17 beta-Estradiol inhibits flow- and acute hypoxia-induced prostacyclin release from perfused endocardial endothelial cells

In search of pulmonary hypertension treatments: Effect of 17β-estradiol on PGI2 pathway in human pulmonary artery

INTRODUCTION

Prostacyclin (PGI₂) is synthetized by PGI₂ synthase (PGIS) and induces vasorelaxation via activation of cyclic AMP (cAMP) generating IP-receptor. Several components of the PGI₂ signaling pathway are reduced in patients with pulmonary hypertension (PH).

AIM

To study the effect of 17β-estradiol (E2) on the PGI₂ signaling pathway in human pulmonary arteries (HPA) and in their smooth muscle cells (hPASMC) derived from Group-3 PH and non-PH patients.

METHODS

Following E2-treatments of isolated HPA and cultured hPASMC, we measured: 6-keto-Prostaglandin F_1α (PGI₂ stable metabolite) by ELISA, PGIS and IP protein levels by Western blot and HPA vasorelaxations with an organ bath system.

RESULTS

Incubation with E2 (24/48 h, doses ≥ 10 nM) significantly increased the expression of PGIS in hPASMC derived from both PH (65-98%) and non-PH (21-33%) patients, whereas incubation with E2 (2 h, 0.1 and 1 µM) increased 6-keto-PGF_1α production in HPA from Group-3 PH patients only, and did not affect 6-keto-PGF_1α production in hPASMC from either non-PH or Group-3 PH patients. Increases in IP receptor expression were observed following 10 mM E2-treatment of hPASMC from non-PH (33% after 48 h) and Group-3 PH (23% after 24 h) patient lungs. Finally, preincubation with 100 nM E2 significantly increased arachidonic acid-induced vasorelaxation of HPA from non-PH patient lungs but not of HPA from Group-3 PH patient lungs.

CONCLUSION

E2-treatment may help to restore the PGI₂-pathway in Group-3 PH.

Differential effects of oral and transdermal menopausal hormone therapy on prostacyclin and thromboxane in platelets

Abstract Menopausal hormone therapies (MHT) may increase thrombotic risk but modulate endothelial function and reduce development of vascular lesions. This study compared effects of MHT on prostanoid-modulated adenosine triphosphate (ATP) secretion from platelets in relationship with endothelial reactive hyperemia (RH) index and carotid intima medial thickness (CIMT). Participants were healthy, recently menopausal women of the Kronos Early Estrogen Prevention Study (KEEPS) randomized to one of three treatments: oral conjugated equine estrogen (oCEE, 0.45 mg/day), transdermal 17β-estradiol (tE2, 50 μg/day) each with intermittent oral progesterone or placebo pills and patch (PL). Prostacyclin and thromboxane A2 were assessed by quantification of their stable metabolites (6-keto-prostaglandin F1α, 6-k-PGF1α; thromboxane B2, TXB2), using ELISA. Dense granule ATP secretion from activated platelets was determined by bioluminescence; RH and CIMT were determined by fingertip tonometry and ultrasound, respectively. After 48 months of treatment, platelet content of 6-k-PGF1α and TXB2 was significantly lower in oCEE compared to the PL. Inhibition of ATP secretion by exogenous activation of cAMP associated with platelet 6-k-PGF1α (r = -0.41, P = 0.04) and TXB2 (r = 0.71, P = 0.0005) only in the oCEE group. Serum and platelet content of 6-k-PGF1α and TXB2 associated positively in the PL and tE2 groups. Serum 6-k-PGF1α positively associated with RH in the oCEE group (r = 0.73, P = 0.02), while serum TXB2 positively associated with CIMT in the tE2 group (r = 0.64, P = 0.01). Thus, oCEE and tE2 differentially affect prostanoid-mediated platelet secretory pathways but alone would not account for an increased thrombotic risk for oral MHT. Furthermore, platelet-derived prostanoids may contribute to RH and vascular remodeling in healthy menopausal women.

Endothelium-derived neuregulin protects the heart against ischemic injury

BACKGROUND

Removal of cardiac endothelial cells (EC) has been shown to produce significant detrimental effects on the function of adjacent cardiac myocytes, suggesting that EC play a critical role in autocrine/paracrine regulation of the heart. Despite this important observation, the mediators of the protective function of EC remain obscure. Neuregulin (NRG, a member of the epidermal growth factor family) is produced by EC and cardiac myocytes contain receptors (erbB) for this ligand. We hypothesized that NRG is an essential factor produced by EC, which promotes cardioprotection against ischemic injury.

