Analysis of vasocontractile responses to endothelin-1 in rabbit retinal vessels using an ETA receptor antagonist and an ETB receptor agonist

Intraocular Adeno-Associated Virus-Mediated Transgene Endothelin-1 Delivery to the Rat Eye Induces Functional Changes Indicative of Retinal Ischemia-A Potential Chronic Glaucoma Model

Endothelin-1 (ET-1) overactivity has been implicated as a factor contributing to glaucomatous neuropathy, and it has been utilized in animal models of retinal ischemia. The functional effects of long-term ET-1 exposure and possible compensatory mechanisms have, however, not been investigated. This was therefore the purpose of our study. ET-1 was delivered into rat eyes via a single intravitreal injection of 500 µM or via transgene delivery using an adeno-associated viral (AAV) vector. Retinal function was assessed using electroretinography (ERG) and the retinal expression of potentially compensatory genes was evaluated by means of qRT-PCR. Acute ET-1 delivery led to vasoconstriction and a significant reduction in the ERG response. AAV-ET-1 resulted in substantial transgene expression and ERG results similar to the acute ET-1 injections and comparable to other models of retinal ischemia. Compensatory changes were observed, including an increase in calcitonin gene-related peptide (CGRP) gene expression, which may both counterbalance the vasoconstrictive effects of ET-1 and provide neuroprotection. This chronic ET-1 ischemia model might be especially relevant to glaucoma research, mimicking the mild and repeated ischemic events in patients with long-term vascular dysfunction. The compensatory mechanisms, and particularly the role of vasodilatory CGRP in mitigating the retinal damage, warrant further investigation with the aim of evaluating new therapeutic strategies.

Endothelin 1-induced retinal ganglion cell death is largely mediated by JUN activation

Glaucoma is a neurodegenerative disease characterized by loss of retinal ganglion cells (RGCs), the output neurons of the retina. Multiple lines of evidence show the endothelin (EDN, also known as ET) system is important in glaucomatous neurodegeneration. To date, the molecular mechanisms within RGCs driving EDN-induced RGC death have not been clarified. The pro-apoptotic transcription factor JUN (the canonical target of JNK signaling) and the endoplasmic reticulum stress effector and transcription factor DNA damage inducible transcript 3 (DDIT3, also known as CHOP) have been shown to act downstream of EDN receptors. Previous studies demonstrated that JUN and DDIT3 were important regulators of RGC death after glaucoma-relevant injures. Here, we characterized EDN insult in vivo and investigated the role of JUN and DDIT3 in EDN-induced RGC death. To accomplish this, EDN1 ligand was intravitreally injected into the eyes of wildtype, Six3-cre⁺Jun^fl/fl (Jun^-/-), Ddit3 null (Ddit3^-/-), and Ddit3^-/-Jun^-/- mice. Intravitreal EDN1 was sufficient to drive RGC death in vivo. EDN1 insult caused JUN activation in RGCs, and deletion of Jun from the neural retina attenuated RGC death after EDN insult. However, deletion of Ddit3 did not confer significant protection to RGCs after EDN1 insult. These results indicate that EDN caused RGC death via a JUN-dependent mechanism. In addition, EDN signaling is known to elicit potent vasoconstriction. JUN signaling was shown to drive neuronal death after ischemic insult. Therefore, the effects of intravitreal EDN1 on retinal vessel diameter and hypoxia were explored. Intravitreal EDN1 caused transient retinal vasoconstriction and regions of RGC and Müller glia hypoxia. Thus, it remains a possibility that EDN elicits a hypoxic insult to RGCs, causing apoptosis via JNK-JUN signaling. The importance of EDN-induced vasoconstriction and hypoxia in causing RGC death after EDN insult and in models of glaucoma requires further investigation.

