Effect of angiotensin II on pulse wave velocity in humans is mediated through angiotensin II type 1 (AT(1)) receptors

The objective of this study was to examine the effect of angiotensin II (Ang II) and angiotensin II type 1 (AT(1)) receptor blockade on pulse wave velocity (PWV) in healthy humans. We studied nine young male volunteers in a double-blind randomised crossover design. Carotid-femoral PWV (an index of arterial stiffness) was measured by using a Complior machine. Subjects were previously treated for 3 days with once-daily dose of either a placebo or valsartan 80 mg. On the third day, they were infused with either placebo or 5 ng/kg/min of Ang II over 30 min. Subjects thus received placebo capsule + placebo infusion (P), valsartan + placebo infusion (V), placebo + Ang II infusion (A), and valsartan + Ang II infusion (VA) combinations. Heart rate (HR), blood pressure and PWV were recorded at baseline and then every 10 min during infusion and once after the end of infusion. There were significant increases in systolic blood pressure (SBP), diastolic blood pressure (DBP) and mean arterial pressure (MAP) with A compared with P (P = 0.002, P = 0.002, P = 0.001 respectively). These rises in blood pressure were completely blocked by valsartan. A significant rise in PWV by A was seen compared with P (8.38 +/- 0.24 vs 7.48 +/- 0.24 m/sec, P = 0.013) and was completely blocked by valsartan; VA compared with P (7.27 +/- 0.24 vs 7.48 +/- 0.24 m/sec, P = NS). Multiple linear regression analysis showed that blockade of Ang II induced increase in blood pressure by valsartan contributed to only 30% of the total reduction in Ang II induced rise in PWV (R(2) = 0.306). The conclusions were that valsartan completely blocks the effect of Ang II on PWV. The effect of Ang II on PWV is mediated through AT(1)receptors. Reduction in PWV by Ang II antagonist is not fully explained by its pressure lowering effect of Ang II and may be partially independent of its effect on blood pressure.

Evidence for changes in the tachyphylactic property of recombinant angiotensin II AT(1) receptor expressed in CHO cells

The manifestation of tachyphylaxis to angiotensin II in Chinese hamster ovary (CHO) cells expressing the rat angiotensin II AT(1) receptor was investigated. The cells were transfected with a cDNA fragment containing the complete coding region of the angiotensin II AT(1A) receptor gene, as well as 56 bp of its 3'- and 52 bp of its 5'-untranslated regions. These cells (CHO-AT(1)) responded to angiotensin II by increases in intracellular Ca(2+) concentration and inositol phosphate turnover, which were inhibited upon repeated administrations, characterizing the tachyphylaxis phenomenon. In contrast to smooth muscle cells, which are rendered tachyphylactic to angiotensin II but not to [2-lysine]angiotensin II ([Lys(2)]angiotensin II), this analogue induced responses in CHO-AT(1) cells that were also inhibited upon repeated administrations. A smooth muscle cell line, which showed tachyphylaxis only to angiotensin II, became tachyphylactic also to [Lys(2)]angiotensin II after transfection with the angiotensin II AT(1) receptor gene. Our findings suggest that posttranscriptional control directed by the 3'- or the 5'-untranslated regions in the angiotensin II AT(1) receptor gene may play a role in modulating the signal transduction pathways involved in the mechanism of angiotensin II tachyphylaxis.

beta-Arrestin scaffolding of the ERK cascade enhances cytosolic ERK activity but inhibits ERK-mediated transcription following angiotensin AT1a receptor stimulation

beta-Arrestins are cytosolic proteins that mediate homologous desensitization of G protein-coupled receptors (GPCRs) by binding to agonist-occupied receptors and by uncoupling them from heterotrimeric G proteins. The recent finding that beta-arrestins bind to some mitogen-activated protein (MAP) kinases has suggested that they might also function as scaffolds for GPCR-stimulated MAP kinase activation. To define the role of beta-arrestins in the regulation of ERK MAP kinases, we examined the effect of beta-arrestin overexpression on ERK1/2 activation and nuclear signaling in COS-7 cells expressing angiotensin II type 1a receptors (AT1aRs). Expression of either beta-arrestin1 or beta-arrestin2 reduced angiotensin-stimulated phosphatidylinositol hydrolysis but paradoxically increased angiotensin-stimulated ERK1/2 phosphorylation. The increase in ERK1/2 phosphorylation in beta-arrestin-expressing cells correlated with activation of a beta-arrestin-bound pool of ERK2. The beta-arrestin-dependent increase in ERK1/2 phosphorylation was accompanied by a significant reduction in ERK1/2-mediated, Elk1-driven transcription of a luciferase reporter. Analysis of the cellular distribution of phospho-ERK1/2 by confocal immunofluorescence microscopy and cellular fractionation revealed that overexpression of beta-arrestin resulted in a significant increase in the cytosolic pool of phospho-ERK1/2 and a corresponding decrease in the nuclear pool of phospho-ERK1/2 following angiotensin stimulation. beta-Arrestin overexpression resulted in formation of a cytoplasmic pool of beta-arrestin-bound phospho-ERK, decreased nuclear translocation of phospho-ERK1/2, and inhibition of Elk1-driven luciferase transcription even when ERK1/2 was activated by overexpression of cRaf-1 in the absence of AT1aR stimulation. These data demonstrate that beta-arrestins facilitate GPCR-mediated ERK activation but inhibit ERK-dependent transcription by binding to phospho-ERK1/2, leading to its retention in the cytosol.

