Mechanical load opposes angiotensin-mediated decrease in vascular alpha 1-adrenoceptors

Increased angiotensin II coupled with decreased Adra1a expression enhances cardiac hypertrophy in pregnancy-associated hypertensive mice

Cardiac hypertrophy is a crucial risk factor for hypertensive disorders during pregnancy, but its progression during pregnancy remains unclear. We previously showed cardiac hypertrophy in a pregnancy-associated hypertensive (PAH) mouse model, in which an increase in angiotensin II (Ang II) levels was induced by human renin and human angiotensinogen, depending on pregnancy conditions. Here, to elucidate the factors involved in the progression of cardiac hypertrophy, we performed a comprehensive analysis of changes in gene expression in the hearts of PAH mice and compared them with those in control mice. We found that alpha-1A adrenergic receptor (Adra1a) mRNA levels in the heart were significantly reduced under PAH conditions, whereas the renin-angiotensin system was upregulated. Furthermore, we found that Adra1a-deficient PAH mice exhibited more severe cardiac hypertrophy than PAH mice. Our study suggests that Adra1a levels are regulated by renin-angiotensin system and that changes in Adra1a expression are involved in progressive cardiac hypertrophy in PAH mice.

Balloon injury alters alpha-adrenoceptor expression across rat carotid artery wall

alpha(1)-Adrenoceptors (AR) mediate growth factor-like activity of catecholamines on vascular smooth muscle cells (SMC) and adventitial fibroblasts. This trophic activity is strongly augmented by balloon injury, contributes significantly to subsequent proliferation, wall hypertrophy and lumen loss and is mediated by alpha(1A)- and alpha(1B)-AR. However, it is not known how injury augments adrenergic trophic activity. The aim of the present study was to examine alpha-AR expression in rat carotid artery and to test the hypothesis that balloon injury augments a(1)-AR expression. 2. Neointima, media and adventitia were isolated at various days after balloon injury of rat carotid artery and subjected to quantitative reverse transcription-polymerase chain reaction and radioligand binding. Cultured SMC were also studied. 3. Transcripts for alpha(1A)-, alpha(1B)-, alpha(1D)- and alpha(2D)-AR were expressed in different proportions in media and adventitia from uninjured carotid artery. Injury caused a reduction by as much as 85% at day 4 in all alpha-AR mRNA (but not cyclophilin) in both the media and adventitia. In both layers, expression returned to control by day 21 for alpha(2D)-AR and by day 42 for alpha(1A)-AR, but remained reduced by 25-50% for alpha(1B)- and alpha(1D)-AR at 42 days. alpha(1)-Adrenoceptor transcripts in the neointima at 21 and 42 days after injury were expressed at levels more than 80% lower than in the media or adventitia of uninjured carotid; alpha(2D)-AR mRNA was undetectable. The density of total alpha(1)-AR binding sites was similar in the media and adventitia of uninjured carotid. Density was reduced by approximately 60% in the intima-media and adventitia 21 days after injury. To examine possible mechanisms, early passaged cultured SMC were studied that express alpha(1D)- and alpha(1B)-AR at levels similar to in vivo but that do not express other alpha-AR. Basic fibroblast growth factor caused downregulation of alpha(1D)-AR mRNA and alpha(1)-AR density, without affecting mRNA half-life, whereas transforming growth factor-beta1 had no effect. Neither growth factor altered alpha(1B)-AR message expression. 4. These data demonstrate that: (i) carotid artery expresses the same four alpha-AR genes and similar total alpha(1)-AR density in the SMC media and fibroblast-rich adventitia; and (ii) injury induced enhancement of adrenergic trophic activity is not caused by upregulation of alpha(1)-AR, but, instead, is associated with a generalized reduction in alpha-AR expression.

Induction of a hypertrophic growth status of coronary smooth muscle cells is associated with an overexpression of TGF-beta