METHODS AND RESULTS

We demonstrate that human cardiac EC express and release NRG in response to hypoxia-reoxygenation. Under conditions where hypoxia--reoxygenation causes significant cardiac myocyte cell death, NRG can significantly decrease apoptosis of isolated adult ventricular myocytes. Coculturing adult murine myocytes with human umbilical vein, murine lung microvascular, or human coronary artery EC can also protect myocytes against hypoxia--reoxygenation--induced apoptosis. These protective effects are abolished by NRG gene deletion or silencing of NRG expression in EC. Finally, endothelium-selective deletion of NRG in vivo leads to significantly decreased tolerance to ischemic insult, as demonstrated by impaired postischemic contractile recovery in a perfused whole-organ preparation and larger infarct sizes after coronary artery ligation.

CONCLUSION

Together, these data demonstrate that EC-derived NRG plays an important role in cardiac myocyte protection against ischemic injury in the heart and supports the idea that manipulation of this signaling pathway may be an important clinical target in this setting.

Estrogen modulates permeability and prostaglandin levels in the rabbit urinary bladder

The purpose of this study was to characterize barrier function, hypoxic damage and prostaglandin levels in the urinary bladder in response to estrogen deficiency. Female New Zealand White rabbits were separated into three groups: ovariectomized; sham-operated; and ovariectomized treated with estrogen. Barrier function was compromised in the ovariectomized group compared to that of the sham group and the ovariectomized group treated with estrogen. Urinary bladders of ovariectomized animals showed higher concentrations of hypoxic markers than controls, localized primarily in the urothelium. Levels of 6-keto-PGF(1alpha) and PGF(2alpha) were significantly higher in smooth muscle than the ovariectomized animals and PGE(2) levels were significantly lower in the mucosa of ovariectomized animals. These results suggest that estrogen deficiency induces a loss of barrier function and an increase in hypoxia. The estrogen-dependent decreases of prostaglandin PGE(2) in the urothelium correlate with loss of barrier function, suggesting estrogen regulation of PGE(2) may contribute to maintenance of urothelial function.

The role of estrogen in cardiovascular disease

Cardiovascular disease is the number one cause of death among women, accounting for nearly 50% of female deaths. Statistics show that women on average develop cardiovascular disease 10 to 15 years later in life than men, and that the risk may increase after menopause. This observation has led to much speculation as to what physiological change(s) associated with menopause is responsible for the higher risk of atherosclerosis. Estrogen, with its potential as a cardioprotective agent and as an immunomodulator of the inflammatory response in atherosclerosis, has received the most attention. Understanding the mechanisms that lead to these differences may allow beneficial therapeutic intervention to enhance this effect in females and evoke this protection in males. This review will do the following: (1) characterize mechanisms of atherosclerosis, (2) explore the role of estrogen-replacement therapy, (3) define the effect of gender on inflammation, (4) compare and contrast the effects of estrogen and testosterone on endothelial functional, and (5) suggest mechanistic based therapeutic opportunities.

Cardiac endothelial-myocardial signaling: its role in cardiac growth, contractile performance, and rhythmicity

Experimental work during the past 15 years has demonstrated that endothelial cells in the heart play an obligatory role in regulating and maintaining cardiac function, in particular, at the endocardium and in the myocardial capillaries where endothelial cells directly interact with adjacent cardiomyocytes. The emerging field of targeted gene manipulation has led to the contention that cardiac endothelial-cardiomyocytal interaction is a prerequisite for normal cardiac development and growth. Some of the molecular mechanisms and cellular signals governing this interaction, such as neuregulin, vascular endothelial growth factor, and angiopoietin, continue to maintain phenotype and survival of cardiomyocytes in the adult heart. Cardiac endothelial cells, like vascular endothelial cells, also express and release a variety of auto- and paracrine agents, such as nitric oxide, endothelin, prostaglandin I(2), and angiotensin II, which directly influence cardiac metabolism, growth, contractile performance, and rhythmicity of the adult heart. The synthesis, secretion, and, most importantly, the activities of these endothelium-derived substances in the heart are closely linked, interrelated, and interactive. It may therefore be simplistic to try and define their properties independently from one another. Moreover, in relation specifically to the endocardial endothelium, an active transendothelial physicochemical gradient for various ions, or blood-heart barrier, has been demonstrated. Linkage of this blood-heart barrier to the various other endothelium-mediated signaling pathways or to the putative vascular endothelium-derived hyperpolarizing factors remains to be determined. At the early stages of cardiac failure, all major cardiovascular risk factors may cause cardiac endothelial activation as an adaptive response often followed by cardiac endothelial dysfunction. Because of the interdependency of all endothelial signaling pathways, activation or disturbance of any will necessarily affect the others leading to a disturbance of their normal balance, leading to further progression of cardiac failure.