Effect of avosentan (SPP-301) in porcine ciliary arteries

PURPOSE

To investigate the vasoactive effect of ET(A)-endothelin receptor antagonists avosentan (SPP-301) and BQ-123 in isolated porcine ciliary arteries with and without endothelium. To investigate the effect of avosentan on the endothelin-1 induced contractions in comparison with BQ-123 and BQ-788 (ET(B)-endothelin receptor antagonist) in isolated porcine ciliary arteries with and without endothelium.

METHODS

Vessels were placed in a myograph system to measure isometric forces. In a first set of experiments, quiescent vessels were exposed, cumulatively, to increasing concentrations of avosentan and BQ-123 (10(-9) M-3 × 10(-6) M). In a second set of experiments, quiescent vessels were first incubated with avosentan (10(-6) M and 10(-8) M), BQ-123 (10(-6) M), and BQ-788 (10(-6) M), respectively. Then the vessels were exposed, cumulatively, to increasing concentrations of endothelin-1 (10(-12) M-3 × 10(-8) M). Each set of experiments was conducted in the vessels with and without endothelium.

RESULTS

Cumulative concentrations of avosentan and BQ-123 had no vasoactive effect in quiescent vessels. Avosentan had a strong inhibitory effect on the endothelin-1-induced contractions. The inhibitory effect of 10(-6) M avosentan was significantly stronger than the effect of 10(-8) M avosentan. The effect of avosentan (10(-6) M) tended to be stronger than the effect of BQ-123 (10(-6) M). To a lesser extent, BQ-788 also had an inhibitory effect on the endothelin-1-induced contractions.

CONCLUSIONS

Avosentan has a strong inhibitory effect on the endothelin-1-induced contractions. Blockade of ET receptors is potentially an attractive target in many eye diseases including glaucoma. Further studies are needed to evaluate the usefulness of endothelin blockers in ophthalmology.

An Endothelin Type A Receptor Antagonist Reverses Upregulated VEGF and ICAM-1 Levels in Streptozotocin-Induced Diabetic Rat Retina

Diabetic retinopathy, a cause of blindness, is often associated with the upregulation of vascular endothelial growth factor (VEGF) in the retina. Recently, leukocyte adhesion (leukostasis) is claimed for the occlusion of retinal capillary vascularity, which ultimately assists in the progression of diabetic retinopathy. In addition, intercellular adhesion molecule-1 (ICAM-1), a representative factor for leukostasis, is increased in diabetic retina. Endothelin (ET)-1, a potent vasoconstrictor peptide, is closely linked to the pathogenesis of diabetic retinopathy. Different therapeutic interventions concerning VEGF have already been proposed to prevent diabetic retinopathy. However, no study has yet reported concerning the effects of ET-1 receptor antagonist on the upregulated VEGF and ICAM-1 in morphologically intact diabetic retina. The current study investigated the effect of ET(A) receptor antagonist (TA-0201; 1 mg kg(-1) day(-1)) on the expressions of VEGF and ICAM-1 in rat diabetic retina. Diabetes was induced by intraperitoneal injection of streptozotocin (70 mg/kg) in Sprague-Dawley rats, whereas control rats (Cont) received only citrate buffer. After 1 week, the streptozotocin-administered rats were randomly divided into two groups: ET(A) receptor antagonist-treated group (DM+TA-0201) and saline-treated group (DM+vehicle). After the treatment for 4 weeks, the retina was removed from the eyeball. In DM+vehicle group, the VEGF expression of retina was significantly increased (33.5 pg/mg) in comparison with that in the Cont group (25.1 pg/mg), and the upregulation of VEGF was reversed in DM+TA-0201 group (26.9 pg/mg), a phenomenon consistent with the change in VEGF mRNA levels. The expression of retinal ICAM-1 was increased in DM+vehicle group (55.1 pg/mg) compared with Cont group (43.8 pg/mg), and ET antagonism completely blocked this increase (43.8 pg/mg). Moreover, an increased leukostasis by 3.3-fold in DM+vehicle retina was returned to the control level by ET antagonism. In the current study, there was no obvious retinal morphological alteration from both the hematoxylin and eosin staining and the FITC-dextran angiography. Thus, ET(A) receptor antagonist might be useful in preventing the progression of diabetic retinopathy, as evidenced by suppressing the increase in VEGF and ICAM-1 levels as well as leukostasis in morphologically intact diabetic retina.

