The effects of endothelin-1 on isolated bovine ciliary muscles

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.

Modulation of Iris Sphincter and Ciliary Muscles by Urocortin 2

Urocortin 2 (UCN2) is a peptide related to corticotropin-releasing factor, capable of activating CRF-R2. Among its multisystemic effects, it has actions in all 3 muscle subtypes. This study’s aim was to determine its potential role in two of the intrinsic eye muscle kinetics. Strips of iris sphincter (rabbit) and ciliary (bovine) muscles were dissected and mounted in isometric force-transducer systems filled with aerated-solutions. Contraction was elicited using carbachol (10-6 M for iris sphincter, 10-5 M for ciliary muscle), prior adding to all testing substances. UCN2 induced relaxation in iris sphincter muscle, being the effect maximal at 10-7 M concentrations (-12.2 % variation vs. control). This effect was abolished with incubation of indomethacin, antisauvagine-30, chelerytrine and SQ22536, but preserved with L-nitro-L-arginine. In carbachol pre-stimulated ciliary muscle, UCN2 (10-5 M) enhanced contraction (maximal effect of 18.2 % increase vs. control). UCN2 is a new modulator of iris sphincter relaxation, dependent of CRF-R2 activation, synthesis of prostaglandins (COX pathway) and both adenylate cyclase and PKC signaling pathways, but independent of nitric oxide production. Regarding ciliary muscle, UCN2 enhances carbachol-induced contraction, in higher doses.

Nifedipine enhances the relaxant effects of cyclo-oxygenase inhibitors on the bovine ciliary muscle

BACKGROUND/AIMS

The inhibition of cyclo-oxygenase (COX) enzymes and the blockade of Ca (2+) channels play an important role in the regulation of smooth muscle relaxation. This study was designed to investigate the relaxant effects of celecoxib, DFU (5,5-dimethyl-3-(3-fluorophenyl)-4-(4-methylsulphonyl)phenyl-2(5H)-furanone), and indomethacin, cyclo-oxygenase (COX-1 and -2) inhibitors, in the absence or presence of a nifedipine, L-type Ca(2+) channel blocker, on bovine ciliary muscle.

METHODS

Ciliary muscle strips (n = 12) were mounted in organ baths and tested for changes in isometric tension in response to celecoxib, DFU, and indomethacin. The relaxant effects of celecoxib, DFU, and indomethacin on carbachol-induced contractions in the presence or absence of nifedipine were investigated.

RESULTS

Celecoxib (10(-7)-10(-4) M), DFU (10(-7)-10(-4) M), indomethacin (10(-7)-10(-4) M), and nifedipine (10(-7)-10(-4) M) inhibited the carbachol-induced contractions in a concentration-dependent manner. The E(max) value of indomethacin was significantly higher than the E(max) values of celecoxib and DFU in ciliary muscle (P < 0.05), with no significant change in pD(2) values (P > 0.05). The relaxation responses by celecoxib, DFU, and indomethacin were significantly increased in the presence of nifedipine (10(-6) M). There were no significant differences between pEC50 and values of celecoxib, DFU, and indomethacin in the absence of nifedipine (10(-6) M) (P > 0.05), but E(max)values were significantly increased (P < 0.05).

CONCLUSIONS

These results suggest that the celecoxib, DFU, and indomethacin cause relaxation in ciliary muscle precontracted with carbachol. Blockade of calcium channels with nifedipine in ciliary muscle may increase the relaxant effect of celecoxib, DFU, and indomethacin. The topical or systemic use of celecoxib, DFU, and indomethacin with nifedipine can cause blurred near vision due to ciliary muscle relaxation, and in ocular pain conditions caused by ciliary spasm, the pain can be decreased more easily by combined use of these drugs.