Angiotensin I-converting enzyme inhibition increases cardiac catecholamine content and reduces monoamine oxidase activity via an angiotensin type 1 receptor-mediated mechanism

Antihypertensive and cardioprotective effects of angiotensin I-converting enzyme (ACE) inhibitors are well established and have usually been attributed to the inhibition of angiotensin II (ANG)-mediated effects at vascular or ventricular (angiotensin type 1) AT(1) receptors. One other important mechanism involves ANG-induced interactions with the sympathetic nervous system, which might include alterations of cardiac catecholamine concentrations during ACE inhibition due to a modulation of monoamine oxidase (MAO) activity. Tissue catecholamines were studied in spontaneously hypertensive rats that were long-term treated with captopril (50 or 0.5 mg/kg/day), enalapril (10 or 0.1 mg/kg/day), an AT(1) receptor antagonist (candesartan-cilexetil, 3 mg/kg/day), or a calcium antagonist (mibefradil, 18 mg/kg/day). The kinetic parameters of MAO were then determined in vitro in the presence of ANG, captopril, enalaprilat, or candesartan. Noradrenaline and adrenaline contents were doubled in the left ventricle by captopril, enalapril, or candesartan independently of hypotensive potency but not in liver or cortex. In parallel, cardiac MAO activity was reduced by all doses of captopril (49/29%), enalapril (52/24%), or candesartan (38%). Mibefradil, which does not interact with the renin-angiotensin system, did not alter cardiac catecholamines or MAO activity when an equipotent antihypertensive dose was applied. In vitro MAO activity was not influenced by ANG, enalaprilat, or captopril at concentrations of up to 1 mM. It is concluded that diminished AT(1) receptor stimulation decreases cardiac MAO activity, probably by regulating MAO expression, since ANG, ACE inhibitors, and AT(1) antagonists had no effect on MAO activity in vitro. This action contributes to an increase in cardiac catecholamine content that may improve cardiac sympathetic control during therapy.

Comparative effects of chronic ACE inhibition and AT1 receptor blocked losartan on cardiac hypertrophy and renal function in hypertensive patients

The present study describes the effects of losartan and the angiotensin-converting enzyme inhibitor enalapril on blood pressure, echocardiographically calculated left ventricular mass, renal function evaluated by glomerular filtration rate and quality of life. The renin-angiotensin-aldosterone system is of importance for cardiovascular growth. There is substantial experimental documentation in animals that the angiotensin II antagonist, losartan, decreases the cardiac hypertrophy response caused by elevated arterial pressure as well as intravascular volume overload. However, data in humans is scarce. This is a 3-year, randomised, double-blind study with parallel group design in 50 patients with essential hypertension. The results show that both drugs reduced blood pressure equally effectively, and also left ventricular mass (P < 0.001). After 3 years of treatment glomerular filtration rate significantly increased with losartan (P < 0.005). Serum uric acid fell modestly although significantly, dose-dependent in losartan patients compared with an increase in enalapril patients. A fall in serum potassium from the pre-study period was observed in all patients. There was no difference between treatments in terms of patient satisfaction on quality of life. Both drugs have relatively similar hormonal and haemodynamic effect, with an excellent tolerability profile; they appear to induce comparable blood pressure falls in hypertensive patients in particular, therapy based on specific Ang II blockade may offer advantages in high risk hypertensives if left ventricular hypertrophy is present. Both enalapril and losartan, in improving the renal function attenuating the intrarenal effects of angiotensin II, might be able to reverse the pathophysiology of essential hypertensive kidney disease, and should be first-choice drugs in the treatment of essential hypertension.

Angiotensin II activates a bumetanide sensitive increase in 86RB+ efflux in the rat heart

We previously reported that alpha-adrenoceptor (AR) stimulation of the isolated perfused rat heart increased the efflux of 42K+ and the K+ analogue 86Rb+. The main part of this increase was bumetanide sensitive, indicating an activation of the Na+/K+/2Cl- cotransporter. The purpose of the present study was to investigate the effects of angiotensin II (1-100 nmol/l) and the protein kinase C (PKC) activator PMA (phorbol-12-myristate-13-acetate, 1-1000 nmol/l) on 86Rb+ efflux from isolated rat hearts and to compare the effects with the effect of the alpha1- AR agonist phenylephrine (30 micromol/l) in the presence of a beta-AR antagonist. Phenylephrine increased the 86Rb+ efflux rate by 47+/-4.1% (n=5, p<0.001). Angiotensin II induced a maximal increase in 86Rb+ efflux rate of 13+/-1.6% (n=12, p<0.0001). The effect of angiotensin II was totally eliminated by bumetanide (50 micromol/l). PMA decreased the 86Rb+ efflux rate by 23+/-7.0 % (n=7, p=0.02) and this effect of PMA was not sensitive to bumetanide. Pre-treatment of the hearts with PMA for 30 min did not influence the response to phenylephrine. In conclusion, angiotensin II stimulation, but not PKC activation by PMA increased the 86Rb+ efflux rate in isolated rat hearts, but the effect was smaller than that of alpha1- AR stimulation. The effect of angiotensin II was completely abolished by bumetanide indicating an activation of the Na+/K+/2Cl- -cotransporter.