Role of protein kinase C in the expression of endothelin converting enzyme-1

Increased expression of endothelin converting enzyme-1 (ECE-1) is associated with diabetic nephropathy. The molecular mechanisms underlying this association, as yet unknown, possibly involve protein kinase C (PKC) pathways. In the present study, we examined the effects of high glucose and PKC activation on ECE-1 expression in primary human umbilical vein endothelial cells (HUVECs) and in HUVEC line (EA.hy926). Increasing glucose concentration, but not mannitol, from 5.5-22.2 mmol/liter for 3 d, enhanced prepro endothelin-1 (ET-1) mRNA expression, ET-1 levels, ECE-1 protein, and mRNA expressions by 7, 4, 20, and 2.6-fold, respectively. High glucose increased ECE-1 protein expression dose and time dependently. By Western blot analysis, PKC-beta1, -beta2, and -delta isoform levels were significantly increased relative to other isoforms when glucose level was increased. Treatment with Rottlerin, a PKC-delta isoform inhibitor, reduced significantly the glucose-induced ET-1 secretion, and ECE-1 protein expression, but (S)-13-[(dimethylamino)methyl]-10,11,14,15-tetrahydro-4,9:16,21-dimetheno 1H,1(3)H-dibenzo[e,k]pyrrolo[3,4-h] (1, 4, 3) oxadiaza-cyclohexadecene-1,3(2H)-dione or Gö6976, specific PKC-beta and -alpha inhibitors, respectively, did not. Overexpression of PKC-delta but not PKC-alpha or -beta1 isoforms by adenovirus vector containing the respective cDNA in HUVECs incubated with 5.5 mmol/liter glucose, increased in parallel PKC proteins, and glucose-induced endothein-1 and ECE-1 protein expression by 4- to 6-fold. These results show that enhanced ECE-1 expression induced by hyperglycemia is partly due to activation of the PKC-delta isoform. Thus, inhibition of this PKC isoform may prevent diabetes-related increase in ET-1.

Functional and molecular characterization of the endothelin system in retinal arterioles

PURPOSE

Activation of the endothelin (ET) system has been implicated in the pathogenesis of retinal ischemic disease. Although ET-1, the predominant endogenous isoform of ET, has been shown to cause constriction of retinal vessels, the expression and functional significance of its synthesis and the involved specific ET receptors in retinal arterioles remain unknown. The authors examined the roles of ET(A) and ET(B) receptors and of endothelin-converting enzyme (ECE)-1 in ET-1-induced vasomotor responses of single retinal arterioles.

METHODS

To exclude systemic confounding effects, porcine retinal arterioles were isolated for vasoreactivity and molecular studies.

RESULTS

Isolated and pressurized retinal arterioles developed basal tone and constricted in a manner dependent on concentration to ET-1. ET-1 precursor big ET-1 elicited time-dependent vasoconstriction over 20 minutes, which was blocked by the ECE-1 inhibitor phosphoramidon. ET(A) receptor antagonist BQ123 inhibited most (approximately 90%) of vasoconstrictions to ET-1 and big ET-1. ET(B) receptor agonist sarafotoxin also elicited concentration-dependent constriction of retinal arterioles but with significantly less potency than ET-1. ET(B) receptor antagonist BQ788 abolished vasoconstriction to sarafotoxin but only slightly reduced responses to ET-1 and big ET-1. Protein and mRNA expressions of ET(A), ET(B), and ECE-1 were detected in retinal arterioles. Immunohistochemistry revealed ET(A) and ET(B) receptors predominantly in smooth muscle and ECE-1 predominantly in endothelium and smooth muscle.

