Angiotensin II desensitization and Ca2+ and Na+ fluxes in vascular smooth muscle cells

Regional vascular response to ProAngiotensin-12 (PA12) through the rat arterial system

ProAngiotensin-12 (PA12) is the most recent peptide to be identified as a functional component of the renin-angiotensin system (RAS). PA12 is reported to constrict rat coronary arteries and the aorta, dependent upon angiotensin II-converting enzyme 1 (ACE1) and chymase. The current study employed myography to determine the direct vascular effects of PA12 on a range of isolated rat arteries extending from the core to periphery. PA12 significantly constricted the descending thoracic aorta, right and left common carotid arteries, abdominal aorta and superior mesenteric artery, with little effect on the femoral and renal arteries. AngII was found to produce similar responses to PA12 when administered at the same dose. A potency gradient in response to PA12 was clearly apparent, with vessels in closest proximity to the heart responding with the greatest constriction; while constrictive potency was lost further form the heart. Inhibition of ACE1 and chymase both significantly attenuated PA12-induced vasoconstriction, with chymostatin displaying lesser potency. We postulate ACE1 primarily regulates RAS activity within the circulation, while chymase may have an important role in local, tissue-based RAS activity.

The Angiotensin II AT1 Receptor Structure-Activity Correlations in the Light of Rhodopsin Structure

The most prevalent physiological effects of ANG II, the main product of the renin-angiotensin system, are mediated by the AT1 receptor, a rhodopsin-like AGPCR. Numerous studies of the cardiovascular effects of synthetic peptide analogs allowed a detailed mapping of ANG II's structural requirements for receptor binding and activation, which were complemented by site-directed mutagenesis studies on the AT1 receptor to investigate the role of its structure in ligand binding, signal transduction, phosphorylation, binding to arrestins, internalization, desensitization, tachyphylaxis, and other properties. The knowledge of the high-resolution structure of rhodopsin allowed homology modeling of the AT1 receptor. The models thus built and mutagenesis data indicate that physiological (agonist binding) or constitutive (mutated receptor) activation may involve different degrees of expansion of the receptor's central cavity. Residues in ANG II structure seem to control these conformational changes and to dictate the type of cytosolic event elicited during the activation. 1) Agonist aromatic residues (Phe8 and Tyr4) favor the coupling to G protein, and 2) absence of these residues can favor a mechanism leading directly to receptor internalization via phosphorylation by specific kinases of the receptor's COOH-terminal Ser and Thr residues, arrestin binding, and clathrin-dependent coated-pit vesicles. On the other hand, the NH2-terminal residues of the agonists ANG II and [Sar1]-ANG II were found to bind by two distinct modes to the AT1 receptor extracellular site flanked by the COOH-terminal segments of the EC-3 loop and the NH2-terminal domain. Since the [Sar1]-ligand is the most potent molecule to trigger tachyphylaxis in AT1 receptors, it was suggested that its corresponding binding mode might be associated with this special condition of receptors.

EFFECT OF PROPOFOL ON VASOCONSTRICTION AND CALCIUM MOBILIZATION INDUCED BY ANGIOTENSIN II DIFFERS IN AORTAS FROM NORMOTENSIVE AND HYPERTENSIVE RATS

1. Angiotensin (Ang) II is a potent vasopressor agent, involved in the short-term control of arterial blood pressure during anaesthesia. The aim of the present study was to test the hypothesis that propofol, a widely used intravenous anaesthetic agent, could alter the arterial response to AngII and to evaluate its effect in genetic hypertension. 2. We studied the effect of increasing concentrations of propofol (5.6 x 10-7 to 5.6 x 10-4 mol/L) on aortic ring maximal isometric tension elicited by AngII and on AngII-induced Ca2+ mobilization in aortic smooth muscle cells from Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). 3. Maximal tension developed by aortic rings from WKY rats was greater than that developed by rings from SHR. In both WKY rats and SHR, propofol at concentrations from 5.6 x 10-6 mol/L decreased maximal tension induced by AngII in a concentration-dependent manner. The magnitude of inhibition was higher in SHR than in WKY rats, whereas pD2 values were not different. In addition, Ca2+ mobilization induced by AngII was inhibited by propofol in a concentration-dependent manner, with the same magnitude and pD2 values. 4. These results suggest that the arterial response to AngII may be altered during propofol anaesthesia, particularly in hypertension.