Downregulation of angiotensin II type 1 receptor by hydrophobic 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors in vascular smooth muscle cells

3-Hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase inhibitors, so-called statins, reduce the relative risk of a major coronary event by lowering the serum cholesterol level. In addition, statins may confer beneficial effects by cholesterol-lowering independent mechanisms, which are incompletely characterized. Because angiotensin II (Ang II) plays crucial roles in the pathogenesis of cardiovascular diseases, we examined the effect of statins on the expression of the Ang II type 1 receptor (AT(1)-R) in cultured vascular smooth muscle cells (VSMCs). Cerivastatin and fluvastatin reduced the AT(1)-R mRNA and the AT(1)-R protein levels; however, pravastatin lacked this effect. Cerivastatin and fluvastatin suppressed the AT(1)-R promoter activity measured by luciferase assay but did not affect AT(1)-R mRNA stability, suggesting that the suppression occurs at the transcriptional level. Coincubation of VSMCs with mevalonate or geranylgeranyl pyrophosphate but not with farnesyl pyrophosphate reversed the cerivastatin-induced AT(1)-R downregulation. Overexpression of dominant-negative Rho A also suppressed AT(1)-R mRNA expression. Treatment with cerivastatin for 24 hours reduced the calcium response of VSMCs to Ang II. Taken together, statins downregulate AT(1)-R expression through a mevalonate-dependent, geranylgeranyl pyrophosphate-dependent, and Rho A-dependent manner and attenuate the biological function of Ang II. Downregulation of AT(1)-R may contribute to the cholesterol-independent beneficial effect of statins on the cardiovascular system.

Selective potentiation of N-type calcium channels by angiotensin II in rat subfornical organ neurones

1. Here we have characterized Ca(2+) currents in rat subfornical organ neurones, and their modulation by angiotensin II. Currents were of the high voltage-activated (HNA) subtype, as the threshold for activation was near -30 mV (mid-point potential (V(50)) of activation -14 mV). Using Ba(2+) as the charge carrier, little inactivation was observed, and it occurred only at depolarized potentials (V(50) of inactivation -12 mV). More inactivation was observed using Ca(2+) as the charge carrier, indicating that Ca(2+)-dependent inactivation plays a role in regulating Ca(2+) channel function in subfornical organ (SFO) neurones. 2. The net Ba(2+) current could be blocked by Cd(2+) (EC(50) 1.6 microM), confirming that currents are of the HVA variety. By using selective antagonists, we identified the presence of both L- and N-type channels; 20 microM nifedipine blocked 22 +/- 1 % of the current, while omega-conotoxin GVIA blocked 65 +/- 7 %, indicating that these currents make up the net current through Ca(2+) channels. 3. Angiotensin II potentiated the inward current throughout the range of activation. Using depolarizing voltage ramps, 1 nM angiotensin potentiated the peak current by 14 +/- 5 %. We then used selective blockade of the HVA component currents; 20 microM nifedipine failed to prevent the potentiation by angiotensin II (12 +/- 5 %), while blocking N-type channels with omega-conotoxin GVIA blocked the facilitation by ANG (2.3 +/- 2 %). Losartan (1 microM) prevented the actions of ANG on the inward current (1.6 +/- 1 %), indicating that the selective effects of ANG on N-type channels in SFO neurones are mediated by AT(1) receptors.

Upregulation of alpha(8)beta(1)-integrin in cardiac fibroblast by angiotensin II and transforming growth factor-beta1

Using a novel pharmacological tool with (125)I-echistatin to detect integrins on the cell, we have observed that cardiac fibroblasts harbor five different RGD-binding integrins: alpha(8)beta(1), alpha(3)beta(1), alpha(5)beta(1), alpha(v)beta(1), and alpha(v)beta(3). Stimulation of cardiac fibroblasts by angiotensin II (ANG II) or transforming growth factor-beta1 (TGF-beta1) resulted in an increase of protein and heightening by 50% of the receptor density of alpha(8)beta(1)-integrin. The effect of ANG II was blocked by an AT(1), but not an AT(2), receptor antagonist, or by an anti-TGF-beta1 antibody. ANG II and TGF-beta1 increased fibronectin secretion, smooth muscle alpha-actin synthesis, and formation of actin stress fibers and enhanced attachment of fibroblasts to a fibronectin matrix. The alpha(8)- and beta(1)-subunits were colocalized by immunocytochemistry with vinculin or beta(3)-integrin at focal adhesion sites. These results indicate that alpha(8)beta(1)-integrin is an abundant integrin on rat cardiac fibroblasts. Its positive modulation by ANG II and TGF-beta1 in a myofibroblast-like phenotype suggests the involvement of alpha(8)beta(1)-integrin in extracellular matrix protein deposition and cardiac fibroblast adhesion.

