Inhibitory action of betaxolol, a beta 1-selective adrenoceptor antagonist, on voltage-dependent calcium channels in guinea-pig artery and vein

Betaxolol as a Potent Inhibitor of NDM-1-Positive E. coli That Synergistically Enhances the Anti-Inflammatory Effect in Combination with Meropenem

With significant human and economic losses, increasing bacterial resistance is a serious global threat to human life. Due to their high efficacy, broad spectrum, and cost-effectiveness, beta-lactams are widely used in the clinical management of bacterial infection. The emergence and wide spread of New Delhi metallo-β-lactamase (NDM-1), which can effectively inactivate β-lactams, has posed a challenge in the design of effective new antimicrobial treatments. Medicine repurposing is now an important tool in the development of new alternative medicines. We present a known glaucoma therapeutic, betaxolol (BET), which with a 50% inhibitory concentration (IC50) of 19.3 ± 0.9 μM significantly inhibits the hydrolytic activity of the NDM-1 enzyme and may represent a potential NDM-1 enzyme inhibitor. BET combined with meropenem (MEM) showed bactericidal synergism in vitro. The efficacy of BET was further evaluated against systemic bacterial infections in BALB/c mice. The results showed that BET+MEM decreased the numbers of leukocytes and inflammatory factors in peripheral blood, as well as the organ bacterial load and pathological damage. Molecular docking and kinetic simulations showed that BET can form hydrogen bonds and hydrophobic interactions directly with key amino acid residues in the NDM-1 active site. Thus, we demonstrated that BET inhibited NDM-1 by competitively binding to it and that it can be developed in combination with MEM as a new therapy for the management of infections caused by medicine-resistant bacteria.

Selectivity of pharmacological tools: implications for use in cell physiology. A review in the theme: Cell signaling: proteins, pathways and mechanisms

Pharmacological inhibitors are frequently used to identify the receptors, receptor subtypes, and associated signaling pathways involved in physiological cell responses. Based on the effects of such inhibitors conclusions are drawn about the involvement of their assumed target or lack thereof. While such inhibitors can be useful tools for a better physiological understanding, their uncritical use can lead to incorrect conclusions. This article reviews the concept of inhibitor selectivity and its implication for cell physiology. Specifically, we discuss the implications of using inhibitor vs. activator approaches, issues of direct vs. indirect pathway modulation, implications of inverse agonism and biased signaling, and those of orthosteric vs. allosteric, competitive vs. noncompetitive, and reversible vs. irreversible inhibition. Additional problems can result from inconsistent estimates of inhibitor potency and differences in potency between cell-free systems and intact cells. These concepts are illustrated by several examples of inhibitors displaying affinity for related but distinct targets or even unrelated targets. Of note, many of the issues being addressed are also applicable to genetic inhibition strategies. The main practical conclusion following from these concepts is that investigators should be critical in the choice of inhibitor, its concentrations, and its mode of application. When this advice is adhered to, small-molecule pharmacological inhibitors can be important experimental tools in the hand of physiologists.

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.

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.

Effects of landiolol on mechanical and metabolic changes in rat reperfused ischaemic hearts

1. The aim of the present study was to clarify the effects of landiolol, a short-acting selective beta(1)-adrenoceptor blocking agent, on mechanical and metabolic changes in postischaemic perfused hearts. 2. Rat isolated hearts (n = 30) were randomly separated into non-ischaemic or ischaemic groups. The latter group was further divided into Krebs'-Henseleit solution (KHS)- and landiolol (30, 100 or 300 micromol/L)-treated groups. Ischaemic hearts were subjected to 25 min global ischaemia and 20 min reperfusion under atrial pacing. Time-course changes in left ventricular (LV) end-diastolic pressure (LVEDP), LV developed pressure (LVDP), peak positive velocity of change of LV pressure (LVdP/dt(max)) and coronary flow were observed along with tissue contents of adenosine triphosphate (ATP), creatine phosphate, inorganic phosphate (Pi), malondialdehyde (MDA) and lactate dehydrogenase (LDH) release in coronary effluent. The effects of landiolol on rat isolated aortic preparations under KCl contraction were also investigated. 3. Ischaemia-reperfusion significantly impaired cardiodynamics, such as LVEDP, LVDP and LVdP/dt(max), decreased myocardial ATP content and increased Pi and LDH release. In the 30 micromol/L landiolol-treated group, cardiovascular parameters impaired by ischaemia-reperfusion and increased LDH release were further exacerbated and myocardial MDA content was significantly increased. In the 300 micromol/L landiolol-treated group, cardiac contractile dysfunction was improved and myocardial MDA, ATP and Pi contents were preserved. All measurements in the 100 micromol/L landiolol-treated group were similar to those in the ischaemic KHS group. Furthermore, significant relaxations of isolated aortic preparations were obtained with landiolol 30-1000 micromol/L, suggesting a possible calcium antagonism with landiolol. 4. In conclusion, landiolol, at low concentrations, aggravated myocardial ischaemia-reperfusion injuries, whereas at high concentrations it ameliorated them. The former effect may be mediated by the production of reactive oxygen species, whereas the latter may involve calcium antagonist activity.

