Angiotensin II-induced endothelium-dependent relaxation of fowl aorta

Renin-angiotensin system in vertebrates: phylogenetic view of structure and function

Renin substrate, biological renin activity, and/or renin-secreting cells in kidneys evolved at an early stage of vertebrate phylogeny. Angiotensin (Ang) I and II molecules have been identified biochemically in representative species of all vertebrate classes, although variation occurs in amino acids at positions 1, 5, and 9 of Ang I. Variations have also evolved in amino acid positions 3 and 4 in some cartilaginous fish. Angiotensin receptors, AT₁ and AT₂ homologues, have been identified molecularly or characterized pharmacologically in nonmammalian vertebrates. Also, various forms of angiotensins that bypass the traditional renin-angiotensin system (RAS) cascades or those from large peptide substrates, particularly in tissues, are present. Nonetheless, the phylogenetically important functions of RAS are to maintain blood pressure/blood volume homeostasis and ion-fluid balance via the kidney and central mechanisms. Stimulation of cell growth and vascularization, possibly via paracrine action of angiotensins, and the molecular biology of RAS and its receptors have been intensive research foci. This review provides an overview of: (1) the phylogenetic appearance, structure, and biochemistry of the RAS cascade; (2) the properties of angiotensin receptors from comparative viewpoints; and (3) the functions and regulation of the RAS in nonmammalian vertebrates. Discussions focus on the most fundamental functions of the RAS that have been conserved throughout phylogenetic advancement, as well as on their physiological implications and significance. Examining the biological history of RAS will help us analyze the complex RAS systems of mammals. Furthermore, suitable models for answering specific questions are often found in more primitive animals.

Angiotensin AT2 receptor activates the cyclic-AMP signaling pathway in eel

A unique angiotensin type 2 receptor (AT2) that induces a cAMP signaling pathway was cloned and characterized for the first time in fish, Anguilla japonica. Phylogeny and synteny results showed that the AT2s among fishes and tetrapods share the same origin despite a sub-cluster formation among eel, salmon, and zebrafish. The eel AT2 was expressed abundantly in the spleen and localized at straight arterioles and ellipsoid regions prior to the sinusoid, suggesting a role in the regulation of microcirculation and/or immune response. Various angiotensin (Ang) peptides, including Ang II, Ang III, and Ang IV, were detected in the spleen by a radioimmunoassay coupled with HPLC separation, and these endogenous peptides stimulated a cAMP signaling, which has no crosstalk with cGMP pathway. The common and contrasting features of AT2 between fishes and mammals imply some ancestral characters of AT2, which are important information for receptor binding and evolutionary studies.

Maturation of the angiotensin II cardiovascular response in the embryonic White Leghorn chicken (Gallus gallus)

Angiotensin II (Ang II) is an important regulator of cardiovascular function in adult vertebrates. Although its role in regulating the adult system has been extensively investigated, the cardiovascular response to Ang II in embryonic vertebrates is relatively unknown. We investigated the potential of Ang II as a regulator of cardiovascular function in embryonic chickens, which lack central nervous system control of cardiovascular function throughout the majority of incubation. The cardiovascular response to Ang II in embryonic chickens was investigated over the final 50% of their development. Ang II produced a dose-dependent increase in arterial pressure on each day of development studied, and the response increased in intensity as development progressed. The Ang II type-1 receptor nonspecific competitive peptide antagonist [Sar(1) ile(8)] Ang II blocked the cardiovascular response to subsequent injections of Ang II on day 21 only. The embryonic pressure response to Ang II (hypertension only) differed from that of adult chickens, in which initial hypotension is followed by hypertension. The constant level of gene expression for the Ang II receptor, in conjunction with an increasing pressure response to the peptide, suggests that two Ang II receptor subtypes are present during chicken development. Collectively, the data indicate that Ang II plays an important role in the cardiovascular development of chickens; however, its role in maintaining basal function requires further study.

P2X purinoceptors mediate an endothelium-dependent hyperpolarization in longitudinal smooth muscle of anterior mesenteric artery in young chickens

BACKGROUND AND PURPOSE

The chicken anterior mesenteric artery contains an outer longitudinal smooth muscle layer, whose neural regulation remains to be elucidated. ATP evokes a depolarization in the smooth muscle through P2Y purinoceptors. However, there may be an additional inhibitory regulation because blockade of P2Y purinoceptors converts the depolarization to hyperpolarization. The objective of the present study was to examine the mechanism underlying this hyperpolarization.