Relaxant effects of β-adrenoceptor agonist formoterol and BRL 37344 on bovine iris sphincter and ciliary muscle

We investigated the relaxant effect of beta2-adrenoceptor agonist formoterol and beta3-adrenoceptor agonist BRL 37344 on bovine iris sphincter and ciliary muscle and measured cAMP and cGMP levels. Iris sphincter (n = 16) and ciliary muscle (n = 16) strips were mounted in organ baths and tested for changes in isometric tension in response to formoterol and BRL 37344. Their relaxant effects on serotonin-induced contractions in the presence or absence of metoprolol, ICI 118.551 and SR 59230A (beta1-, beta2-, beta3-adrenoceptor antagonist, respectively) were investigated. Their effects on cAMP and cGMP levels in iris sphincter (n = 12) and ciliary muscle (n = 12) were evaluated. Formoterol (10(-11)-10(-5) M) and BRL 37344 (10(-10)-10(-5) M) decreased the serotonine-induced contractions in a concentration-dependent manner. Emax values of formoterol were significantly higher than those of BRL 37344 in iris sphincter and ciliary muscle, with no significant change in pD2 values. The relaxation responses by formoterol and BRL 37344 were antagonized with ICI 118.551 (10(-6) M) and SR 59230A (10(-6) M). cAMP levels of formoterol- and BRL 37344-treated tissues were significantly higher than those of the control tissues. cGMP levels of BRL 37344-treated tissues were significantly higher than those of control tissues, but this effect of BRL 37344 was less significant than its effect on cAMP levels. beta-adrenoceptor relaxation responses in bovine iris sphincter and ciliary muscle are mediated by a mixed population of beta2- and beta3-adrenoceptor subtypes, with a predominant contribution of cAMP. Potency of formoterol and BRL 37344 was similar, but efficacy of formoterol was better than BRL 37344.

Pharmacological and functional characterization of endothelin receptors in bovine trabecular meshwork and ciliary muscle

To clarify the potential role of endothelin-1 (ET-1) in the pathogenesis of glaucoma, the endothelin receptors expressed in bovine trabecular meshwork (TM) and ciliary muscle (CM) were identified. TM and CM strips were subjected to ET-1 as well as to specific endothelin receptor antagonists. In both tissues BQ123, a specific ET-A receptor antagonist, substantially inhibited ET-1-induced contraction. BQ788, a specific ET-B receptor antagonist, showed only moderate effects. Both ET receptor types were detected in bovine TM and CM using Western blot analysis. ET-1 produced an increase in intracellular calcium in cultured TM cells. This effect was inhibited by BQ123, but not by BQ788. Thus, although both receptors are present, the ET-A receptor appears to play the predominant role in mediating contraction in both the TM and CM, while the ET-B receptor seems to contribute little to the overall ET-1 effect.

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.

Endothelin: is it a contributor to glaucoma pathophysiology?

Endothelin is a vasoactive peptide that has been shown to play an important role in vascular homeostasis. Recently, endothelin and its receptors have been found in ocular tissues where it appears to have a regulatory function. Endothelin is found in both the aqueous and vitreous humors and its concentration is elevated in glaucoma patients and in animal models of glaucoma. In the current review, the authors present information about the distribution of endothelin and endothelin receptors in the eye and the ocular actions of endothelins. Specifically, endothelin/aqueous humor dynamics, endothelin/nitric oxide interactions, endothelin and ischemia, and endothelin/optic nerve head effects. Observations concerning the potential role of endothelin in glaucoma pathophysiology is presented and discussed relative to its effects on the optic nerve head and in relation to glaucoma theories.

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.

Advances in glaucoma diagnosis and therapy for the next millennium: new drugs for trabecular and uveoscleral outflow

Advances in our understanding of the physiology and molecular biology of the trabecular and uveoscleral outflow pathways of the eye will lead to the development of new approaches for glaucoma therapy. Therapies of the future will target the structures and enzymes involved in maintaining cell shape and cell-cell and cell-extracellular matrix interactions. Altering the extracellular matrix in the ciliary muscle has been important in the intraocular pressure lowering effects of prostaglandins and will be developed further as an approach to enhancing outflow through the trabecular meshwork. Gene therapy may be used to enhance or suppress the endogenous targets that are ultimately responsible for the outflow enhancement triggered by these agents.