Update on cardiovascular disease in post-menopausal women

Cardiovascular disease (CVD), and in particular coronary artery heart disease (CAHD), is the leading cause of morbidity and mortality in women. Until recently, most of our knowledge about the pathophysiology of CVD in women - and, subsequently, management guidelines - were based on studies conducted mostly in men. While similar mechanisms operate to induce CVD in women and men, gender-related differences exist in the anatomy and physiology of the myocardium, and sex hormones modify the course of disease in women. Women, more than men, have their initial manifestation of CAHD as angina pectoris; are likely to be referred for diagnostic tests at a more advanced stage of disease, and are less likely than men to have corrective invasive procedures. The overall morbidity and mortality following the initial ischaemic heart event is worse in women, and the case fatality rate is greater in women than in men. Also, the relative impact of impaired vasoreactivity of the coronary artery, increased viscosity of the blood and dysregulation of automaticity and arrhythmia, is greater in women than in men. The most effective means of decreasing the impact of CVD on women's health is by an active approach from childhood to proper principles of healthcare in order to modify the contribution of specific risk factors. The latter include obesity, abnormal plasma lipid profile, hypertension, diabetes mellitus, cigarette smoking, sedentary lifestyle, increased blood viscosity, augmented platelet aggregability, stress and autonomic imbalance. The use of lipid-lowering drugs has not been adequately studied in women but reports from studies conducted mostly in men do predict an advantage also to women. Oestrogen deficiency after spontaneous or medically induced menopause is an important risk factor for CVD and CAHD. Observational and mechanistic data suggest a role for oestrogen replacement after menopause for primary, and possibly secondary, prevention of CVD. However, two recent prospective trials suggest that treatment de novo with hormone replacement of older post-menopausal women after an acute coronary event may not confer cardiovascular protection and may increase the risk of thromboembolic disease. Results of ongoing long-term studies may determine the beneficial role of hormone replacement versus potential risks involved with this treatment.

Sex differences and the effects of sex hormones on hemostasis and vascular reactivity

Thrombus formation and vasospasm are involved in the initiation of acute ischemic events in the heart. Gender differences in persons with coronary artery disease and the incidence of myocardial ischemia have been clearly documented. In addition, it is well established that sex hormones influence the risk of developing coronary artery disease. Epidemiologic studies suggest that estrogen may exert a protective effect, yet the results of recently completed and ongoing prospective trials of estrogen and hormone (estrogen + progesterone) replacement suggest that these hormones can increase thrombotic events in postmenopausal women. This review focuses on sex (gender) differences in hemostasis and vascular reactivity and on the influence that sex hormones have on these physiologic systems. This review takes the novel approach of focusing on sex differences in hemostasis and vascular reactivity in healthy premenopausal women and men of a similar age. By comparing men and women in this age group, the confounding issues of age, pathology, or decline in sex hormone levels are avoided. Animal and in vitro investigations pertinent to examining potential cellular mechanism(s) of sex hormones in mediating these sex differences are discussed. We assume there is a relationship between the normal physiologic and pathologic effects of sex hormones; elucidating sex differences in normal cardiovascular function will help clarify the basis for sex differences in the incidence and manifestations of coronary heart disease and will aid in the future development of gender-specific therapies for cardiovascular disease.