Desensitization of type 1 angiotensin II receptor subtypes in the rat kidney

Differences involving serine residues in the sequence of the carboxyl-terminal tail of type 1 angiotensin II (Ang II) receptor subtypes AT(1A) and AT(1B) suggest differences in desensitization ability. We examined the Ang II-induced homologous desensitization patterns of both receptor subtypes in freshly isolated renal structures: glomerulus (Glom), afferent arteriole, and cortical thick ascending limb (CTAL), whose content in each subtype mRNA is different, by measuring variations in intracellular calcium concentration. A preexposure to a maximal dose of Ang II, followed by a second application of the same concentration, induced: 1) a complete desensitization in Glom, where AT(1A) and AT(1B) mRNAs were expressed in similar proportions, and 2) no or partial desensitization in afferent arteriole and CTAL, where AT(1A) mRNA was predominant. In the absence of nephron structure containing only AT(1B) mRNA, we studied rat anterior pituitary cells that exhibit high content in this subtype and observed that desensitization was not complete. In Glom, CTAL, and pituitary cells, desensitization proceeded in a dose-dependent manner. In Glom and CTAL, desensitization occurred via a PKC-independent mechanism. These results suggest that desensitization does not depend on the nature of Ang II receptor subtype but either on the proportion of each subtype in a given cell and/or on cell specific type. This could allow adaptive biological responses to Ang II appropriate to the specific function of a given cell type.

Angiotensin II enhances tubuloglomerular feedback via luminal AT(1) receptors on the macula densa

BACKGROUND

Recent studies have revealed angiotensin II subtype 1 (AT1) receptors on macula densa cells, raising the possibility that angiotensin II (Ang II) could enhance tubuloglomerular feedback (TGF) by affecting macula densa cell function. We hypothesized that Ang II enhances TGF via activation of AT1 receptors on the luminal membrane of the macula densa.

METHODS

Rabbit afferent arterioles and the attached macula densa were simultaneously microperfused in vitro, keeping pressure in the afferent arteriole at 60 mm Hg.

RESULTS

The afferent arteriole diameter was measured while the macula densa was perfused with low NaCl (Na+, 5 mmol/L; Cl-, 3 mmol/L) and then with high NaCl (Na+, 79 mmol/L; Cl-, 77 mmol/L) to induce a TGF response. When TGF was induced in the absence of Ang II, the afferent arteriole diameter decreased by 2.4 +/- 0.5 microm (from 17.3 +/- 1.0 to 14.9 +/- 1.2 microm). With Ang II (0.1 nmol/L) present in the lumen of the macula densa, the diameter decreased by 3.8 +/- 0.7 microm (from 17.3 +/- 1.0 to 13.5 +/- 1.2 microm, P < 0.05 vs. TGF with no Ang II, N = 8). To test whether Ang II enhances TGF via activation of AT1 receptors on the luminal membrane of the macula densa, Ang II plus losartan (1 micromol/L) was added to the lumen. Losartan itself did not alter TGF. When TGF was induced in the absence of Ang II and losartan, the afferent arteriole diameter decreased by 2.3 +/- 0.3 microm (from 15.9 +/- 1.0 to 13.6 +/- 1.2 microm). When Ang II and losartan were both present in the macula densa perfusate, the diameter decreased by 2.4 +/- 0.4 microm (from 15.8 +/- 0.9 to 13.4 +/- 0.7 microm, P> 0.8 vs. TGF with no Ang II and losartan, N = 7). We then examined whether AT2 receptors on the macula densa influence the effect of luminal Ang II on TGF. When TGF was induced in the absence of Ang II plus PD 0123319-0121B (1 micromol/L), the afferent arteriole diameter decreased by 2.4 +/- 0.2 microm (from 17.0 +/- 0.9 to 14.6 +/- 0.8 microm). When Ang II and PD 0123319-0121B were both present in the macula densa lumen, the diameter decreased by 3.9 +/- 0.2 microm (from 16.8 +/- 0.9 to 12.9 +/- 0.9 microm, P < 0.001 vs. TGF with no Ang II and PD 0123319-0121B, N = 8). PD 0123319-0121B itself had no effect on TGF. To assure that this effect of Ang II was not due to leakage into the bath, losartan was added to the bath. When TGF was induced in the absence of Ang II with losartan in the bath, the afferent arteriole diameter decreased by 2.8 +/- 0.5 microm (from 19.3 +/- 1.2 to 16.5 +/- 0.8 microm). After Ang II was added to the macula densa perfusate and losartan to the bath, the diameter decreased by 4.0 +/- 0.7 microm (from 18.9 +/- 1.1 to 14.9 +/- 0.5 microm, P < 0.01 vs. TGF with no Ang II in the lumen and losartan in the bath, N = 8).

CONCLUSIONS

These results demonstrate that Ang II enhances TGF via activation of AT1 receptors on the luminal membrane of the macula densa.