M3-type muscarinic receptors predominantly mediate neurogenic quick contraction of bovine ciliary muscle

1. The present experiments were designed to investigate which subtypes of muscarinic receptors are involved in the neurogenic quick contraction of bovine ciliary muscle in connection to quick eye focal accommodation. 2. Transmural electrical stimulation (TES) produced a transient contraction, which was abolished in the presence of 3 x 10(-7) M tetrodotoxin and 10(-6) M atropine, but greatly augmented by 3 x 10(-7) M physostigmine. 3. The exogenously applied acetylcholine (ACh: 10(-9) to 3 x 10(-6) M) produced a concentration-dependent contraction, which was competitively antagonized by 10(-6) M atropine and augmented by 3 x 10(-7) M physostigmine, but unaffected by 3 x 10(-7) M tetrodotoxin. 4. The magnitude and time to peak of the maximal contraction produced by TES were significantly greater (1267.5 +/- 86.0 mg, P < 0.005) and shorter (9.0 +/- 0.2 sec, P < 0.005) than corresponding values (97.0 +/- 9.9 mg and 20.3 +/- 2.1 sec, respectively) of the phasic contraction caused by exogenously applied 10(-5) M ACh, at which concentration the agonist caused the maximal contraction. The velocity (140.6 +/- 7.8 mg/sec) of the transient contraction caused by TES was approximately 28-fold greater than that of the phasic contraction caused by ACh (5.1 +/- 0.9 mg/sec). 5. The contractions produced by TES were greatly attenuated by 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) as an M3 antagonist and slightly by pirenzepine as an M1 antagonist (20.2 +/- 7.9% inhibition at the highest concentration), but not by methoctramine (MET) as an M2 antagonist. The IC50 value (-log M) for 4-DAMP was determined to be 7.17 +/- 0.14. 6. Scatchard plot analysis of [3H]-quinuclidinylbenzilate (QNB) binding revealed that the binding sites constituted a single population with a Kd of 31.2 +/- 0.8 pM and a Bmax of 895.5 +/- 93.2 fmol/mg protein. The activity in inhibiting [3H]-QNB binding was most potent with 4-DAMP (-log Ki = 7.98 +/- 0.02), but less potent with pirenzepine (-log Ki = 6.43 +/- 0.04) and MET (-log Ki = 7.32 +/- 0.16). 4-DAMP was approximately 35- and 5-fold more potent than pirenzepine and MET in terms of -log Ki values, respectively, suggesting the predominant localization of M3 receptor subtypes in the bovine ciliary muscle membrane. 7. These results suggest that TES produces a neurogenic quick contraction of the bovine ciliary muscle, which would be mediated mainly by ACh released from the intramural nerve terminals and subsequent excitation of M3 receptor subtypes localized on the ciliary muscle cells, and that neurogenic quick contraction of the ciliary muscle is possibly involved in part in eye focal accommodation.

Endothelin receptor A is expressed and mediates the [Ca2+]i mobilization of cells in human ciliary smooth muscle, ciliary nonpigmented epithelium, and trabecular meshwork

PURPOSE

To identify which endothelin receptor subtype is expressed and is functional in the human ciliary body and trabecular meshwork, tissues that regulate aqueous humor dynamics.

METHODS

Immunocytochemistry was used to characterize the primary culture cells of normal human ocular cells. Endothelin receptor gene expression was probed with reverse transcription of polymerase chain reaction (RT-PCR). Intracellular calcium ([Ca2+]i) mobilization was measured with video image microscopy using Fura-2AM as a fluorescent probe.