Effect and Mechanism of Betaxolol and Timolol on Vascular Relaxation in Isolated Rabbit Ciliary Artery

PURPOSE

In order to clarify the vasodilatory mechanism of betaxolol and timolol, we studied the effects of these drugs in isolated rabbit ciliary arteries.

METHODS

Rabbit ciliary artery specimens were mounted in a double myograph system, and betaxolol, timolol, or another agent was introduced into the organ chamber. The mechanical response of the arteries was studied using an isometric tension recording method. The intracellular free calcium concentration [Ca2+]i was also measured using fluorescence photometry.

RESULTS

Betaxolol and timolol induced dose-dependent relaxation in the rabbit ciliary arteries precontracted by high-K+ Krebs solution. The minimum concentrations required to cause relaxation were 10 microM of betaxolol, and 30 microM of timolol. At the maximum concentration of 1 mM, betaxolol induced almost complete relaxation of the ciliary arteries, whereas timolol induced approximately 70% relaxation. These actions were not inhibited by pretreatment with 100 microM NG-nitro-l-arginine methylester (L-NAME), a nitric oxide synthase inhibitor, or by denudation of the vascular endothelium. However, 300 microM of betaxolol or timolol decreased the [Ca2+]i of the vascular smooth muscle, an action similar to that of diltiazem, a typical L-type voltage calcium-channel blocker.

CONCLUSIONS

Betaxolol, a selective beta1-adrenoceptor antagonist, and timolol, a nonselective beta-adrenoceptor antagonist, both frequently used in the medical management of glaucoma, decrease [Ca2+]i by acting as Ca2+ channel blockers, thus causing relaxation of isolated rabbit ciliary artery.

Comparative studies of AJG049, a novel Ca2+ channel antagonist, on voltage-dependent L-type Ca2+ currents in intestinal and vascular smooth muscle

BACKGROUND AND PURPOSE

Antagonists of Ca2+ channels reduce contraction of intestinal smooth muscle but also affect vascular smooth muscle. We have therefore examined the effects of AJG049, a newly synthesized antagonist for regulation of gut motility, on voltage-dependent L-type Ca2+ channels, in vascular and intestinal smooth muscle, comparing AJG049 with two other Ca2+ channel antagonists, verapamil and diltiazem.

EXPERIMENTAL APPROACH

Affinities of AJG049 for various types of voltage-dependent Ca2+ channels were examined by binding studies. Effects of AJG049 on voltage-dependent inward Ca2+ (or Ba2+) currents (ICa or IBa) in dispersed smooth muscle cells from guinea-pig ileum, colon and mesenteric artery were measured using conventional whole-cell configurations.

KEY RESULTS

In binding studies, AJG049 showed a high affinity for the diltiazem-binding site of L-type Ca2+ channels. In whole-cell configuration, AJG049 suppressed ICa in ileal myocytes, with concentration-, voltage-and use-dependencies. AJG049 shifted the steady-state inactivation curve of ICa to the left. The order of potency to inhibit ICa in ileal myocytes was AJG049>verapamil>diltiazem. AJG049 also suppressed IBa in guinea-pig mesenteric arterial myocytes, showing concentration- and voltage-dependencies and the potency order for this action was also AJG049>verapamil>diltiazem. For the relative ratio of Ki values between ileal and mesenteric arterial myocytes, the order was AJG049>diltiazem>>verapamil.

