Inhibition of endothelin-1 and KCL-induced increase of [CA2+]i by antiglaucoma drugs in cultured A7r5 vascular smooth-muscle cells

Influence of Trace Elements on Neurodegenerative Diseases of The Eye-The Glaucoma Model

Glaucoma is a heterogeneous group of chronic neurodegenerative disorders characterized by a relatively selective, progressive damage to the retinal ganglion cells (RGCs) and their axons, which leads to axon loss and visual field alterations. To date, many studies have shown the role of various elements, mainly metals, in maintaining the balance of prooxidative and antioxidative processes, regulation of fluid and ion flow through cell membranes of the ocular tissues. Based on the earlier and current research results, their relationship with the development and progression of glaucoma seems obvious and is increasingly appreciated. In this review, we aimed to summarize the current evidence on the role of trace elements in the pathogenesis and prevention of glaucomatous diseases. Special attention is also paid to the genetic background associated with glaucoma-related abnormalities of physiological processes that regulate or involve the ions of elements considered as trace elements necessary for the functioning of the cells.

Calcium channels and their blockers in intraocular pressure and glaucoma

Several factors besides high intraocular pressure assumed to be associated with the development and progression of glaucoma, and calcium channel blockers (CCBs) have been an anticipated option for glaucoma treatment by improving ocular perfusion and/or exerting neuroprotective effects on retinal ganglion cells with safety established in wide and long-term usage. Decrease in IOP has been reported after topical application of CCBs, however, the effect is much smaller and almost negligible after systemic application. Various CCBs have been reported to increase posterior ocular blood flow in vivo and to exert direct neuroprotection in neurons in vitro. Distribution of the drug at a pharmacologically active concentration in the posterior ocular tissues across the blood-brain barrier or blood-retina barrier, especially in the optic nerve head and retina where the ganglion cells mainly suffer from glaucomatous damage, is essential for clinical treatment of glaucoma. Improved visual functions such as sensitivity in the visual field test have been reported after administration of CCBs, but evidences from the randomized studies have been limited and effects of CCBs on blood flow and direct neuroprotection are hardly distinguished from each other.

Preservative use in topical glaucoma medications

Benzalkonium chloride (BAK) is the principal preservative employed in topical ocular hypotensive medications, although alternative compounds recently have begun to be employed or examined. Individual clinical trials have shown that exposure to BAK concentrations contained in ophthalmic solutions does not produce adverse sequelae in the majority of glaucoma patients, but concerns continue with regard to its long-term use. These concerns have resulted from an extensive research effort, including preclinical studies with in vitro and in vivo models, as well as recent clinical investigations dedicated specifically to this issue. The aim of this systematic literature review of both preclinical and clinical data was to determine the relevance of these findings to clinical practice. Most preclinical studies reported negative effects of BAK exposure, but with few exceptions, BAK concentrations and exposure times greatly exceeded those likely to be experienced by patients, given the normal physiological dilution by the tear film. In addition, consistent evidence of BAK-related toxicity did not emerge from our review of dedicated clinical investigations. Thus, taken together, current evidence supports the safety of BAK for most glaucoma patients, although subpopulations with abnormal tearing may benefit from alternative preservative compounds or preservative-free formulations. Further studies to identify these populations are needed.

Mechanism of carteolol-induced cytosolic Ca2+ mobilization in cultured vascular smooth muscle cells

An increase in cytosolic calcium concentration triggers intracellular signal transduction in vascular cells, which then regulates the vascular contraction. In the present study, the regulatory mechanism of carteolol on the intracellular free Ca(2+) ([Ca(2+)](i)) mobilization was investigated in cultured A7r5 vascular smooth muscle cells. The A7r5 cells were cultured and loaded with fura-2-AM, which was used as a Ca(2+) sensitive fluorescent probe. In both the presence and absence of external Ca(2+), carteolol increased [Ca(2+)](i) with a dose-dependent manner in A7r5 cells at concentrations between 608 microM and 6.08 microM. In a Ca(2+)-containing buffer, carteolol-induced [Ca(2+)](i) showed an initial peak followed by a secondary and persistent plateau. Pretreatment of the cells with La(3+), the plasma membrane Ca(2+) pump inhibitor, and nifedipine, a L-type Ca(2+) channel inhibitor, both partially restrained the carteolol-induced initial peak in [Ca(2+)](i) by 92% and 86%, respectively. Pretreatment of the cells with adrenoceptor antagonists, prazosin inhibited the [Ca(2+)](i) response by 80%, and propranolol enhanced the response by 61%. In the Ca(2+-)-free buffer, pretreatment of the cells with carteolol inhibited the endoplasmic reticulum Ca(2+) pump inhibitor of thapsigargin-induced [Ca(2+)](i) increase by 97%. Pretreatment of the cells with thapsigargin also inhibited the carteolol-induced [Ca(2+)](i) rise by 98%. The internal Ca(2+) release induced by the carteolol was partially inhibited by U73122 (phospholipase C inhibitor) and aristolochic acid, quinacrine (phospholipase A(2) inhibitors). After incubation of carteolol in the Ca(2+)-free buffer, the addition of CaCl(2) increased the Ca(2+) influx, implying that the release of Ca(2+) from internal stores further induced capacitative Ca(2+) entry. These results suggest that carteolol-induced [Ca(2+)](i) increase is mediated by the initial influx via the alpha(1)-adrenoceptor, L-type Ca(2+) channel, nonselective calcium entry channels and release of Ca(2+) from an intracellular store, which is mainly in the endoplasmic reticulum followed by capacitative Ca(2+) entry but decrease via the beta(2)-adrenoceptor. The intracellular Ca(2+) release was also modulated by phospholipase A(2), C-coupled events.