RESULTS

Identities of primary cultures, human ciliary smooth muscle (HCSM), ciliary nonpigmented epithelial (HCE), and trabecular meshwork (HTM) cells were confirmed by immunocytochemistry, using cell-specific markers and observing typical cell morphologies. The presence of endothelin receptor A (ETA) was detected with RT-PCR in all three types of cells. The mRNA phenotype was verified with restriction enzyme BamHI digestion. No ETB receptor subtype expression was detected with RT-PCR under the cell culture conditions used. The [Ca2+]i of HCSM cells was increased from 57 +/- 7 nM to 328 +/- 108 nM (n = 23; mean +/- SE; P < 0.05) by 1 nM endothelin-1 (ET-1). In HCE cells, [Ca2+]i increased from 40 +/- 3 nM to 90 +/- 10 nM (n = 55) (P < 0.001) with the same concentration of ET-1. Similarly, ET-1 (1 nM) increased the [Ca2+]i from 51 +/- 6 nM to 185 +/- 47 nM (n = 19) (P < 0.001) in the HTM cells. The agonist for ETB, S6c, had no effect on [Ca2+]i transients in all three cell types. No ETB receptor expression was detected in these cell types under the experimental and culture conditions.

CONCLUSION

ETA receptor is expressed and is possibly responsible for mediating the signal for [Ca2+]i mobilization by ET-1 in human ciliary smooth muscle, ciliary nonpigmented epithelial cells, and trabecular meshwork cells.

Nitric oxide relaxes bovine ciliary muscle contracted by carbachol through elevation of cyclic GMP

It is generally accepted that nitric oxide relaxes vascular smooth muscles by activating guanylyl cyclase, which in turn increases cyclic 3':5' guanosine monophosphate level. Despite the physiological significance of nitric oxide, very few studies have attempted to characterize the mode of action of this mediator in ciliary muscles. Therefore, the present experiments were designed to investigate whether or not the relaxation induced by sodium nitroprusside as a donor of nitric oxide is accompanied by the increase in cyclic 3':5' guanosine monophosphate level in the bovine ciliary muscle, and these responses are affected by methylene blue as an inhibitor of guanylyl cyclase and 3-isobutyl-1-methylxanthine as an inhibitor of phosphodiesterases. The relaxation activity of exogenous 8-bromo-cyclic 3':5' guanosine monophosphate was also determined. Sodium nitroprusside produced a concentration-dependent relaxation in the bovine ciliary muscle strips which had been contracted by carbachol as a cholinergic agonist. Relaxation in response to sodium nitroprusside was accompanied by a significant (P<0.05 and P<0.005) increase in cyclic 3':5' guanosine monophosphate level. The relaxation response and the increase in cyclic 3':5' guanosine monophosphate caused by sodium nitroprusside were significantly (P<0.01 and P<0.05) augmented by the pretreatment with 3-isobutyl-1-methylxanthine, and were significantly (P<0.005 and P<0.05) attenuated in the presence of methylene blue. The exogenously applied 8-bromo-cyclic 3':5' guanosine monophosphate relaxed the ciliary muscle strips in a concentration-dependent manner. These results suggest that nitric oxide causes relaxation of the bovine ciliary muscle through the activation of guanylyl cyclase and an increase in cyclic 3':5' guanosine monophosphate level.

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

PURPOSE

Nitric oxide (NO) relaxes ciliary smooth muscle, and endothelin-1 (ET-1) is reported to regulate ciliary muscle tone. Despite the physiological significance of nitric oxide and ET-1, very few studies have attempted to characterize the mutual modes of action of these mediators in this tissue. Thus, the present experiments were designed to investigate a possible relaxation mechanism of nitric oxide in bovine ciliary muscle that has been contracted by ET-1.

METHODS

The effects of sodium nitroprusside (SNP), as a nitric oxide donor, methylene blue, as an inhibitor of guanylate cyclase, and 8-bromo-cyclic GMP on the bovine ciliary muscle contracted with ET-1 were examined. The changes in cyclic GMP level and relaxation, in response to SNP alone or in combination with 3-isobutyl-1-methylxanthine (IBMX) as a nonselective inhibitor of phosphodiesterases, were also determined.