Heparin and heparan sulfate disaccharides bind to the exchanger inhibitor peptide region of Na+/Ca2+ exchanger and reduce the cytosolic calcium of smooth muscle cell lines. Requirement of C4-C5 unsaturation and 1--> 4 glycosidic linkage for activity

Heparin and heparan sulfate fragments, obtained by bacterial heparinase and heparitinases, bearing an unsaturation at C4-C5 of the uronic acid moiety, are able to produce up to 80% reduction of the cytosolic calcium of smooth muscle cell lines. Unsaturated disaccharides from chondroitin sulfate, dermatan sulfate, and hyaluronic acid are inactive, indicating that, besides the unsaturation of the uronic acid, a vicinal 1 --> 4 glycosidic linkage is needed. An inverse correlation between the molecular weight and activity is observed. Thus, the ED(50) of the N-acetylated disaccharide derived from heparan sulfate (430 Da) is 88 microm compared with 250 microm of the trisulfated disaccharide (650 Da) derived from heparin. Except for enoxaparin (which contains an unsaturation at the non-reducing end and 1 --> 4 glycosidic linkage), other low molecular weight heparins and native heparin are practically inactive in reducing the cytosolic calcium levels. Thapsigargin (sarcoplasmic reticulum Ca(2+)-ATPase inhibitor), vanadate (cytoplasmic membrane Ca(2+)-ATPase inhibitor), and nifedipine and verapamil (Ca(2+) channel antagonists) do not interfere with the effect of the trisulfated disaccharide upon the decrease of the intracellular calcium. A significant decrease of the activity of the trisulfated disaccharide is observed by reducing extracellular sodium, suggesting that the fragments might act upon the Na(+)/Ca(2+) exchanger promoting the extrusion of Ca(2+). This was further substantiated by binding experiments and circular dichroism analysis with the exchanger inhibitor peptide.

Desensitization of angiotensin II: effect on [Ca2+]i, inositol triphosphate, and prolactin in pituitary cells

Activation of pituitary angiotensin (ANG II) type 1 receptors (AT1) mobilizes intracellular Ca2+, resulting in increased prolactin secretion. We first assessed desensitization of AT1 receptors by testing ANG II-induced intracellular Ca2+ concentration ([Ca2+](i)) response in rat anterior pituitary cells. A period as short as 1 min with 10(-7) M ANG II was effective in producing desensitization (remaining response was 66.8 +/- 2.1% of nondesensitized cells). Desensitization was a concentration-related event (EC(50): 1.1 nM). Although partial recovery was obtained 15 min after removal of ANG II, full response could not be achieved even after 4 h (77.6 +/- 2.4%). Experiments with 5 x 10(-7) M ionomycin indicated that intracellular Ca2+ stores of desensitized cells had already recovered when desensitization was still significant. The thyrotropin-releasing hormone (TRH)-induced intracellular Ca2+ peak was attenuated in the ANG II-pretreated group. ANG II pretreatment also desensitized ANG II- and TRH-induced inositol phosphate generation (72.8 +/- 3.5 and 69.6 +/- 6.1%, respectively, for inositol triphosphate) and prolactin secretion (53.4 +/- 2.3 and 65.1 +/- 7.2%), effects independent of PKC activation. We conclude that, in pituitary cells, inositol triphosphate formation, [Ca2+](i) mobilization, and prolactin release in response to ANG II undergo rapid, long-lasting, homologous and heterologous desensitization.

Transforming growth factor-beta1 modulates angiotensin II-induced calcium release in vascular smooth muscle cells from spontaneously hypertensive rats

OBJECTIVES

To investigate the role of transforming growth factor-beta1 (TGF-beta1) on Ca2+-dependent mechanisms elicited by angiotensin II in aortic vascular smooth muscle cells (VSMC) of Wistar- Kyoto (WKY) rats and spontaneously hypertensive rats (SHR).

METHODS

Cai2+ release induced by angiotensin II (1 micromol/ l) was studied in cultured VSMC isolated from the aortas of 6-week-old WKY rats and SHR. Intracellular Ca2+ (Cai2+) was assessed in Fura-2 loaded cells using fluorescent imaging microscopy. Angiotensin II receptors were analysed by binding studies.

RESULTS

Pretreatment of VSMC for 24 h with TGF-beta1 significantly increased angiotensin II-induced Cai2+ mobilization from internal stores in SHR, while Ca2+ influx was not altered. This effect involves tyrosine kinase and is not due to an increase in angiotensin II binding sites, or a change in the affinity of the receptors. By contrast, TGF-beta1 did not modify the response of VSMC from WKY rats to angiotensin II.

CONCLUSIONS

These results help our understanding of the interactions between the pathways activated by TGF-beta1 and the G protein-coupled receptor signalling pathway, and their role in genetic hypertension.