CONCLUSIONS AND IMPLICATIONS

These results show that AJG049 inhibits L-type Ca2+ channels mainly through the diltiazem-binding site(s). From our results, AJG049 showed a little selectivity for these Ca2+ channels in intestinal smooth muscle.

Effects of hypoxia on the vasodilator activity of nifedipine and evidence of secondary pharmacological properties

The effects of hypoxia on the vasodilator response of endothelium-denuded rat aortic rings to the calcium channel blocker, nifedipine, were examined. Under normoxic conditions, nifedipine (10(-8)-3x10(-6) M) attenuated the contractility of noradrenaline precontracted rings in a concentration-dependent manner, although the sensitivity was less than what occurs with K+ precontracted tissues. Under hypoxic conditions there was no relaxation by nifedipine. When a concentration-response curve to noradrenaline was constructed before and in the presence of a high concentration of nifedipine (10(-5) M), the response to noradrenaline was unaffected in both normoxic and hypoxic conditions. When noradrenaline was replaced by phenylephrine (10(-8)-10(-5) M), the maximum tension was reduced in the presence of nifedipine to 59+/-6% of the pre-nifedipine value. Repetition of the experiment in the presence of cocaine (10(-5) M) revealed the inhibitory effect of nifedipine on noradrenaline-induced contraction, the maximum contraction in the presence of nifedipine falling significantly (P<0.005) to 67+/-6% of the pre-nifedipine response. When propranolol (10(-7) M) was present in the bath, the maximum contraction to noradrenaline was significantly (P<0.05) reduced by nifedipine to 55+/-4% of its previous value. The fact that nifedipine was able to inhibit phenylephrine-induced contractions and relax noradrenaline-precontracted aortic rings confirms its calcium channel blocking activity. The failure to inhibit noradrenaline when added prior to the noradrenaline-induced contractions suggests an opposing effect in addition to calcium channel blockade, which cancels out the attenuation of noradrenaline--but not phenylephrine-induced contractions. When neuronal uptake of noradrenaline was blocked with cocaine or beta-adrenoceptors were blocked with propranolol, the inhibitory effect of nifedipine against noradrenaline-induced contractions was revealed. This suggests that the additional property was due to blockade of neuronal reuptake or antagonism at beta-adrenoceptors. This study also showed that nifedipine is ineffective as a vasodilator in the rat aorta under hypoxic conditions.

Protective effects of betaxolol in eyes with kainic acid-induced neuronal death

In the present study, we investigated whether betaxolol, a selective beta1-adrenoceptor antagonist, has neuroprotective effect on kainic acid (KA)-induced retinal damage. Neurotoxicities were induced in adult male rats by intravitreal injection of KA (total amount, 6 nmol). To examine the neuroprotective effects of betaxolol, rats were pretreated with betaxolol topically 60 min before KA injection to the rat eyes and twice daily for 1, 3, and 7 days after KA injection. The neuroprotective effects of betaxolol were estimated by measuring the thickness of the various retinal layers, and by counting the number of choline acetyltransferase (ChAT)- and tyrosine hydroxylase (TH)-positive cells in each retinal layer. The retina is highly vulnerable to KA-induced neuronal damage. Morphometric analysis of retinal damage in KA injected eyes, the thickness of the retinal layers decreased markedly after KA injection period of both 3 and 7 days. Furthermore, the numbers of ChAT- and TH-positive cells were significantly reduced by intravitreal injection of KA. However, when two drops of betaxolol, once before KA injection and twice daily for 7 days after KA injection, were continuously administered, the reductions in the retinal thickness and the retinal ChAT- and TH-positive cells were significantly attenuated. The present study suggests that topically applied betaxolol has neuroprotective effect on the retinal cell damage due to KA-induced neurotoxicity.