EXPERIMENTAL APPROACH

Membrane potentials of longitudinal smooth muscle of the chicken mesenteric artery were recorded with a microelectrode technique. Perivascular nerves were stimulated by applying electrical field stimulation (EFS).

KEY RESULTS

EFS induced a hyperpolarization in preparations obtained from 5-week-old chickens, whereas it evoked a depolarization in those from 12-week-old chickens. The EFS-evoked hyperpolarization in 5-week-old chickens was blocked by a non-specific purinoceptor antagonist, suramin, and by a specific P2X purinoceptor antagonist, pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid. Desensitization of the P2X purinoceptor with its agonist alpha,beta-MeATP significantly suppressed EFS-evoked hyperpolarization. Blockade of the P2Y purinoceptor did not affect EFS-evoked hyperpolarization. The application of the NOS inhibitor Nomega-nitro-L-arginine methyl ester or the removal of the endothelium inhibited the hyperpolarization. The application of the nitric oxide (NO) donor sodium nitroprusside mimicked the hyperpolarization. Reverse transcriptase-PCR showed that P2X purinoceptors are expressed in the endothelium of the anterior mesenteric artery.

CONCLUSIONS AND IMPLICATIONS

Hyperpolarization in the longitudinal smooth muscle of the chicken anterior mesenteric artery was induced by ATP. ATP released from perivascular nerves may act on P2X purinoceptors in the endothelium and thereby stimulate NO production.

Do incremental increases in blood pressure elicit neointimal plaques through endothelial injury?

Fowl (males more than females) show maturation-dependent rises in blood pressure (BP) and formation of neointimal plaques (NPs), resembling balloon catheter injury-induced neointima, in the abdominal aorta (AbA) just above the bifurcation. The plaque comprises neointimal cells containing abundant endoplasmic reticulum and extracellular matrix. Hence, we investigated whether rapid incremental BP increases in male chicks trigger NP formation, possibly via endothelial injury in hemodynamically selective areas. In 6-wk-old chicks (n = 8) treated 4 wk with solvent (Sv; minipump) or arginine supplement (Arg; 0.3% in drinking water), BP increased from 140 +/- 5 to 159 +/- 4 (Sv) and from 138 +/- 4 to 157 +/- 3 (Arg) mmHg, whereas propranolol treatment (Prop, 8 mg.kg(-1).day(-1); minipump) prevented the rise. Arg and Prop groups had, respectively, 73% and 77% smaller (P < 0.05) NP areas and 19% and 25% less (P < 0.01) AbA medial thickness than Sv controls. In 16-wk-old cockerels, established BP remained high after Sv and Arg treatments. In the Prop group, BP decreased, but neither NP area nor medial thickness was lower than in the Sv group, whereas the Arg group showed greater NP area and medial thickness. Pulse pressure, determined by intravascular transducer, increased as the pulse wave descended the aorta. The results suggest that maturation-dependent rises in BP in chicks may trigger NP formation in the lower segment of the AbA, which was prevented by inhibition of BP increase, or via a possible increase in nitric oxide availability. BP reduction exerts no effect once BP reaches a plateau. Involvement of endothelial injury leading to NP formation and hemodynamic forces selective for the lesion-prone area remain to be determined.

Maturation-dependent changes of angiotensin receptor expression in fowl

An angiotensin (ANG) receptor homologous to the type 1 receptor (AT1) has been cloned in chickens (cAT1). We investigated whether cAT1 expression in various tissues shows maturation/age-dependent changes. cAT1 mRNA levels detected in renal glomeruli [in situ hybridization (ISH)] and kidney extract (RT-PCR) are significantly (P < 0.01) higher in 19-day embryos (EB) than in chicks (CH, 2-3 wk) and pullets/cockerels (PL/CK, 14-16 wk). The levels in adrenal glands (concentrated in subcapsular regions) are high in EB and further increased in CH and PL/CK. cAT1 mRNA is also detectable in smooth muscle (SM)/adventitia of EB and CH aorta and in the adventitia, but not SM, from PL/CK aortas. The endothelia from small arteries and arterioles, but not from aorta, express cAT1 mRNA (ISH). In all age groups, ANG II induces profound endothelium-dependent relaxation of abdominal aorta, partly (37-47%) inhibitable (P < 0.01) by Nomega-nitro-l-arginine methyl ester (l-NAME, 10(-4) M), suggesting the presence of ANG receptor in endothelium. l-NAME-resistant ANG II relaxation, examined in a limited number of EB or CH aortas, was reduced by 125 mM K+ or apamin plus charybdotoxin. The results suggest that 1) cAT1 is present in kidney, adrenal gland, and vascular endothelium (heterogeneity exists among arteries) of EB, CH, and PL/CK, and in aortic SM/adventitia of EB/CH but only in adventitia of PL/CK; 2) levels of cAT1 gene expression change during maturation in a tissue-specific manner; and 3) ANG II-induced relaxation may be partly attributable to nitric oxide and potassium channel activation.