The effect of propofol on angiotensin II-induced Ca(2+) mobilization in aortic smooth muscle cells from normotensive and hypertensive rats

We studied the effect of propofol (5.6-560 micromol/L; 1-100 microg/mL) on the mechanisms involved in Ca(2+) mobilization elicited by angiotensin II (AngII) in Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats. We studied the variations in intracellular Ca(2+) ([Ca(2+)](i)) concentrations in cultured aortic vascular smooth muscle cells (VSMCs) isolated from 6-wk-old WKY and SHR rats loaded with the Ca(2+)-sensitive fluorescent dye, Fura-2, using fluorescent imaging microscopy. In the absence of external Ca(2+), AngII (1 micromol/L) induced a transient [Ca(2+)](i) mobilization from internal stores that was larger in SHR than in WKY rats. Ca(2+) influx was assessed after external Ca(2+) (1 mmol/L) reintroduction. Propofol (1-100 microg/mL) added 5 min before the experiments did not alter AngII-induced Ca(2+) release from internal stores in either strain. By contrast, Ca(2+) influx elicited by AngII was significantly decreased by propofol. This effect occurred at a smaller concentration of propofol in the SHR than in the WKY rats. When Ca(2+) stores were depleted by exposure of cells to thapsigargin, an inhibitor of the sarcoendoplasmic reticulum Ca(2+)-ATPase, reintroduction of Ca(2+) to the medium induced a capacitative Ca(2+) influx of similar magnitude than that elicited by AngII. This influx was also significantly decreased by propofol at 100 microg/mL ( WKY: 27 +/- 3% of control values, n = 107; SHR: 16 +/- 3%, n = 47; P < 0.001). In conclusion, propofol decreased AngII-induced Ca(2+) influx through voltage-independent channels, without altering Ca(2+) release from internal stores in aortic VSMCs. The hypertensive rats were found to be more sensitive to the effect of propofol than the normotensive rats. This suggests that the response of VSMCs to AngII may be altered by propofol.

IMPLICATIONS

In rat aortic vascular smooth muscle cells, propofol reduced angiotensin II-elicited Ca(2+) entry through capacitative Ca(2+) channels without altering Ca(2+) release from intracellular stores. Spontaneously hypertensive rats were more sensitive to these effects of propofol than normotensive rats. The response of vascular smooth muscle cells to angiotensin II may be altered by propofol.

Extracellular signal-regulated kinase pathway is involved in basic fibroblast growth factor effect on angiotensin II-induced Ca(2+) transient in vascular smooth muscle cell from Wistar-Kyoto and spontaneously hypertensive rats

We studied the effect of basic fibroblast growth factor (b-FGF) on different Ca(2+) mechanisms elicited by angiotensin II (Ang II) in normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). Intracellular Ca(2+) (Ca(2+)(i)) variations were studied in cultured vascular smooth muscle cells (VSMCs) isolated from the aorta of 5- to 6-week-old WKY rats and SHR. Ca(2+)(i) was assessed in Fura-2-loaded cells with fluorescent imaging microscopy. Ang II subtype 1 receptor activation by Ang II (1 micromol/L) induced a transient increase in Ca(2+)(i) that was partially attenuated by genistein, a tyrosine kinase inhibitor. Pretreatment of VSMCs with b-FGF for 24 hours markedly stimulated the Ang II-induced Ca(2+)(i) release from the internal stores in WKY rats, whereas it was without effect in SHR. This was not consequent to a change in the affinity of Ang II subtype 1 receptors or an increase in their density. Inhibition of mitogen-activated protein kinase with PD 98059 reduced this stimulatory effect of the cytokine in the WKY rats. On the other hand, b-FGF stimulated the Ang II-induced Ca(2+) influx in both strains. Similar results were observed when Ca(2+) influx was induced with thapsigargin. Genistein and PD 98059 abolished the effect of b-FGF. These results show for the first time that b-FGF regulates Ca(2+) mechanisms induced by Ang II and that this regulation is different in SHR than in normotensive control animals. The extracellular signal-regulated kinase cascade is implicated in this cross-regulation with G protein-signaling pathway at 2 levels and possibly more: 1 at the tyrosine kinases and the other downstream of the extracellular signal-regulated kinase family. These results may prove useful in understanding the interaction between these 2 pathways and their implication in genetic hypertension.