Estradiol decreases IGF-1 and IGF-1 receptor expression in rat aortic smooth muscle cells. Mechanisms for its atheroprotective effects

Insulin-like growth factor (IGF-1) is a potent mitogen for vascular smooth muscle cells. Both IGF-1 and its receptor have been shown to be highly expressed in atherosclerotic lesions. Here we investigated whether part of the vasculoprotective properties of E(2) may be mediated by its negative regulation of the IGF-1 system. HeLa cells, which do not contain endogenous estrogen receptors (ER), were transiently transfected with IGF-1R promoter constructs with or without a plasmid encoding human ERalpha or ERbeta and treated with 100 nm 17beta-estradiol (E(2)) for 24 h. E(2) treatment decreased basal luciferase activity by 51%, and this effect was dependent on co-expression of ERalpha, whereas no repression was observed with ERbeta. A mutation within the DNA binding domain of the ERalpha abolished the repressor function of the ER receptor. Similarly, E(2) decreased IGF-1R transcription by 21% in rat aortic smooth muscle cells (RASMC), which express endogenous ER. This effect was specific for E(2), because it was inhibited by an antiestrogen and because progesterone did not have any effect on IGF-1R expression in HeLa or RASMC transfected with progesterone receptor. Accordingly, E(2) decreased IGF-1R and IGF-1 mRNA in RASMC by 47% and 33%. Western blot analysis and radioligand binding studies showed that E(2) also dose-dependently decreased IGF-1R protein expression in RASMC by 40% and 30%, respectively, and that IGF-1 protein was reduced by 43%. Repression of IGF-1R promoter activity by a combination of ERalpha and E(2) did not appear to be mediated via direct binding of ER to the IGF-1R promoter but rather by inhibition of SP1 binding to the IGF-1R promoter. Thus, E(2) down-regulates IGF-1R and IGF-1 expression in vascular smooth muscle cells. This may have important implications for the understanding of the beneficial effects of estrogen in the cardiovascular system.

Multiple mechanisms are involved in the acute vasodilatory effect of 17beta-estradiol in the isolated perfused rat heart

The purpose of this study was to define the dose-dependent effects of 17beta-estradiol on coronary flow and cardiac function in isolated rat hearts and to identify the mechanisms involved in its vasodilator action. Hearts from female and male Wistar rats were perfused at constant pressure (100 mm Hg). Stereoisomer specificity and the mechanism of vasodilation by 17beta-estradiol were examined in female rat hearts. Function was measured by a left ventricular (LV) balloon and coronary flow (CF) with an ultrasonic flowmeter. 17Beta-estradiol at 10(-6), 5 x 10(-6), and 10(-5) M increased CF in female hearts by 5 +/- 2, 27 +/- 4 (p < 0.05 vs. baseline), and 40 +/- 4% (p < 0.05 vs. baseline), respectively. The effect of 17beta-estradiol in hearts from male rats was similar but less pronounced compared with females [deltaCF 8 +/- 3, 19 +/- 3 (p < 0.05 vs. baseline)] and 25 +/- 7% (p < 0.05 vs. baseline; p < 0.05 vs. female 17beta-estradiol). Maximum vasodilation by the stereoisomer 17alpha-estradiol was significantly smaller [deltaCF 5 +/- 3, 4 +/- 3 (p < 0.05 vs. female 17beta-estradiol) and 14 +/- 1% (p < 0.05 vs. baseline; p < 0.05 vs. female 17beta-estradiol)] for 10(-6), 5 x 10(-6), and 10(-5) M. Pretreatment with the NO-synthesis inhibitor Nomega-methyl-L-arginine (10(-4) M) had no effect on the maximal vasodilator response to 17beta-estradiol (10(-5) M) [deltaCF 36 +/- 6% (p < 0.05 vs. baseline)]. When hearts were pretreated with the prostaglandin-synthesis inhibitor diclofenac (10(-6) M), the maximal vasodilator effect of 17beta-estradiol was partially attenuated [deltaCF 12 +/- 7% (p < 0.05 vs. female 17beta-estradiol)]. Similarly, pretreatment with the K+ATP-blocker glibenclamide (10(-6) M) partially inhibited the maximal vasodilator effect of 17beta-estradiol [deltaCF 22 +/- 6% (p < 0.05 vs. baseline; p < 0.05 vs. female 17beta-estradiol)]. Pretreatment with the Ca2+ channel antagonist nifedipine (7.2 x 10(-8) M) completely blocked the vasodilator effect. In isolated perfused rat hearts, 17beta-estradiol induced marked acute coronary vasodilation; this effect is in part gender specific, and in female hearts, largely stereoisomer specific. The dilator effect is mediated predominantly by calcium channel blockade, but prostaglandin release and K+ATP channel activation also are involved. In the isolated perfused rat heart, NO production does not contribute to the acute vasodilator effect of 17beta-estradiol.