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

Over contraction of vascular smooth muscle may result in ischemia to ocular neuronal cells and deteriorate the glaucoma. The purpose of this study was to investigate the inhibitory effects of various commercial antiglaucoma drugs including brimonidine, dipivefrin, betaxolol, timolol, levobunolol, carteolol, brinzolamide, dorzolamide, unoprostone, latanoprost, pilocarpine, and preservative benzalkonium chloride on endothelin-1(ET-1) and KCl-induced increase of intracellular free Ca2+ ([Ca2+]i) in cultured rat A7r5 vascular smooth muscle cells. These drugs were diluted from original concentrations to 1/100, 1/1000, and 1/10000. [Ca2+]i mobility was analyzed by spectrofluorometry after loading with fura-2-AM. Betaxolol, timolol, levobunolol, and carteolol were found to inhibit KCl-induced release of [Ca2+]i in a dose-dependent manner. High concentrations of betaxolol, timolol, levobunolol, carteolol, and unoprostone also inhibited ET-1-induced increase of [Ca2+]i in A7r5 cells. However, ET-1- and KCl-induced increase of [Ca2+]i was not diminished by other drugs including brimonidine, dipivefrin, brinzolamide, dorzolamide, latanoprost, pilocarpine, and benzalkonium chloride. These results indicate that high concentrations of unoprostone and beta-adrenergic blocking agents including betaxolol, timolol, levobunolol, and carteolol may inhibit ET-1-induced increase of [Ca2+]i. The mechanism may be mediated by inhibition of extracellular calcium influx via blocking of L-type voltage-dependent Ca2+ channel in A7r5 cells.

Betaxolol attenuates retinal ischemia/reperfusion damage in the rat

This study was performed to elucidate the protection afforded by post-treatment with Betoptic (0.25% betaxolol) against neuronal cell damage after ischemia/reperfusion insult in rats. Betaxolol was applied topically after the start of reperfusion and its effect was evaluated by morphometry and choline acetyltransferase immunoreactivity of retinas at 7 days after reperfusion. In non-treated eyes, the thickness of the inner plexiform layer decreased markedly after a reperfusion period of both 3 and 7 days. However, when eyes were treated with betaxolol after ischemia/reperfusion injury, both the reduction of the inner plexiform layer thickness and the retinal choline acetyltransferase immunoreactivity were significantly attenuated. These findings suggest that betaxolol is an efficient neuroprotective agent and prevents the retinal cell damage induced by ischemic injury in rats.

Beta-adrenergic receptor agonists and antagonists counteract LPS-induced neuronal death in retinal cultures by different mechanisms

Treatment with lipopolysaccharide (LPS) for 72 h was shown to dose-dependently increase nitric oxide production from 6-day-old retinal cultures. Cell death, as determined by lactate dehydrogenase (LDH) release and an increase in neuronal labelling for TUNEL, was elevated concurrently. During treatment there was an increase of both inducible nitric oxide synthase and glial fibrillary acidic protein labelling in glial cells and a reduction in the number of gamma-aminobutyric acid-positive neurones. The NOS inhibitors, N-nitro-L-arginine methyl ester, dexamethasone and indomethacin potently inhibited both nitric oxide stimulation and cell death caused by LPS. In this study, the beta(2)- (ICI-18551), beta(1)- (betaxolol) and mixed beta(1)/beta(2)- (timolol, metipranolol) adrenergic receptor antagonists were all shown to attenuate LPS-induced LDH release from these cultures, but to have no effect on LPS-stimulated nitric oxide production. This effect was mimicked by the calcium channel blocker, nifedipine. Interestingly, the beta-adrenergic receptor agonists, salbutamol, arterenol and isoproterenol were also able to attenuate cell death caused by LPS. Moreover, these compounds also inhibited LPS-stimulated nitric oxide release. These studies suggest that LPS stimulates nitric oxide release from cultured retinal glial cells and that this process leads to neurone death. beta-adrenergic receptor agonists prevent the effects of LPS by inhibiting the stimulation of nitric oxide production. The data also suggest that beta-adrenergic receptor antagonists can attenuate LPS-induced death of neurones, but that these compounds act in a manner that is neurone-dependent, is mimicked by blockade of calcium channels and is independent of the stimulation of nitric oxide release.