CONCLUSIONS

ET-1 elicits constriction of retinal arterioles predominantly through the activation of smooth muscle ET(A) receptors. Endogenous production of ET-1 from vascular ECE-1 is sufficient to evoke ET(A) receptor-dependent constriction in retinal arterioles.

Vascular effects of newer cardiovascular drugs: focus on nebivolol and ACE-inhibitors

Alterations in the function and structure of the blood vessel wall account for most clinical events in the coronary and cerebrovascular circulation such as myocardial infarction and stroke. Cardiovascular drugs may exert beneficial effects on the vascular wall both at the level of the endothelium and vascular smooth muscle cells. Therefore, endothelial mediators, in particular nitric oxide (NO) and endothelin (ET), are of special interest. Drugs can modulate the expression and actions of NO, a vasodilator with antiproliferative and antithrombotic properties, and of ET, a potent vasoconstrictor and proliferative mitogenic agent. The most successful drugs in this context are statins and angiotensin-converting enzyme (ACE)-inhibitors. While statins increase the expression of NO synthase. ACE-inhibitors increase the release of NO via bradykinin-mediated mechanisms. Antioxidant properties of drugs are also important, as oxidative stress is crucial in atherosclerotic vascular disease. These properties may explain part of the effects of calcium antagonists and ACE-inhibitors. Indeed, angiotensin II stimulates NAD(P)H oxidases responsible for the formation of superoxide, which inactivates NO. ACE-Inhibitors thus increase the bioavailability of NO. Newer cardiovascular drugs such as nebivolol are able to directly stimulate NO release from the endothelium both in isolated arteries and in the human forearm circulation. ET receptor antagonists may exert beneficial effects in the vessel wall by preventing the effects of ET at its receptors and by reducing ET production. In summary, cardiovascular drugs have important effects on the vessel wall, which may be clinically relevant for the prevention and treatment of cardiovascular disease.

The effects of intravitreally injected endothelin-1 on the iris-ciliary body microvasculature in rabbits

PURPOSE

We previously reported that intravitreal injection of 0.5 microg of endothelin-1 (ET-1) caused both a sustained reduction of intraocular pressure (IOP) and decreased aqueous production in the rabbit eye. On the theory that these effects might have resulted from a sustained reduction of blood flow to the ciliary body due to ET-1-mediated vasoconstriction, in the present study we attempted to determine if ET-1 causes any changes in the vascular caliber of the iris-ciliary body.

METHODS

ET-1 solution (0.5 microg) was injected into the vitreous of one eye of each of 10 albino rabbits; the same amount of vehicle was injected into the contralateral eyes. One h following these injections in five of the rabbits and 24 h following them in the other five rabbits, ocular microvascular castings were obtained under controlled physiologic conditions, and the amount of vasoconstriction of the arterioles branching from the major arterial circle of the iris (MAC) and supplying the iris-ciliary body was measured by a scanning electron microscope and expressed as a percentage.

RESULTS

The ET-1 caused a statistically significant focal vasoconstriction in the treated eyes as compared with the contralateral, control eyes (9.9% at 1 h and 6.2% at 24 h; both P = . 0001).

CONCLUSIONS

Intravitreally injected ET-1 caused statistically significant, but only mild vasoconstriction of the arterioles supplying the ciliary processes.

Thyrotropin-like immunoreactivity in human retina: immunoreactive co-localization in ganglion cells and perivascular fibers