Estrogen replacement inhibits intimal hyperplasia and the accumulation and effects of transforming growth factor beta1

BACKGROUND

The role of estrogens in providing atheroprotection has been well documented in both epidemiologic and experimental studies. This phenomenon has traditionally been attributed to the beneficial lipid-modifying effects of estrogens. Previous studies have used models of either diet- or injury-induced atherosclerosis. As such, the interrelationship between estrogens, lipids, and atherosclerosis remains unclear. We hypothesized that estrogens are atheroprotective independent of changes in serum lipids by directly influencing the accumulation and effects of the peptide growth factor transforming growth factor beta1 (TGF-beta1).

MATERIAL AND METHODS

Thirteen female sheep (8 years old) were randomized to sham, ovariectomy, or ovariectomy with 17beta-estradiol replacement. Serum lipid levels were serially measured. At 9 months, necropsy was performed with histologic morphometric analysis of the aortoiliac bifurcation. Levels of TGF-beta1 were determined in serum and aortic tissue. Human aortic smooth muscle cells were isolated and cultured.

RESULTS

Serum triglyceride, lipoprotein a, and total, low-density lipoprotein, and high-density lipoprotein cholesterol levels were similar and normal between groups. Ovariectomy resulted in aortoiliac intimal hyperplasia compared with sham (P < 0.001) and hormone replacement (P < 0.001) groups. Compared with ovariectomy, estrogen replacement attenuated aortic accumulation of TGF-beta1 (P < 0.02). In vitro, estradiol potentiated TGF-beta1 inhibition of human vascular smooth muscle cell (VSMC) proliferation and increased TGF-beta1 release in stimulated VSMCs (P < 0.001).

CONCLUSIONS

Without dietary manipulation, ovarian ablation induces aortic intimal hyperplasia in the ewe. Estradiol abrogates this response independently of its effects on serum lipids. Hormone replacement decreases the accumulation of TGF-beta1, suggesting that estrogens may provide atheroprotection both by modifying local production and by modulating the influence of TGF-beta1 on VSMC growth.

Growth factor activation of the estrogen receptor in vascular cells occurs via a mitogen-activated protein kinase-independent pathway

The classical estrogen receptor ERalpha mediates many of the known cardiovascular effects of estrogen and is expressed in male and female vascular cells. Estrogen-independent activation of ERalpha is known to occur in cells from reproductive tissues, but has not been investigated previously in vascular cells. In this study, transient transfection assays in human saphenous vein smooth muscle cells (HSVSMC) and pulmonary vein endothelial cells (PVEC) demonstrated ERalpha-dependent activation of estrogen response element-based, and vascular endothelial growth factor-based reporter plasmids by both estrogen-deficient FBS (ED-FBS) and EGF. In nonvascular cells, ERalpha-mediated gene expression can be activated via mitogen-activated protein (MAP) kinase- induced phosphorylation of serine 118 of ERalpha. However, in vascular cells, we found that pharmacologic inhibition of MAP kinase did not alter EGF-mediated ERalpha activation. In addition, a mutant ER containing an alanine-for-serine substitution at position 118 was activated to the same degree as the wild-type receptor by ED-FBS and EGF in both HSVSMC and PVEC. Furthermore, constitutively active MAP kinase kinase (MAPKK) activated ERalpha in Cos1 cells as expected, but MAPKK inhibited ER activation in PVEC. We conclude that growth factors also stimulate ERalpha-mediated gene expression in vascular cells, but find that this occurs via a MAP kinase-independent pathway distinct from that reported previously in nonvascular cells.

The biology of estrogen-mediated repair of cardiovascular injury

Women appear to be protected from cardiovascular disease until the onset of menopause. Considerable evidence supports the atheroprotective effects of endogenous and supplemental estrogens. The beneficial effects of estrogens on lipid metabolism cannot wholly explain this phenomenon. Accumulating data suggest that estrogen may act at the cellular and molecular level to influence atherogenesis. The purpose of this review is to examine lipid-independent mechanisms of estrogen-mediated atheroprotection after cardiovascular injury.

