Nitric oxide-induced ciliary muscle relaxation during contraction with endothelin-1 is mediated through elevation of cyclic GMP

Therapeutic Effects of Anthocyanins for Vision and Eye Health

Anthocyanin (AC) is widely used as supplement of eye health in Europe and in East Asia. In this review, I describe AC effects to clarify the mechanism is important in order to understand the effects of AC on vision health. The bioavailability of AC is quite low but, reported as intact form and many kinds of metabolite. And AC passes through the blood-aqueous fluid barrier and blood-retinal barrier. In vitro study, AC had a relaxing effect on ciliary muscle which is important to treat both myopia and glaucoma. And AC stimulate the regeneration of rhodopsin in frog rod outer segment. Furthermore, AC could inhibit the axial length and ocular length elongation in a negative lens-induced chick myopia model. In addition, we summarized clinical studies of AC intake improved dark adaptation and transient myopic shift and the improvement on retinal blood circulation in normal tension glaucoma patients.

Regulation of intraocular pressure by soluble and membrane guanylate cyclases and their role in glaucoma

Glaucoma is a progressive optic neuropathy characterized by visual field defects that ultimately lead to irreversible blindness (Alward, 2000; Anderson et al., 2006). By the year 2020, an estimated 80 million people will have glaucoma, 11 million of which will be bilaterally blind. Primary open-angle glaucoma (POAG) is the most common type of glaucoma. Elevated intraocular pressure (IOP) is currently the only risk factor amenable to treatment. How IOP is regulated and can be modulated remains a topic of active investigation. Available therapies, mostly geared toward lowering IOP, offer incomplete protection, and POAG often goes undetected until irreparable damage has been done, highlighting the need for novel therapeutic approaches, drug targets, and biomarkers (Heijl et al., 2002; Quigley, 2011). In this review, the role of soluble (nitric oxide (NO)-activated) and membrane-bound, natriuretic peptide (NP)-activated guanylate cyclases that generate the secondary signaling molecule cyclic guanosine monophosphate (cGMP) in the regulation of IOP and in the pathophysiology of POAG will be discussed.

Inverse relationship between nitric oxide synthases and endothelin-1 synthesis in bovine corpus luteum: interactions at the level of luteal endothelial cell

Endothelin-1 (ET-1) and nitric oxide (NO) play pivotal roles in corpus luteum (CL) function. The present study examined the interplay between NO and ET-1 synthesis in the bovine CL. We found similar inducible and endothelial NO synthase (iNOS and eNOS, respectively) activities in the young CL (d 1-5) expressing the highest levels of both eNOS and iNOS mRNA. These values later declined at mid-cycle (d 8-15) and remained low at later stages (d 16-18). Luteolysis, initiated by prostaglandin F2alpha analog administration, further reduced NOS mRNA and by 24 h, NOS values dropped to approximately 15% of those at mid-cycle. eNOS protein levels followed a similar pattern to its mRNA. Because endothelial cells (ECs) are the main site for ET-1 and NO production in the CL, we examined the direct effects of the NO donor, NONOate on luteal ECs (LECs). Elevated NO levels markedly decreased ET-1 mRNA, and peptide concentrations in cultured and freshly isolated LECs in a dose-dependent manner. In agreement, NOS inhibitor, NG-nitro-l-arginine methyl ester, stimulated ET-1 mRNA expression in these cells. Interestingly, NO also up-regulated prostaglandin F2alpha receptors in LECs. These data show that there is an inverse relationship between NOS and ET-1 throughout the CL life span, and imply that this pattern may be the result of their interaction within the resident LECs. NOS are expressed in a physiologically relevant manner: elevated NO at an early luteal stage is likely to play an important role in angiogenesis, whereas reduced levels of NO during luteal regression may facilitate the sustained up-regulation of ET-1 levels during luteolysis.