ANG II-induced Ca(2+) increase in smooth muscle cells from SHR is regulated by actin and microtubule networks

We hypothesized that the cytoskeletal network in vascular smooth muscle cells (VSMC) is critical to the signaling pathways from angiotensin (ANG) II-receptor subtype 1 (AT(1)) activation to intracellular Ca(2+) (Ca(2+)(i)) release from internal stores and Ca(2+) influx. This was tested in spontaneously hypertensive rats (SHR), in which differences were reported in cultured aortic VSMC Ca(2+)(i) regulation and G protein function compared with those in normotensive Wistar-Kyoto (WKY) rats. In cultured aortic VSMC, disorganization of actin filaments with cytochalasin D (2 micromol/l) decreased the ANG II-induced Ca(2+)(i) release from internal stores and the ANG II-induced Ca(2+) influx in SHR in a reversible fashion, whereas it was without effect in WKY rats. On the other hand, blocking the dynamic state of the microtubule network significantly reduced ANG II-induced Ca(2+)(i) release from internal stores but was without effect on Ca(2+) influx in either SHR or WKY rats. This study demonstrates for the first time that, in the SHR, actin filaments play a major role in linking AT(1)-receptor activation to both Ca(2+)(i) release mechanisms and capacitative Ca(2+) influx. Furthermore, a functionally intact microtubule system is a necessary prerequisite for ANG II-induced Ca(2+)(i) release in both strains.

Action of angiotensin II, 5-hydroxytryptamine and adenosine triphosphate on ionic currents in single ear artery cells of the rabbit

1. Angiotensin II, 5-hydroxytryptamine (5-HT) and adenosine triphosphate (ATP) evoked a transient inward current in isolated single car artery cells of rabbit held at -60 mV by whole cell voltage clamp in physiological saline using a KCL-containing pipette solution. Under these conditions agonist did not activate a calcium-dependent potassium current. 2. Responses to each agonist were transient and desensitized rapidly. Inward current at -60 mV holding potential was not abolished by blockade of voltage-dependent calcium channels or by buffering intracellular calcium with BAPTA, a calcium chelator, or following depletion of intracellular calcium stores with ryanodine. 3. The shape of the current-voltage relationships and the reversal potentials of the current induced by angiotensin II, 5-HT and ATP were similar under a variety of ionic conditions. Agonist-induced current was unaffected by replacing intracellular chloride with citrate ions or by replacing intracellular sodium with caesium or extracellular sodium with barium or calcium. Replacement of extracellular sodium with Tris shifted the reversal potential in all cases by around 30 mV negatively. 4. These data suggest that angiotensin II, 5-HT and ATP activate similar cationic conductances which are relatively non-selective allowing mono- and divalent cations to cross the smooth muscle cell membrane. These channels may allow the influx of calcium under physiological conditions.

Evidence that [Sar1]angiotensin II behaves differently from angiotensin II at angiotensin AT1 receptors in rabbit aorta

Three peptide analogues, [Sar1]angiotensin II, angiotensin II and [Asn1, Val5]angiotensin II, that act at angiotensin AT1 receptors were compared in an isolated rabbit aorta assay. Significant differences have been found among them in agonist profiles and agonist-antagonist interactions with losartan, a nonpeptide antagonist selective for AT1 receptors. Most significantly, underestimation of the antagonist potency for losartan with a flat Schild plot was obtained with [Sar1]angiotensin II. These findings were confirmed in further examinations with representative peptide antagonists including [Sar1,Ala8]angiotensin II. The failure of PD123177, a nonpeptide antagonist selective for AT2 binding sites, to induce any significant difference in the complex antagonism of [Sar1,Phe(Br5)8]angiotensin II to angiotensin II appeared to rule out significant involvement of AT2 binding sites in the differences observed among the agonists, as well as in the complex antagonism. On the basis of the present findings it is speculated that either a saturable agonist removal process or heterogeneous sub-populations of AT1 receptors may be involved.

Angiotensin II-induced tachyphylaxis in aortas of normo- and hypertensive rats: changes in receptor affinity

Angiotensin II-induced tachyphylaxis was found to be associated with changes in agonist affinity (Ka) and EC50 values, as assessed by using Furchgott's equation derived for the determination of full agonist affinity. The diminished affinity during tachyphylaxis was observed in aorta ring preparations from both Wistar-Kyoto and spontaneously hypertensive rats. Noradrenaline (10(-9) M) reduced the increase in the Ka value during tachyphylaxis in both strains. The results suggest that tachyphylaxis occurs at the level of the receptor, resulting in changes in the affinity of the ligand for the receptor and in the coupling efficiency of the receptor system. The results also support the probable role of modulators acting on allosteric receptor sites.