Estrogens and atherosclerosis

Estrogens prevent heart disease in women and have also been shown to retard atherogenesis in animal models. Estrogens may act at several steps in the atherogenic process to prevent cardiovascular disease. Some of the benefits of estrogens can be ascribed to their ability to favorably alter the lipoprotein profile, i.e. increase high-density lipoprotein and decrease low-density lipoprotein, and also to their ability to prevent oxidative modification of low-density lipoprotein. Other beneficial effects of estrogens include direct actions on the vascular endothelium and vascular smooth muscle, leading to a decrease in the expression of adhesion molecules involved in monocyte adhesion to endothelial cells, and to a decrease in certain chemokines involved in monocyte migration into the subendothelial space. Estrogens may also affect the later stages of atherogenesis. Finally, estrogens may modify the behavior of atherosclerotic vessels by altering their reactivity and thereby promoting vasodilation, and this may also partly account for their ability to prevent clinical events due to cardiovascular disease.

Human vascular smooth muscle cells express an estrogen receptor isoform

In women, estrogen (E2) exerts a clinically relevant anti-atherogenic effect. The atheroprotective effects of E2 are mediated both by E2-induced changes in systemic factors and by direct effects of E2 on the blood vessel wall. In studies to characterize E2 signaling pathways in vascular smooth muscle cells (VSMC), we recently demonstrated that human VSMC express a functional estrogen receptor [1]. In the present study, we applied a reverse transcription/PCR-based strategy to identify isoforms of the E2 receptor in human VSMC. We now report that in addition to the classical E2 receptor, human VSMC derived from both mammary artery and saphenous vein express an estrogen receptor isoform containing an in-frame deletion of Exon 4 (ER delta 4). RNase protection assays confirm the presence of ER delta 4 message in VSMC and demonstrate it is nearly as abundant as the classical E2 receptor. Transient transfection experiments in VSMC and HeLa cells demonstrate that, in contrast to the classical 67 kDa nuclear-localized E2 receptor, ER delta 4: (a) is a 55 kDa protein that is widely distributed throughout the cell; (b) does not transactivate an E2 response element-driven reporter plasmid in response to E2; and (c) does not modulate transactivation of the ERE-reporter by the classical (wild type) estrogen receptor. Thus, human VSMC express an E2 receptor isoform that does not appear to alter gene transcription. The presence of a novel isoform of the E2 receptor may have important implications for studies of E2-mediated signaling in VSMC.

An in vivo model of chronic optic nerve ischemia: the dose-dependent effects of endothelin-1 on the optic nerve microvasculature

The purpose of this study was to evaluate the effects induced by chronic microapplication of endothelin-1 on the anterior optic nerve microvasculature and to determine the dose-response characteristics of endothelin-1 on this vascular bed. Daily dosages between 4.69 x 10(-4) and 9.0 x 10(-1) micrograms/day of endothelin-1 were delivered continually over 3 days, and at a constant flow rate, to the perineural region of the anterior optic nerve of 15 albino rabbits via osmotically-driven minipumps. The vasomotor effect of local endothelin-1 on the microvasculature of the optic nerve was examined using intraluminal microvascular corrosion casting technique. The vasomotor effects were quantified by measuring the relative amount of vasoconstriction of the arterioles supplying the anterior optic nerve (primary and secondary branches of the short posterior ciliary arteries). The average constriction was calculated for the endothelin-treated eyes and the untreated, contralateral eyes. The mean vasoconstriction in the endothelin treated eyes ranged from 14.7% to 30.0% and was highly correlated with the logarithmic value of the daily dose of endothelin-1 (R2 = 0.59, p = 0.00083). The interocular difference (between treated and untreated eyes) of the optic nerve vasoconstriction ranged from 0-19% (mean +/- SD: 7.23 +/- 5.7%). This interocular difference also correlated highly with the log of the daily endothelin-1 dosage (R2 = 0.80; p < 0.0001). By additionally accounting for the weight and sex in a multiple linear regression function, the correlation was markedly improved (R2 = 0.92; p < 0.0001). In conclusion, the microvasculature supplying the anterior optic nerve of the rabbit demonstrates a dose-dependent vasoconstriction with chronic local application of endothelin-1. This in vivo, experimental model offers a titratable method with which the effects of chronic vasoconstriction and vascular insufficiency on the optic nerve can be examined.