Delphinidin-3-rutinoside relaxes the bovine ciliary smooth muscle through activation of ETB receptor and NO/cGMP pathway

Delphinidin-3-rutinoside (D3R) is the major anthocyanin component in blackcurrant (Ribes nigrum L.) fruits. We investigated the relaxation mechanism of D3R in bovine ciliary smooth muscle (CM). D3R at a concentration of 10(-5) m produced a sustained and progressive relaxation during the contraction induced by endothelin (ET)-1 in the bovine CM specimens. After the pre-treatment with D3R, the anthocyanin exerted an inhibitory effect on the ET-1-induced contraction with a concomitant increase in cyclic GMP production and decreased phosphorylation ratio of myosin light chain (RLC). The inhibitory effect of D3R was significantly attenuated in the presence of either N(G)-nitro-L-arginine (NOARG) as a nitric oxide synthase (NOS) inhibitor, carboxy-PTIO as a NO scavenger, ODQ as an inhibitor of guanylyl cyclase, or BQ788 as a selective ET(B) receptor antagonist. The atteuation with NOARG was reversed by the addition of excess L-arginine. However, iberiotoxin as a Ca2+-activated K+ channel inhibitor, propranolol as a beta-adrenoceptor antagonist, and indomethacin as a cyclooxygenase inhibitor failed to modify the inhibitory effect of D3R. Scatchard plot analysis revealed that the [125I]-ET-1 binding site constituted a single population with Kd of 54.5+/-4.6 nm and maximum binding site (B(max)) of 168.4+/-25.4 fmol/mg protein in the ciliary epithelium (CE), and Kd of 141.7+/-18.0 nm and B(max) of 357.7+/-35.8 fmol/mg protein in CM. [125I]-ET-1 binding was completely displaced by BQ788 with K(i) values of 56.7+/-10.8 pm in CE and 93.4+/-23.3 pm in CM. Meanwhile, partial displacement (approximately 40%) was observed by BQ123 as a selective ET(A) receptor antagonist in both preparations. ET(B) receptor was predominant subtype in CE and CM, whereas kinetics of the binding was different in two preparations. These results suggest that D3R possibly stimulates ET(B) receptors to produce/release NO, and results in an inhibition of myosin RLC phosphorylation and/or acceleration of dephosphorylation, thereby causing relaxation and producing an inhibitory effect on the ET-1-induced contraction in the bovine CM.

Eyeing endothelins: a cellular perspective

Endothelin is an endogenous vasoactive peptide that is considered among the most potent vasoconstrictor substances known. In addition to its vascular effects, endothelins and their receptors have been shown to be present in the eye and to have a number of ocular actions that may be important for ocular homeostasis, but, in excess can be a potential contributor to ocular neuropathy in glaucoma. The current review focuses on the cellular and molecular aspects of endothelins and its receptors in the eye with an emphasis on its relationship to ocular function and its potential role in the etiology of glaucoma pathophysiology.

A novel in vitro model for screening and evaluation of anti-asthenopia drugs

Patients suffering asthenopia are steadily increasing with an expanding use of visual display terminals such as computers. An attempt was made to develop an in vitro model for asthenopia. Ciliary muscle removed from eyeballs of a rabbit was stimulated with acethylcholine, resulting in contraction of the muscle. Repeated stimulations caused decreased contraction, which may be related to fatiguing of ciliary muscle and hence asthenopia. Treatment of the repeatedly stimulated muscle with cyanocobalamin restored contraction dose-dependently. Thus, the model developed in this study can be used to screen drug candidates for treating asthenopia.

Involvement of accumulated endogenous NOS inhibitors and decreased NOS activity in the impaired neurogenic relaxation of the rabbit proximal urethra with ischaemia

1. We examined the effect of ischaemia on the neurogenic and nitric oxide (NO)-mediated urethral relaxation. 2. Rabbits were divided into control and urethral ischaemia (UI) groups, which was prepared by the partial occlusion of bilateral iliac arteries using blood vessel occluders. 3. Neurogenic and NO-mediated proximal urethral relaxation induced by electrical field stimulation (EFS) was greatly impaired in the UI group, while relaxation by sodium nitroprusside (SNP) as a NO donor showed no difference between the two groups. Pretreatment with L-arginine significantly improved but did not normalize the impaired relaxation in the UI group. Not only basal level, but also stimulated production of cyclic GMP with EFS, were significantly decreased in the UI group. 4. The tissue contents of N(G)-methyl-L-arginine (L-NMA) and asymmetric N(G), N(G)-dimethyl-L-arginine (ADMA) in the proximal urethra were increased following ischaemia. While L-arginine and symmetric N(G), N'(G)-dimethyl-L-arginine (SDMA) contents remained unchanged. Exogenously applied authentic L-NMA and ADMA (1 -- 100 microM) concentration-dependently inhibited the EFS-induced urethral relaxation in the control group. The inhibition with L-NMA and ADMA was undetectable in the presence of 3 mM L-arginine. 5. The Ca(2+)-dependent NOS activity in the urethra from the UI group was significantly lower than that from the control group and was not restored by an addition of 3 mM L-arginine. 6. These results suggest that the impaired neurogenic and NO-mediated urethral relaxation with ischaemia is closely related to the increased accumulation of L-NMA and ADMA and decreased NOS activity, which would result in an accelerated reduction in NO production/release.