Effects of Topical Travoprost and Unoprostone on Optic Nerve Head Circulation in Normal Rabbits

PURPOSE

To evaluate the effect, and the duration of the effect, of topically administrated travoprost and unoprostone on optic nerve head (ONH) circulation in Dutch rabbits.

METHODS

First, travoprost (0.004% solution) or unoprostone (0.12% solution) was unilaterally instilled once, or once daily (travoprost) or twice daily (unoprostone) for 7 days in Dutch rabbits. The ONH tissue blood velocity (NB(ONH)) was measured using the laser speckle method at 30 and 60 min after a single instillation of travoprost or unoprostone, and before and at 1, 6, and 12 hr (travoprost or unoprostone) and 24 hr (travoprost only) after the last instillation of the aforementioned 7-day instillation regimen. Second, similar experiments were conducted with indomethacin (5 mg/kg) pretreatment.

RESULTS

Both travoprost and unoprostone significantly increased NB(ONH) only in the treated eyes after a single instillation (p = 0.011 to 0.038); this effect was abolished by indomethacin pretreatment. In the 7-day instillation regimen, NB(ONH) was increased by 11%, 40%, 17%, 16%, and 12% only in the treated eyes just before and at 1, 6, 12, and 24 hr after the final instillation of travoprost, respectively, and increased by 10%, 25%, 13%, and 14% only in the treated eyes just before and at 1, 6, 12 hr after the final instillation of the unoprostone, respectively.

CONCLUSIONS

Topical travoprost or unoprostone significantly increased the ONH blood velocity with a single instillation and the effect persisted for 24 hr after a 7-day instillation. The effects of these drugs against retinal and ONH circulation are probably associated with the production of endogenous prostaglandins.

Local effect of topical FP-receptor agonists on retinal vessels of the ipsilateral posterior retina in normal rabbit eyes

PURPOSE

To determine whether a topically instilled prostaglandin analogue inhibits endothelin-1 (ET-1)-induced vasoconstrictive effects in the posterior retina by its local effects, and the duration of the effect in normal rabbit eyes.

METHODS

Travoprost, a potent selective FP-agonist, or unoprostone, a prostone that also has a weak non-selective FP-agonistic activity, solution was instilled once, or once daily (travoprost) or twice daily (unoprostone) for 7 days in one randomly chosen Dutch rabbit eye, and vehicle in the contralateral eye. ET-1 was intravitreously injected in both eyes 30 min after a single instillation of a test drug or its vehicle, or just after, 30, 60, 90 or 180 min after the final instillation of a 7-day instillation, and fundus photographed before, 30 and 60 min after the injection to study whether difference was seen in the ET-1-induced constriction of retinal vessels between the drug- and vehicle-instilled eyes. The same experimental procedures were conducted with indomethacin pretreatment.

RESULTS

In the rabbit eyes where travoprost was instilled for 7 days, the ET-1-induced constriction of retinal vessels was significantly inhibited only on the drug-treated side, when ET-1 was injected 30 or 60 min after the final instillation (P = 0.026-0.005), which was abolished by indomethacin pretreatment. A single instillation of travoprost or unoprostone and 7-day instillation of unoprostone showed no effect.

CONCLUSIONS

After a 7-day instillation in normal rabbit eyes, topical travoprost suppressed ET-1-induced vasoconstrictive effects only in the ipsilateral posterior retina by its local effect; this effect was maintained at least for 30 min and mediated by endogenous prostaglandins.