Postnatal development of presynaptic receptors that modulate noradrenaline release in mice

The objective of the study was to clarify the postnatal development of the following transmitter release-modulating receptors of noradrenergic neurons in mice: alpha2-adrenoceptors, muscarinic, opioid and cannabinoid receptors (inhibitory), beta-adrenoceptors and receptors for angiotensin II and bradykinin (facilitatory). Wildtype (NMRI) and in some cases alpha2A/D-adrenoceptor-deficient mice aged 1 day (P1) or 8-16 weeks (adults) were used. Hippocampal and occipito-parietal cortex slices and sympathetically innervated tissues (atria and vas deferens) were preincubated with [3H]-noradrenaline and then superfused and stimulated electrically. Stimulation led to distinct increases in tritium efflux which were abolished by tetrodotoxin or removal of calcium. Concentration-response curves of appropriate agonists and in the case of alpha2-autoreceptors antagonists were determined. For beta-adrenoceptors and angiotensin receptors, the interaction of agonists with antagonists was also examined. Results demonstrate that alpha2A/D-autoreceptors operate already at P1 whereas nonalpha2A/D-autoreceptors, presumably alpha2C, develop later. Of the various heteroreceptors, those of brain noradrenergic neurons (OP3 and ORL1) modulate the release of [3H]-noradrenaline at least as effectively at P1 as in adults. Those of peripheral sympathetic neurons (muscarinic, probably mainly M2, OP1, OP2, OP3, CB1, AT1 and B1), in contrast, operate less effectively or not at all at P1, with one exception: beta2-adrenoceptors increase the release of [3H]-noradrenaline (atria) to the same extent, irrespective of age. Overall, results indicate that brain and peripheral noradrenergic neurons release their transmitter already shortly after birth. Presynaptic receptor mechanisms mature differentially in the brain and the periphery. Moreover, the various presynaptic receptors differ in their postnatal development and may play differential roles at different ages.

Effects of central angiotensin II receptor antagonism on fetal swallowing and cardiovascular activity

OBJECTIVE

Fetal plasma angiotensin II levels are 10 times the levels found in adults. Despite these high levels, central injection of angiotensin II may stimulate fetal swallowing and increase fetal arterial blood pressure. We postulated that the high rate of spontaneous fetal swallowing and normal fetal pressor regulation may be dependent, in part, on central angiotensin II. In view of the potential dipsogenic role of both type 1 and type 2 angiotensin II receptors, we examined the central effect of the nonselective angiotensin II receptor antagonist saralasin on fetal swallowing and cardiovascular responses.

STUDY DESIGN

Six time-dated pregnant ewes and fetuses were chronically prepared with fetal vascular and intracerebroventricular catheters, electrocorticograms, and esophageal electromyogram electrodes and studied at 130 +/- 1 days' gestation. After an initial 2-hour baseline period (0 to 2 hours), saralasin (1 mL, 64 microg) was injected intracerebroventricularly (2 to 4 hours). After 4 hours the dose of saralasin was repeated together with angiotensin II (1 mL, 6.4 microg), and the fetuses were monitored for a final 2 hours. Four fetuses also underwent an identical control study (on an alternate day) in which saralasin was replaced with artificial cerebrospinal fluid.

RESULTS

Blockade of central angiotensin II receptors by intracerebroventricular saralasin significantly reduced mean (+/- SEM) spontaneous fetal swallowing (1.3 +/- 0.1 to 0.4 +/- 0.1 swallows per minute; P <.001) but did not alter fetal mean blood pressure (50 +/- 5 versus 56 +/- 5 mm Hg). Intracerebroventricular angiotensin II, in the presence of saralasin, did not affect swallowing (0.6 +/- 0.1 swallows per minute) or fetal blood pressure. In the control study, intracerebroventricular artificial cerebrospinal fluid did not change fetal swallowing (0.9 +/- 0.1 versus 1.0 +/- 0.1 swallows per minute), whereas intracerebroventricular angiotensin II significantly increased swallowing activity (1.0 +/- 0.1 versus 2.0 +/- 0.1 swallows per minute; P <.001) and fetal blood pressure (51 +/- 2 to 59 +/- 3 mm Hg; P =.003).

CONCLUSIONS

Tonic activity of central angiotensin II receptor stimulation contributed to the high rate of basal ovine fetal swallowing but not fetal basal blood pressure. Angiotensin II-mediated fetal dipsogenic and pressor responses are a result of specific angiotensin II receptor binding in central brain regions. These results indicate that fetal exposure to angiotensin II antagonists or angiotensin-converting enzyme inhibitors may have adverse effects on fetal and amniotic fluid homeostasis.

