN) throughout the rostro-caudal extent; and (2). the cell bodies and fibers throughout the rostro-caudal extent of the dorsal and ventral division the nucleus ambiguus (NA). Staining in the ventrolateral medulla was restricted to regions ventral to the nucleus ambiguus and dorsal to the lateral reticulate nucleus. The present study is the first to provide a detailed mapping of mGluR8 within the autonomic nuclei of the medulla and suggests that this subtype may be involved in shaping synaptic transmission in these central nuclei.

Frequency dependence of endocytosis in aortic baroreceptor neurons and role of group III mGluRs

Synaptic transmission between baroreceptor afferents and the nucleus tractus solitarius (NTS) is known to exhibit frequency-dependent depression. Reductions in neurotransmitter release and alterations in mechanisms regulating synaptic transmission are hypothesized to be involved in the activity-dependent depression observed in baroreceptor afferent neurons. The present study utilized cultured aortic baroreceptor neurons and the fluorescent dyes FM1-43 and FM2-10 to characterize the process of endocytosis or vesicle retrieval and its dependence on 1) frequency of neuronal activation, 2) metabotropic glutamate receptor (mGluR) activation, and 3) calcium concentrations inside and outside the cell. Endocytosis per spike, measured in fluorescence units after a 10-s stimulus applied at frequencies of 0.5 (53 +/- 4), 1.0 (23 +/- 1), and 10.0 Hz (2.7 +/- 0.2), was significantly depressed at higher frequencies. Blockade of group III mGluRs with (RS)-cyclopropyl-4-phosphonophenylglycine (CPPG) facilitated endocytosis at all frequencies, suggesting that this receptor subtype may be involved in the inhibition of endocytosis. Manipulating the extracellular and intracellular calcium concentrations subsequent to exocytosis had no effect on endocytosis. These results suggest that frequency-dependent depression of endocytosis observed in vitro could contribute to the frequency-dependent depression of baroreceptor afferent neurotransmission and that group III mGluRs inhibit endocytosis.

Cellular mechanisms regulating synaptic vesicle exocytosis and endocytosis in aortic baroreceptor neurons

The purpose of this chapter is to review some of the recent progress in the understanding of the cellular and biophysical mechanisms that are involved in the regulation of arterial baroreceptor neurotransmission. Synaptic depression or fatigue following repeated neuronal stimulation has been shown at central baroreceptor synapses in vivo and in vitro. As most of the central neurons have a limited number of vesicles, vesicle retrieval or endocytosis following exocytosis is thought to play a major role in preserving synaptic transmission. We have hypothesized that central baroreceptor terminals may inhibit their own synaptic transmission via feedback activation of presynaptic metabotropic glutamate receptors (mGluRs). We have analyzed the effects of mGluR autoreceptors (group III mGluRs) on voltage-gated calcium channels using standard patch-clamp techniques and on the process of exocytosis and endocytosis in aortic baroreceptor neurons using the quantitative imaging dye FM1-43 and FM2-10. Usng the whole-cell patch-clamp technique, we have found that activation of group III mGluRs with L-AP4 inhibits peak calcium channel current. Furthermore, activation of group III mGluRs with L-AP4 markedly decreases stimulation-induced exocytosis in aortic baroreceptor neurons, as measured with FM1-43, and inhibits synapsin I phosphorylation. These results suggest that activation of group III mGluRs may inhibit synaptic transmission by (1) inhibiting calcium influx, (2) decreasing synaptic vesicle exocytosis, and (3) modulating the mechanisms governing synaptic vesicle recovery and endocytosis. These effects of mGluRs on baroreceptor synaptic vesicles may contribute to the baroreceptor/nucleus tractus solitarius synaptic depression observed in vivo.

Evaluation of hemodynamics, vascular reactivity and baroreceptor compensation in the insulin resistant Zucker obese rats

Incidence of essential hypertension has been reported to be significantly higher in the population afflicted with non-insulin dependent diabetes mellitus (NIDDM). The present studies were under taken in the insulin resistant, Zucker obese rats to evaluate various factors that could lead to the development of high blood pressure. Direct blood pressure measurements in the conscious obese rats indicated that they were not consistently hypertensive although the blood pressures of the obese rats tended to be higher than that of the control lean rats. However, after Inactin anesthesia blood pressures of the obese rats were significantly elevated which can be related to an increase in sympathetic tone since autonomic ganglionic blockade eliminated the differences between the pressures of the two groups. Under anesthesia, cardiac output per 100 gm body weight was significantly lower indicating inadequate tissue perfusion in the obese rats. In a separate series of studies carried out in conscious rats, reflexly mediated alterations in the heart rate to intravenous phenylephrine and sodium nitroprusside were significantly blunted in the obese rats. These observations which include enhanced central sympathetic discharge, inadequate systemic hemodynamics and attenuation of baroreceptor compensation collectively suggest that the insulin resistant obese rats are in a pre-hypertensive state and could develop sustained hypertension if they are exposed to other risk factors.