RESULTS

Sodium nitroprusside (SNP) produced a concentration-dependent relaxation, which was significantly (p < 0.005) augmented by 10(-5) M 3-isobutyl-1-methylxanthine (IBMX) and significantly (p < 0.005) attenuated by 3 x 10(-5) M methylene blue as an inhibitor of guanylate cyclase. The relaxation in response to SNP was accompanied by an increase in the cyclic 3':5' guanosine monophosphate (cyclic GMP) level, which was again significantly (p < 0.05) augmented by 10(-5) M IBMX and significantly (p < 0.005) attenuated by 3 x 10(-5) M methylene blue. The exogenously applied 8-bromo-cyclic GMP relaxed the ciliary muscle strips during the contraction caused by ET-1.

CONCLUSIONS

These results lead us to assume that NO generated from SNP is closely related to cyclic GMP production via the activation of guanylate cyclase and, in turn, causes a relaxation response in the bovine ciliary muscle contracted with ET-1.

A possible role of endogenous inhibitor for nitric oxide synthesis in the bovine ciliary muscle

The present experiments were designed to investigate the possible role of endogenous methylarginine derivatives such as NG-monomethyl-L-arginine, asymmetrical NG,NG-dimethyl-L-arginine and symmetrical NG,N'G-dimethyl-L-arginine for the nitric oxide synthesis in the bovine ciliary muscle. The contents of asymmetrical NG,NG-dimethyl-L-arginine and symmetrical NG,N'G-dimethyl-L-arginine in the bovine ciliary muscle were determined to be 370.2 +/- 27.6 (n = 5) and 182.4 +/- 22.9 (n = 5) pmoles g-1 wet weight, respectively by means of the automated high-performance liquid chromatography. NG-Monomethyl-L-arginine was below the assay limits. On the basis of the total tissue water content (0.792 +/- 0.006 ml g-1 wet weight, n = 14), the concentrations of asymmetrical NG,NG-dimethyl-L-arginine and symmetrical NG,N'G-dimethyl-L-arginine were tentatively estimated to be (4.7 +/- 0.3) x 10(-7) M (n = 5) and (2.3 +/- 0.3) x 10(-7) M (n = 5), respectively. A23187 (10(-7)-3 x 10(-4) M) produced a concentration-dependent relaxation of the ciliary muscle strips which had been contracted with 10(-5) M carbachol. Authentic asymmetrical NG,NG-dimethyl-L-arginine (3 x 10(-6)-3 x 10(-4) M), but not symmetrical NG,N'G-dimethyl-L-arginine (3 x 10(-4) M), inhibited the 10(-6) M A23187-induced relaxation in a concentration-dependent manner. The inhibition with asymmetrical NG,NG-dimethyl-L-arginine (10(-4) M) was reversed by an addition of 3 x 10(-3) M L-arginine, but not by 3 x 10(-3) M D-arginine. The A23187 (10(-6) M)-induced relaxation was enhanced by 3 x 10(-3) M L-arginine or superoxide dismutase (50 U ml-1), whereas it was inhibited by carboxy-PTIO (3 x 10(-4) M), a scavenger of nitric oxide, or methylene blue (10(-5) M), an inhibitor of guanylate cyclase. The carbachol-induced contraction was enhanced by asymmetrical, NG,NG-dimethyl-L-arginine (10(-5) M) and inhibited by 3 x 10(-3) M L-arginine. Any effect of prostanoid formation during the A23187-induced relaxation was ruled out by using indomethacin (10(-5) M). Sodium nitroprusside (10(-5) M), a donor of nitric oxide, also produced a relaxation, which was inhibited by methylene blue (10(-5) M) or carboxy-PTIO (3 x 10(-4) M) and was augmented by superoxide dismutase (50 U ml-1), but unaffected by asymmetrical NG,NG-dimethyl-L-arginine (3 x 10(-4) M) or L-arginine (3 x 10(-3) M). These results lead us to speculate that the nitric oxide synthesized endogenously from L-arginine may play a role for mediating relaxation of the bovine ciliary muscle and that the endogenous asymmetrical NG,NG-dimethyl-L-arginine may be involved in inhibiting the biosynthesis of nitric oxide when there are increased intracellular concentrations of the methylarginine under certain circumstances.