Angiotensin IV enhances LTP in rat dentate gyrus in vivo

Angiotensins have been shown to play a significant role in a variety of physiological functions including learning and memory processes. Relatively recent evidence supports the increasing importance of angiotensin IV (Ang IV), in many of these functions previously associated only with Ang II, including learning and memory. An interesting hypothesis generated by these results has been that Ang II is a precursor for the production of a more active peptide fragment, Ang IV. Since Ang II impairs learning and memory, when administered directly or released into the hippocampal dentate gyrus, and inhibits long term potentiation (LTP) in medial perforant path-dentate granule cell synapses, as well; it remained to be seen what effects Ang IV had on LTP in these same synapses. Results of this study show clearly that Ang IV significantly enhances LTP, and the enhancement is both dose and time dependent. The following solutions of Ang IV were administered over a five min period, at the end of baseline and before the first tetanus was applied: 2.39, 4.78, and 9.56 nM. An inverted U-type dose related effect was observed. A complex time related effect was observed with a maximum at 5 min, a return to normal LTP at 30 min and a minimum below normal at 90 min, and a return to normal LTP at 120 min. The effects of the 4.78 nM solution were determined at the following intervals between administration and the first tetanus: 5, 15, 30, 60, 90, and 120 min. The enhancement of LTP can be prevented by pretreatment with Divalinal, an Ang IV antagonist, without any effect on normal LTP. Two solutions of Divalinal were used; 5 nM and 5 microM, and the 5 microM was more effective and completely blocked the enhancement of normal LTP. Results were also obtained with 4.78 nM Nle1-Ang IV (Norleucine), an Ang IV agonist. Norleucine was less effective than Ang IV in the enhancement of normal LTP and displayed a similar time course of activity. Both Ang IV and Norleucine produced a significant suppression of normal LTP at 90 min; that remains to be explained. However, the inhibition by Ang IV was dose dependent and was blocked by Divalinal. The fact that the Ang IV enhancement of normal LTP was blocked by losartan, an Ang II AT1 receptor antagonist, is puzzling since Divalinal had no effect on the inhibition of LTP by Ang II.

Pre-treatment with candesartan protects from cerebral ischaemia

Angiotensin II (Ang II) regulates cerebral blood flow by stimulating cerebral vasoconstriction via AT1-receptors. In adult spontaneously hypertensive rats (SHR), the cerebrovascular autoregulatory curve is shifted to the right, in the direction of higher blood pressures, an indication of excessive cerebrovascular vasoconstriction. A restricted capacity to dilate cerebral blood vessels may be responsible for the enhanced vulnerability to cerebrovascular ischaemia during hypertension. We found that chronic treatment with the AT1-receptor antagonist, candesartan, (0.5 mg/kg/day for 14 days, via osmotic minipumps implanted in the subcutaneous tissue) blocked Ang II binding to AT1-receptors in cerebral blood vessels and in brain areas involved in the regulation of cerebrovascular flow, and increased the ratio of lumen-wall area in the middle cerebral artery. Candesartan treatment normalised the lower part of the autoregulatory curve in SHR, and markedly decreased cerebral ischaemia as a consequence of middle cerebral artery occlusion with reperfusion. Protection from ischaemia is related to arterial remodelling, enhanced compensatory vasodilatation in the peripheral area of ischaemia, decreased reduction in cerebral blood flow following the occlusion of a major cerebral blood vessel, and protection from injury in the periphery of the lesion. Our results indicate that pre-treatment with AT1-antagonists such as candesartan could be of benefit in the prevention and treatment of brain ischaemia.

Increased AT(1) receptor heterodimers in preeclampsia mediate enhanced angiotensin II responsiveness

Several examples of functional G-protein-coupled receptor heterodimers have been identified. However, it is not known whether receptor heterodimerization is involved in the pathogenesis of human disorders. Here we show that in preeclamptic hypertensive women, a significant increase in heterodimerization occurs between the AT(1)-receptor for the vasopressor angiotensin II and the B(2)-receptor for the vasodepressor bradykinin. AT(1)-B(2)-receptor heterodimerization in preeclampsia correlated with a 4-5-fold increase in B(2)-receptor protein levels. Expression of the AT(1)-B(2) heterodimer increased the responsiveness to angiotensin II and conferred resistance in AT(1)-receptors to inactivation by reactive oxygen species raised in normotensive and preeclamptic pregnancies. We suggest that AT(1)-B(2) heterodimers contribute to angiotensin II hypersensitivity in preeclampsia. Moreover, we identify preeclampsia as the first disorder associated with altered G-protein-coupled receptor heterodimerization.

Nitric oxide-dependent down-regulation of angiotensin II type 2 receptors during experimental sepsis

OBJECTIVE

The systemic renin-angiotensin system is highly activated during septic shock. This has focused interest in regulation of the adrenal angiotensin II type 2 receptor (AT2) as the target thought to mediate angiotensin II-induced adrenal catecholamine release during experimental sepsis in vivo. In addition, the influence of typical endogenous mediators of sepsis, such as proinflammatory cytokines and nitric oxide, on AT2 receptor expression should be investigated in vitro.

DESIGN

Prospective animal trial followed by a controlled cell culture study.

SETTING

Laboratory of the Department of Anesthesiology.

SUBJECTS

Male Sprague-Dawley rats weighing 200-250 g, PC12 cell line.

INTERVENTIONS

Rats were injected with lipopolysaccharide to stimulate Gram-negative sepsis or lipoteichoic acid to stimulate Gram-positive sepsis. AT2 receptor expression, abundance of the proinflammatory cytokines (interleukin-1beta, tumor necrosis factor-alpha, and interferon-gamma), and nitric oxide synthase II expression have been determined in the adrenal gland. Rat adrenal pheochromocytoma cells were incubated with these cytokines or with the nitric oxide donors sodium nitroprusside or S-nitroso-N-acetylpenicillamine to investigate the regulation of AT2 receptors during severe inflammation on a cellular level.