The present immunocytochemical study has demonstrated immunoreactive thyrotropin-like ganglion cell populations as well as perivascular fibers in the human retina by using specific antiserum. Thyrotropin is a pituitary glycopeptide involved in the synthesis and release of thyroid hormones. The existence and functions of peptides in vertebrate retinas are still not well known. Many authors have reported neuropeptide immunoreactivity in the human retina which have had their functions established in the neuroregulatory processes of vision. Moreover, some authors have reported the possibility that the fiber terminal of peptidergic neurons may also be a blood vessel. The appearance of immunoreactive-cells in human retina, e.g. existence of retinal ganglion cells with thyrotropin-like immunoreactivity, indicates the existence of specific mechanisms that would be mediated by these peptides which are located near immunoreactive ganglion cells. We hypothesize that there is an intrinsic mechanism for blood flow control, mediated by retinal ganglion cells which may regulate vessel diameter according to its luminous stimuli. No-one has demonstrated the presence or the functional existence of thyrotropin-like immunoreactive structures in the vertebrate retina, or on the side of the pituitary-thyroid axis. To the best of our knowledge this is the first time that thyrotropin has been immunocytochemically demonstrated in the human retina. Thus, we suggest that thyrotropin acts as a neuromodulator in the human retina, which is implicated in blood flow control.

Identification of endothelin receptor subtypes in rat retina using subtype-selective ligands

We investigate the existence of endothelin receptor subtypes using subtype selective ligands and the presence of immunoreactive (IR) endothelin (ET)-3 (IR-ET-3) by radioimmunoassay (RIA) in rat retina. Scatchard transformation of saturation binding experiments with [125I]ET-3 revealed specific binding sites with a Kd and Bmax values of 42 +/- 12 pM and 111 +/- 24 fmol/mg of protein, respectively. The Kd was similar to that obtain in previous studies using [125I]ET-1. However, the Bmax was 65% of that obtained with [125I]ET-1. Competitive experiments in the presence of the cyclic pentapeptide BQ123 (selective for ETA receptor) and Sarafotoxin 6C (selective for ETB receptor), demonstrated the existence of ETA and ETB receptors in a ratio of 35:65. The order of potency of ET family peptides was ET-3 = ET-1 > S6C for ETB receptor and ET-1 > ET-3 > BQ123 for ETA receptor. Cross-linking of [125I]ET-1 to retinal membranes with disuccinimidyl suberate and SDS-PAGE followed by autoradiography resulted in the labeling of two bands with apparent molecular masses of 52 and 34 kDa. Similar results were obtained using [125I]ET-3, suggesting that ETA and ETB receptors have similar molecular mass. The 34 kDa band is a proteolytic degradation product of the 52 kDa band. The concentration of IR-ET-3 was 1212 +/- 153 fmol/g wet weight in rat retina. All these data suggest that ETs may play a role in neurotransmission or neuromodulation in the retina, operating on both ETA and ETB receptor subtypes present in this tissue.

Increase in choroidal blood flow in rabbits with endothelin-1 induced transient complete obstruction of retinal vessels

BACKGROUND

In a previous paper, we reported that retinal blood flow (RBF) ceased immediately after injection of 1 nmol endothelin-1 (ET-1) and no recovery of RBF was detected for at least 50 min. In this study, we confirmed the same duration of RBF cessation and measured choroidal blood flow (CBF) for 180 min.

METHODS

We measured CBF in a rabbit model of transient complete obstruction of retinal vessels induced by intravitreal injection of a high dose of ET-1, using the hydrogen clearance method. We also investigated the effects of intravitreal injection of ET-1 on intraocular pressure (IOP), blood pressure, pulse rate and blood gases.

RESULTS

CBF was significantly greater in the ET-1-injected eyes than in the control eyes 40-130 min after injection of ET-1 (P < 0.05). The maximal CBF ratio in the ET-1-injected eyes was 128 +/- 7.4% at 40 min. CBF decreased to the pre-injection level at 140 min after the injection of ET-1. There was no significant change in blood pressure, pulse rate and blood gases throughout this experiment, and there was no significant difference in IOP between ET-1-injected eyes and control eyes.

CONCLUSION

It seems likely that the increase in CBF resulted from some local mechanisms of control that compensated for the decrease in RBF induced by intravitreal injection of ET-1. This model may be useful for investigation of the regulatory system of intraocular circulation, including endothelin receptors.