Celiprolol increases coronary blood flow and reduces severity of myocardial ischemia via nitric oxide release

Celiprolol is a selective beta(1)-adrenoceptor antagonist with antihypertensive actions, which causes renal vasodilation by increasing tissue nitric oxide (NO) levels. The authors tested whether celiprolol increases coronary blood flow (CBF) by increasing cardiac NO release in the ischemic heart in vivo. In open-chest dogs, coronary perfusion pressure of the left anterior descending coronary artery was reduced so that CBF decreased to 60% of control levels, and thereafter, coronary perfusion pressure was maintained constant. Ten minutes after the reduction of coronary perfusion pressure, we infused celiprolol into the left anterior descending coronary artery and measured fractional shortening and lactate extraction ratio as indices of regional myocardial contractility and metabolism. CBF significantly increased from 51.5 mL/100 g/min +/- 1.9 to 67.0 mL/100 g/min +/- 5.1 20 minutes after celiprolol infusion without changes in coronary perfusion pressure, while fractional shortening and lactate extraction ratio increased. Celiprolol also increased cardiac NO release. The L omega-nitroarginine methyl ester, the inhibitor of NO synthase, attenuated the increases in CBF, fractional shortening, lactate extraction ratio, and cardiac NO release due to celiprolol. ICI 118551, a beta(2)-adrenoceptor antagonist, did not blunt the effects of celiprolol and a nonselective beta-adrenoceptor antagonist, propranolol, increased neither CBF nor cardiac NO release, indicating that the effect of celiprolol is independent of beta-adrenoceptor blockade. It was concluded that celiprolol mediates coronary vasodilation and improves myocardial ischemia through NO-dependent mechanisms.

Betaxolol, a beta1-adrenoceptor antagonist, protects a transient ischemic injury of the retina

In the present study, we investigated the protective effects of the topical beta-adrenoceptor antagonist Betoptic((R)) (0.25% betaxolol) in the rat retina following the ischemic injury induced by a transient increase of intraocular pressure (IOP). Like other areas of the central nervous system, the retina is highly vulnerable to ischemic-induced injury. Ischemia was induced in the rat retina by raising the IOP above the systolic blood pressure for 60min. After an ischemia/reperfusion, the thickness of the retinal layers and the immunoreactivities of choline acetyltransferase (ChAT), gamma-amino butyric acid (GABA) and tyrosine hydroxylase (TH) were examined. After a reperfusion period of 7 days, the thickness of both the inner plexiform layer and inner nuclear layer was much decreased. After a reperfusion period of 14-28 days, the thickness of the outer nuclear layer decreased markedly. Moreover, the ChAT and TH immunoreactivity had almost completely disappeared in the retinas after 7 days, while GABA immunoreactivity remained for 28 days. These results suggest that the inner retinal layers are more susceptible to ischemic-induced injury than the outer retinal layer.Histological examination demonstrated protective effects of betaxolol on ischemic-induced retinal damage, which was more substantial in the inner retinal layer. When two drops of betaxolol, once before ischemic injury and twice daily for 28 days after ischemia, were continuously administered, the reductions in the retinal ChAT, GABA and TH immunoreactivities were significantly attenuated. The present study suggests that topically applied betaxolol is an efficient neuroprotective agent and prevents the retinal cell damage induced by ischemic injury in rats.

Effect of betaxolol on aspartate aminotransferase activity in hypoxic rat retina in vitro

We investigated the effect of betaxolol on the decrease of mitochondrial aspartate aminotransferase (mAAT) activity in rat retinas induced by hypoxia in vitro. It is reported that mAAT decreases in ischemic or hypoxic retina and that the decrease is caused by Ca(2+)-dependent proteases such as calpain. Betaxolol is a compound that has beta(1)-adrenergic receptor blocking and voltage-dependent calcium channel blocking properties. The rat eye cups were maintained with Locke's solution saturated with 95% air - 5% CO(2). The eye cups were immersed in glucose-free Locke's solution saturated with 95% N(2) / 5% CO(2) (hypoxic solution). Ninety minutes of hypoxia caused a 20% decrease in mAAT activity. The eye cups incubated with the hypoxic solution containing 1 mM EGTA, 10 micro M MK-801 or 100 micro M betaxolol were protected from the decrease in mAAT activity, so that the residual mAAT activity was 50%, 45% or 40%, respectively, compared to the eye cups incubated in hypoxic solution alone, which had a 100% decrease in mAAT activity. In addition, co-incubation with EGTA and betaxolol had a greater protective effect against the mAAT decrease than a single application. This additive effect of betaxolol was dose-dependent. These results suggested that betaxolol had a protective effect against the decrease of mAAT caused by hypoxia and indicated that betaxolol might inhibit the Ca(2+) release from intracellular Ca(2+) stores.