Review: effects of nitric oxide on eye diseases and their treatment

Both underproduction and overproduction of nitric oxide (NO) could lead to various eye diseases. It is known that endothelial NO synthase (eNOS) and neuronal NOS (nNOS) are activated in normal tissues to produce NO for physiological functions. Thus, underproduction of NO results in various eye diseases which could be corrected by providing NOS substrates or NO donors to lower the intraocular pressure, increase ocular blood flow, relax ciliary muscle, etc. On the other hand, immunological NOS (iNOS) is inducible only in pathological conditions by endotoxins, inflammation, and certain cytokines, such as interleukin-1 (IL-1), IL-6, TNF (tumor necrosis factor) and the like. Once induced, iNOS will produce large amounts of NO for long periods of time, so that NO is converted into NO2, nitrite, peroxynitrite and free radicals to induce pathophysiological actions, such as optic nerve degeneration and posterior retinal degeneration lesion, which lead to glaucoma, retinopathy, age-related macular degeneration (AMD), myopia, cataracts and uveitis. To treat/prevent these eye diseases, inhibitors of iNOS activity and/or iNOS induction could be tried.

Effects of nitric oxide donors and nitric oxide synthase substrates on ciliary muscle contracted by carbachol and endothelin for possible use in myopia prevention

Research has suggested that the development of myopia may possibly be prevented by the use of drugs which facilitate relaxation of the intraocular ciliary muscle. We examined the effects of five nitric oxide-producing agents--two nitric oxide donors, hydralazine and sodium nitrite, and three nitric oxide synthase substrates, L-arginine, L-canavanine, and N-benzoyl-L-arginine ethyl ester--on isolated bovine ciliary muscle maximally contracted with either carbachol or endothelin-1. Of these agents, hydralazine and L-canavanine produced a relaxing effect on endothelin-1-contracted muscle that was significantly greater than relaxing effect on carbachol-contracted muscle. These results indicate that hydralazine and L-canavanine could possibly be used for the prevention of myopia by relaxing the ciliary muscle with few anticholinergic and cycloplegic side effects.

Accelerated intimal hyperplasia and increased endogenous inhibitors for NO synthesis in rabbits with alloxan-induced hyperglycaemia

1. We examined whether endogenous inhibitors of NO synthesis are involved in the augmentation of intimal hyperplasia in rabbits with hyperglycaemia induced by alloxan. 2. Four weeks after the endothelial denudation of carotid artery which had been performed 12 weeks after alloxan, the intimal hyperplasia was greatly augmented with hyperglycaemia. The degree of hyperplasia was assessed using three different parameters of histopathological findings as well as changes in luminal area and intima: media ratio. 3. There were positive and significant correlations between intima:media ratio, plasma glucose, and concentrations of N(G)-monomethyl-L-arginine (L-NMMA) and N(G), N(G)-dimethyl-L-arginine (ADMA) in endothelial cells, that is, the intima:media ratio became greater as plasma glucose and endothelial L-NMMA and ADMA were increased. Furthermore, endothelial L-NMMA and ADMA were increased in proportion to the increase in plasma glucose. 4. In contrast, there were inverse and significant correlations between cyclic GMP production by carotid artery strips with endothelium and plasma glucose, between cyclic GMP production and endothelial L-NMMA and ADMA, and between the intima:media ratio and cyclic GMP production. 5. Exogenously applied L-NMMA and ADMA inhibited cyclic GMP production in a concentration-dependent manner. IC50 values were determined to be 12.1 microM for the former and 26.2 microM for the latter. The cyclic GMP production was abolished after the deliberate removal of endothelium from the artery strips. 6. These results suggest that the augmentation of intimal hyperplasia with hyperglycaemia is closely related to increased accumulation of L-NMMA and ADMA with hyperglycaemia, which would result in an accelerated reduction in NO production/release by endothelial cells.