MEASUREMENTS AND MAIN RESULTS

In the adrenal gland, AT2 receptor expression was down-regulated in both models of sepsis, whereas tissue cytokine concentrations were elevated and nitric oxide synthase II expression was induced. Incubation of PC12 cells with proinflammatory cytokines resulted in a dose-dependent diminished expression of AT2 receptors, which was mimicked by incubation with nitric oxide donors. Blocking of cytokine-induced nitric oxide synthesis by co-incubation of PC12 cells with NG-nitro-l-arginine methyl ester prevented down-regulation of AT2 receptors.

CONCLUSIONS

These findings show that in our model of sepsis, the expression of AT2 receptors in the adrenal gland is down-regulated in a nitric oxide-dependent manner. Because AT2 receptors are thought to be involved in adrenal catecholamine secretion in a stimulatory fashion, the diminished expression of AT2 receptors could play an important role in the pathogenesis of septic shock via impaired angiotensin II-induced adrenal catecholamine release, despite a strong activation of the systemic renin-angiotensin system.

Angiotensin II promotes glucose-induced activation of cardiac protein kinase C isozymes and phosphorylation of troponin I

Activation of the protein kinase C (PKC) family is a potential signaling mechanism by which high ambient glucose concentration modulates the phenotype and physiological function of cells. Recently, the cardiac renin angiotensin system (RAS) has been reported to promote PKC translocation in the diabetic heart via the angiotensin (ANG) II type 1 receptor (AT-1R). To evaluate the molecular events coupled with high glucose-induced PKC translocation and to examine the role of endogenously released ANG II in myocyte PKC signaling, primary cultures of adult rat ventricular myocytes were exposed to normal (5 mmol/l) or high (25 mmol/l) glucose for 12-24 h. Western blot analysis indicated that adult rat ventricular myocytes coexpress six PKC isozymes (alpha, beta(1,) beta(2,) delta, epsilon, and zeta). Translocation of five PKC isozymes (beta(1), beta(2), delta, epsilon, and zeta) was detected in response to 25 mmol/l glucose. Inhibition of phospholipase C with tricyclodecan-9-yl-xanthogenate blocked glucose-induced translocation of PKC-beta(2), -delta, and -zeta. Inhibition of tyrosine kinase with genistein blocked glucose-induced translocation of PKC-beta(1) and -delta, whereas chelation of intracellular Ca(2+) with 1,2-bis(2-aminophenoxy)ethane N,N,N,'N'-tetraacetic acid blocked translocation of PKC-beta(1) and -beta(2). Enzyme-linked immunosorbent assay performed on culture media from myocytes maintained in 25 mmol/l glucose detected a twofold increase in ANG II. Addition of an AT-1R antagonist (losartan; 100 nmol/l) to myocyte cultures blocked translocation of PKC-beta(1), -beta(2), -delta, and -epsilon. Phosphorylation of troponin (Tn) I was increased in myocytes exposed to 25 mmol/l glucose. Losartan selectively inhibited Tn I serine phosphorylation but did not affect phosphorylation at threonine residues. We concluded that 1) 25 mmol/l glucose triggers the release of ANG II by myocytes, resulting in activation of the ANG II autocrine pathway; 2) differential translocation of myocyte PKC isozymes occurs in response to 25 mmol/l glucose and ANG II; and 3) AT-1R-dependent PKC isozymes (beta(1), beta(2), delta, and epsilon) target Tn I serine residues.

[In contrast to captopril and enalapril, losartan does not increase mortality of gerbils after carotid ligation]

OBJECTIVE

To check wether the deleterious effect of enalaprilat administered before unilateral caroid ligation in the gerbil reported by Fernandez et al. (J Cardiovasc Pharmacol 1994; 24: 937) is not a molecule specific effect but an angiotensin converting enzyme inhibitor class effect.

DESIGN AND METHOD

Survival rate of gerbils (an animal with incomplete Willis hexagona) was measured after unilateral carotid ligation with preadministration (2 hours before by gavage) of saline (0.75 ml) (n = 37); losartan (20 mg/kg) (n = 37), enalaparil (10 mg/kg) (n = 37); a combination of losartan and enalapril at the same dose (n = 37); and of captopril (75 mg/kg) (n = 35).

RESULTS

The survival rate of the gerbils 72 hours after carotid ligation was 65% in control, 62% in losartan, 30% in enalapril, 32% in enalapril + losartan, and 32% in captopril groups. Statistical analysis (log rank test) of the Kaplan-Meier survival curves over 72 hours showed no difference between losartan and controls nor between the various groups treated with ACEI. However survival was significantly lower in the ACEI groups than in the group treated by losartan alone (p < 0.02) or controls (p < 0.02). Intraaortic mean arterial pressure was measured in 6 controls, 6 animals treated with losartan and 6 other treated with enalapril. It was comparable in the losartan and enalapril treated animals (65 +/- 2 mm Hg vs 64 +/- 2) but significantly lower than in the controls (77 +/- 2 mmHg) (p < 0.02).