Mechanism of unoprostone-induced cytosolic Ca2+ mobility in cultured porcine corneal endothelial cells

The effect of unoprostone isopropyl on the intracellular free Ca(2+) signaling ([Ca(2+)](i)) in cultured porcine corneal endothelial cells was evaluated by using fura-2-AM as a Ca(2+) probe. In Ca(2+)-containing buffer and Ca(2+)-free buffer, unoprostone increased [Ca(2+)](i) concentration dependently from 28.2 to 0.282microM, diluted from original concentration to 1/100, 1/1000 and 1/10,000. The response was inhibited on extracellular Ca(2+) removal. In Ca(2+)-free buffer, pretreatment of the cells with unoprostone inhibited the endoplasmic reticulum Ca(2+) pump inhibitor thapsigargin-induced [Ca(2+)](i) increase and the mitochondria uncoupler carbonylcyanide m-chlorophenylhydrazone (CCCP)-induced Ca(2+) release by 96 and 95%, respectively. Pretreatment of the cells with thapsigargin or CCCP also inhibited unoprostone-1-induced [Ca(2+)](i) rise by 84 and 57%, respectively. The intracellular calcium release caused by unoprostone was partially inhibited by phospholipase C inhibitor (U73122) and by phospholipase A(2) inhibitor aristolochic acid. After incubation of the cells with unoprostone in Ca(2+)-free buffer, the addition of 5mM CaCl(2) increased Ca(2+) influx, implying that release of Ca(2+) from internal stores caused by unoprostone further induced capacitative Ca(2+) entry. These results suggest that unoprostone-induced [Ca(2+)](i) increase in corneal endothelial cells results from Ca(2+) influx from external buffer and release of Ca(2+) from intracellular stores from the endoplasmic reticulum and mitochondria Ca(2+) stores followed by capacitative Ca(2+) entry. Unoprostone-induced intracellular Ca(2+) release was also modulated by phospholipase A(2) and C-coupled events.

Vasodilatory mechanism of levobunolol on vascular smooth muscle cells

Topical application of levobunolol hydrochloride, a beta-adrenergic antagonist used for treatment of glaucoma, is reported to increase ocular blood flow. We studied the mechanism of levobunolol-induced vasodilation in arterial smooth muscle. The effects of levobunolol or other agents on isolated, pre-contracted rabbit ciliary artery were investigated using an isometric tension recording method. The effects of the same agents on intracellular free calcium ([Ca(2+)](i)) in cultured human aortic smooth muscle cells were also studied by fluorophotometry. Levobunolol relaxed ciliary artery rings that were pre-contracted with either high-K solution, 1microM histamine, 10microM phenylephrine, or 100nM endothelin-1. The relaxation induced by levobunolol was concentration-dependent over the range of 10microM to 0.3mM. Inhibition of endothelial nitric oxide synthase or denudation of the endothelium did not affect this relaxation. Histamine-induced contractions were inhibited by the histamine H(1) antagonist pyrilamine. Radioligand binding experiments showed that levobunolol did not bind to the H(1) receptor. Further, histamine induced transient contraction in Ca(2+)-free solution, and levobunolol inhibited this contraction by 74.6+/-11.0%. In cultured smooth muscle cells in the presence of extracellular Ca(2+), levobunolol significantly inhibited the histamine-induced elevation of [Ca(2+)](i). However, it did not inhibit the increase of [Ca(2+)](i) in histamine-stimulated cells incubated in Ca(2+)-free solution. These results indicate that levobunolol may relax rabbit ciliary artery by two different mechanisms. First, the relaxation could be due to the blockade of Ca(2+) entry through non-voltage-dependent Ca(2+) channels. Second, levobunolol may change the Ca(2+) sensitivity of vascular smooth muscle cells.

Cellular and molecular effects of unoprostone as a BK channel activator

Effects of unoprostone isopropyl (unoprostone), a prostaglandin metabolite analog; latanoprost, a PGF(2alpha) analog; and PGF(2alpha) were examined in HCN-1A cells, a model system for studies of large conductance Ca(2+) activated K(+)(BK) channel activator-based neuroprotective agents. Unoprostone and latanoprost, both used as anti-glaucoma agents, have been suggested to act through FP receptors and have neuroprotective effects. Ion channel activation, plasma membrane polarization, [Ca(2+)](i) changes and protection against long-term irreversible glutamate-induced [Ca(2+)](i) increases were studied. Unoprostone activated iberiotoxin (IbTX)-sensitive BK channels in HCN-1A cells with an EC(50) of 0.6+/-0.2 nM and had no effect on Cl(-) currents. Unoprostone caused IbTX-sensitive plasma membrane hyperpolarization that was insensitive to AL8810, an FP receptor antagonist. In contrast, latanoprost and PGF(2alpha) activated a Cl(-) current sensitive to [Ca(2+)](i) chelation, tamoxifen and AL8810, and caused IbTX-insensitive, AL8810-sensitive membrane depolarization consistent with FP receptor-mediated Ca(2+) signaling Cl(-) current activation. Latanoprost and PGF(2alpha), but not unoprostone, increased [Ca(2+)](i). Unoprostone, PGF(2alpha) only partially, but not latanoprost protected HCN-1A cells against glutamate-induced Ca(2+) deregulation. These findings show that unoprostone has a distinctly different mechanism of action from latanoprost and PGF(2alpha). Whether unoprostone affects the BK channel directly or an unidentified signaling mechanism has not been determined.