Hypertrophy of vascular smooth muscle cells occurs during hypertension-induced remodelling of arteries and during development of arteriosclerosis and restenosis following angioplasty but the pathogenesis of the hypertrophic status is not yet fully understood. In a previous study we demonstrated that the synthetic non-sulfated, non-toxic heparin-mimicking compound RG-13577 is capable of inducing a cell cycle-arrested hypertrophic phenotype of coronary smooth muscle cells. In this study we clarify the mode of action of RG-13577 and demonstrate that the RG-13577-induced hypertrophy is associated with an increased expression of TGF-beta1 as indicated by an increase in TGF-beta1-specific protein and mRNA level. Furthermore we show that RG-13577-treated hypertrophic smooth muscle cells maintain full metabolic activity as indicated by a continuous de novo synthesis of protein and proteoglycans and that the RG-13577-induced growth arrest is caused not only by a higher expression of TGF-beta, but also by a reduced response of RG-treated cells to the mitogenic activity of bFGF, PDGF and EGF. The growth inhibitory activity of RG-13577 is reduced in the presence of neutralizing antibodies against TGF-beta. TGF-beta itself has anti-proliferative activity in serum-depleted medium. The RG-13577 effect is reversible since incubation of hypertrophic cells in RG-13577-free medium restores cell volume and [3H]thymidine incorporation to the values of untreated control cells within 4 days. We conclude, that the active metabolic status of RG-13577-treated cells in association with the overexpression of TGF-beta could promote repair processes of injured arteries after angioplasty without stimulating cell proliferation.

Alpha1-adrenergic hypothesis for pulmonary hypertension

Pulmonary hypertension (PH) is a chronic and disabling condition that affects the pulmonary vasculature. Once PH is diagnosed, the prognosis is generally poor with a rapid downhill course. PH management is largely empirical because the underlying pathophysiologic mechanisms that are responsible for the excessive vasoconstrictor and vascular smooth muscle proliferative responses are poorly understood. Based on new information concerning the role of adrenergic receptors in regulating various cellular functions, a new perspective on the genesis of PH has emerged, along with a unifying hypothesis for the role of alpha1-adrenergic receptors present in the pulmonary vasculature as the major contributor to the pathophysiologic changes associated with PH. Adrenergic receptors that are present on vascular smooth muscle cells regulate vascular tone and growth. The alpha1-adrenergic receptors that are present on the small- and medium-sized pulmonary arteries have a unique and greatly enhanced affinity and activity to alpha1-adrenergic agonists. Under physiologic conditions, this helps in regulating vascular tone and maintains an adequate ventilation/perfusion matching. However, the excessive stimulation of alpha1-adrenergic receptors produces not only smooth muscle contraction but also proliferation and growth. The conditions that produce an increase in alpha1-adrenoreceptor gene synthesis, density, and activity (such as hypoxia or changes in vessel wall pressure) or increase the levels of its agonists (such as norepinephrine, appetite suppressants, or cocaine) greatly enhance pulmonary vascular smooth muscle contractile and proliferative responses and lead to the development of PH. An understanding of the role played by these receptors in the pathophysiology of PH would not only help to avoid the use of alpha1-agonists for appetite suppression and other disease states, but also would help in developing new drugs to block these receptors. A further understanding of the alpha1-adrenoreceptor subtypes present in the pulmonary vasculature, the factors that regulate their expression, and their intracellular signaling pathways would help researchers to devise newer therapeutic strategies and, hopefully, to find a cure for this crippling condition.

Effect of angiotensin II receptor blockade on the interaction between enalaprilat and doxazosin in rat tail arteries

1. Previous work has shown that enalaprilat, an inhibitor of angiotensin-converting enzyme (ACE), potentiated the actions of alpha 1-adrenoceptor antagonists; it was hypothesized that angiotensin II (AngII) modulated the activity of alpha 1-adrenoceptors. This hypothesis was tested in Sprague-Dawley rat isolated perfused tail arteries using the AT1 receptor antagonist losartan and the AT2 receptor antagonist PD123319. 2. Losartan had no alpha 1-adrenoceptor antagonist effects at concentrations below 1 mumol/L. Similarly, losartan (0.1 mumol/L) had no effect on the alpha 1-adrenoceptor antagonist action of doxazosin (1, 10 nmol/L) nor on the potentiation of doxazosin by enalaprilat (1 mumol/L). 3. PD123319 (0.1 mumol/L) had no alpha 1-adrenoceptor antagonist effect but altered the mode of action of the alpha 1-adrenoceptor antagonist doxazosin: PD123319 changed doxazosin from a competitive to a non-competitive antagonist, as evidenced by the reduced slope of the dose-response curve for the alpha 1-adrenoceptor agonist phenylephrine. 4. These results suggest that AngII can modulate alpha 1-adrenoceptor function in rat tail arteries via an indirect action at AT2 receptors. However, the present results do not rule out the involvement of bradykinin, endothelin or prostaglandin in the modulation of alpha 1-adrenoceptor function by angiotensin II.