CONCLUSIONS

In contrast to oral preadministration of enalapril and captopril that of losartan does not increase the mortality of the gerbil after unilateral carotid ligation in spite of the same decrease in systemic blood pressure. Although a lower mortality than in controls was not observed with losartan as in the princeps study of Fernandez, these data are consistent with the demonstration by this author that angiotensin II plays a critical protective role in acute ischemia probably by promoting collateral circulation recruitment through non-AT1 receptors stimulation.

Multiple pathways of angiotensin I conversion and their functional role in the canine penile corpus cavernosum

Multiple pathways of angiotensin (Ang) I conversion and their functional role in the canine penile corpus cavernosum were investigated. Biochemical analysis revealed high activities of angiotensin-converting enzyme (ACE) (6.9 +/- 1.7 mU/mg of protein, mean +/- S.E.M., n = 8) and chymase-like enzyme (4.0 +/- 1.4 mU/mg of protein). Functional recording of isometric tension showed that Ang I (3 x 10(-7) M) induced a tension of 0.17 +/- 0.05 g (n = 5), which was reduced to about 60% by pretreatment with an ACE inhibitor, lisinopril (10(-6) M), and almost completely blocked by lisinopril in combination with a chymase inhibitor, chymostatin (10(-4) M). Binding sites for ACE and Ang II receptors were studied by in vitro autoradiography using 125I-351A and 125I-[Sar1, Ile8]Ang II as ligands, respectively. Dense binding of ACE appeared in the endothelial layer of the corpus cavernosum penis, and Ang II receptors were localized in the trabecular smooth muscle layer. An AT1 receptor antagonist, CV-11974 (10(-6) M), markedly displaced 125I-[Sar1, Ile8]Ang II bindings, indicating that the corpus cavernosum penis contains AT1 receptors exclusively. Immunohistochemical studies demonstrated ACE in the endothelium of the corpus cavernosum penis. Mast cells that produce chymase were present mainly in the cavernosal area. These results demonstrate that chymase, in addition to ACE, is involved in the contraction of canine penile corpus cavernosum through local Ang II formation.

Chronic treatment with amlodipine modulates adrenocortical angiotensin II receptors in spontaneously hypertensive rats

We investigated the effects of long-term treatment with calcium-antagonist, amlodipine, on angiotensin II receptors in the adrenal cortex of spontaneously hypertensive rats (SHR). Seven-week-old male SHR were treated with oral amlodipine (10 mg/kg/day) or vehicle (saline) for four weeks. Age-matched normotensive Wistar-Kyoto (WKY) rats were treated with the vehicle similar to control SHR. Systolic blood pressure (SBP) showed time-dependent increase in SHR but not in WKY rats, while amlodipine treatment significantly reduced the high SBP in SHR. Plasma renin activity was serially increased in SHR, which was further enhanced by amlodipine treatment. But the plasma aldosterone level which was increased in SHR was not changed by amlodipine. Competitive reverse transcriptase-polymerase chain reaction showed that the level of adrenocortical angiotensin II type 1 receptor (AT1R) mRNA progressively decreased in vehicle-treated SHR compared to WKY rats and that 4-week course of amlodipine treatment significantly increased AT1R mRNA in SHR to levels comparable to those in WKY rats. Amlodipine treatment reduced the level of adrenocortical angiotensin II type 2 receptor (AT2R) mRNA in SHR from 8 weeks of age. Thus, chronic amlodipine treatment differently modulates both adrenocortical AT1R and AT2R in SHR in a possibly direct manner.

3-Hydroxy-3-methylglutaryl-CoA reductase inhibitors block calcium-dependent tyrosine kinase Pyk2 activation by angiotensin II in vascular endothelial cells. involvement of geranylgeranylation of small G protein Rap1

We recently reported the calcium-dependent activation of tyrosine kinase Pyk2 by angiotensin II (Ang II) in pulmonary vein endothelial cells (PVEC). Since Pyk2 has no calcium binding domain, and neither Ca(2+) nor Ca(2+)/calmodulin directly activates Pyk2, it is not clear how Ca(2+) transduces the signal to activate Pyk2, a key tyrosine kinase, in the early events of Ang II signaling. In the present study, we investigated the mechanism of the calcium-dependent activation of Pyk2 in response to Ang II by using 3-hydroxy-3-methylglutaryl-CoA reductase inhibitors and isoprenoid intermediates in PVEC. We have obtained substantial evidence indicating that Ang II activates Pyk2 through calcium-mediated activation of the geranylgeranylated small G protein Rap1 and the Rap1 association with Pyk2. Thus, the small G protein Rap1 is an intermediary signaling molecule linking Ang II-induced calcium signal to Pyk2 activation in PVEC. In addition, our results indicate that 3-hydroxy-3-methylglutaryl-CoA reductase inhibitors, a class of cholesterol-lowering drugs, could interrupt Ang II signaling independent of cholesterol lowering in endothelial cells.