Direct effects of acute hypoxia on the reactivity of peripheral arteries of the chicken embryo

In the chicken embryo, acute hypoxemia results in cardiovascular responses, including an increased peripheral resistance. We investigated whether local direct effects of decreased oxygen tension might participate in the arterial response to hypoxemia in the chicken embryo. Femoral arteries of chicken embryos were isolated at 0.9 of incubation time, and the effects of acute hypoxia on contraction and relaxation were determined in vitro. While hypoxia reduced contraction induced by high K(+) to a small extent (-21.8 +/- 5.7%), contractile responses to exogenous norepinephrine (NE) were markedly reduced (-51.1 +/- 3.2%) in 80% of the arterial segments. This effect of hypoxia was not altered by removal of the endothelium, inhibition of NO synthase or cyclooxygenase, or by depolarization plus Ca(2+) channel blockade. When arteries were simultaneously exposed to NE and ACh, hypoxia resulted in contraction (+49.8 +/- 9.3%). Also, relaxing responses to ACh were abolished during acute hypoxia, while the vessels became more sensitive to the relaxing effect of the NO donor sodium nitroprusside (pD(2): 5.81 +/- 0.21 vs. 5.31 +/- 0.27). Thus, in chicken embryo femoral arteries, acute hypoxia blunts agonist-induced contraction of the smooth muscle and inhibits stimulated endothelium-derived relaxation factor release. The consequences of this for in vivo fetal hemodynamics during acute hypoxemia depend on the balance between vasomotor influences of circulating catecholamines and those of the endothelium.

Angiotensin receptors--evolutionary overview and perspectives

The structure of the angiotensin molecule has been well preserved throughout the vertebrate scale with some amino acid variations. Specific angiotensin receptors (AT receptors) that mediate important physiological functions have been noted in a variety of tissues and species. Physiological and pharmacological characterization of AT receptors and, more recently, molecular cloning studies have elucidated the presence of AT receptor subtypes. Comparative studies suggest that an AT receptor subtype homologous to the mammalian type 1 receptor subtype (AT(1)), though pharmacologically distinct, is present in amphibians and birds, whereas AT receptors cloned from teleosts show low homology to both AT(1) and AT(2) receptor subtypes. Furthermore, receptors differing from both the AT(1)-homologue receptor and AT(2) receptor exist in some non-mammalian species. This may suggest that the prototype AT receptor evolved in primitive vertebrates and diverged to more than one type of AT receptor subtype during phylogeny. Furthermore, phenotypic modulation of AT receptors appears to occur during individual development/maturation.

Release of nitric oxide by angiotensin-(1-7) from porcine coronary endothelium: implications for a novel angiotensin receptor

The angiotensin I (AI) metabolite, A(1-7), elicited a concentration-dependent dilator response (ED50 > or = 2 microM) in porcine coronary artery rings which was markedly attenuated by the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine, and abolished after removal of the endothelium. This effect of the heptapeptide was not mimicked by AII, AIII or A(3-8) at comparable concentrations. The A(1-7)-induced relaxation was not affected by AT1 or AT2 receptor blockade or cyclo-oxygenase inhibition, but was attenuated by the B2 receptor antagonist, Hoe 140, and augmented by the angiotensin-converting enzyme (ACE) inhibitor, quinaprilat. These findings suggest that the relaxation to A(1-7) was mediated by the release of NO from the coronary endothelium through activation of an, as yet unidentified, AT receptor, the occupation of which also seems to stimulate the release of vasoactive kinins. Since A(1-7) accumulates during ACE inhibition, this mechanism may contribute to the coronary dilator effect of ACE inhibitors in vivo.