Vasorelaxant effects of racemic betaxolol and its R- and S- isomers on bovine retinal vessels

PURPOSE

RS-Betaxolol (equimolar R- and S- isomers) lowers intraocular pressure and dilates precontracted retinal and posterior ciliary arteries in vitro. Betaxolol's vasorelaxant effect is thought to involve the inhibition of calcium influx into vascular smooth muscle and is unrelated to its stereoselective beta-adrenergic blocking action. The authors assessed the vasodilatory effect of RS-betaxolol on different diameters of bovine retinal arterioles and venules, and these responses were compared with the responses induced by R- and S-betaxolol isomers in vitro.

METHOD

In-vitro preparations of the bovine retinal microcirculatory system were perfused continuously with oxygenated, heparinized physiological salt solution at 37 degrees C. Diameters of retinal arterioles and venules were measured using video imaging. The retinal vessels were preconstricted with 40 mM KCl, and concentration-response curves for vasodilation were obtained for RS-betaxolol, R-betaxolol, and S-betaxolol.

RESULTS

Baseline diameters of first-order (A1) and second-order (A2) branches of retinal arterioles were 50 +/- 0.6 microm and 39 +/- 1 microm, respectively (n = 20), whereas diameters of first-order (V1) and second-order (V2) branches of venules were 75 +/- 0.8 microm and 50 +/- 0.4 mM, respectively (n = 20). The diameters of all sizes of retinal arterioles and venules were significantly reduced (i.e., vasoconstricted) in the presence of 40 mM KCl (n = 20). These preconstricted vessels were relaxed in a dose-dependent manner by cumulative additions of RS-betaxolol, R-betaxolol, and S-betaxolol. The dose-response curves of these compounds were not significantly different.

CONCLUSION

RS-Betaxolol, R-betaxolol, and S-betaxolol were equiactive and produced concentration-dependent vasodilatation of all sizes of retinal arterioles and venules studied.

Comparative actions of insulin sensitizers on ion channels in vascular smooth muscle

Thiazolidinedione and isoxazolidinedione insulin sensitizers activate peroxisome proliferator-activated receptor gamma (PPAR gamma). Some thiazolidinediones modify ion channels in smooth muscles; however, the mechanism by which their actions occur has not been clarified. We, thus, examined the effects of three thiazolidinediones (troglitazone, pioglitazone, and rosiglitazone) and isoxazolidinedione (JTT-501), as well as an intrinsic ligand for PPAR gamma, 15-deoxy-Delta(12,14) prostaglandin J(2) (prostaglandin J(2)), on voltage-operated Ca(2+) currents (I(Ca)), voltage-dependent K(+) currents (I(Kv)), and Ca(2+)-activated K(+) currents (I(Kca)), to clarify whether a thiazolidinedione structure or PPAR gamma activation is related to their actions on ion channels. The whole-cell patch clamp method was used to record currents in smooth muscle cells from guinea-pig mesenteric arteries. Thiazolidinediones inhibited I(Ca) in a dose-dependent manner (troglitazone>pioglitazone=rosiglitazone). Troglitazone (> or =1 microM) and rosiglitazone (100 microM), but not pioglitazone, inhibited I(Kv). Rosiglitazone (> or =10 microM) enhanced, troglitazone (> or =1 microM) inhibited, and pioglitazone did not affect I(Kca). A high concentration of JTT-501 (100 microM) inhibited I(Ca), I(Kv), and I(Kca) to a similar extent. Prostaglandin J(2) enhanced I(Kca), but affected neither I(Ca) nor I(Kv). In summary, the three thiazolidinediones and isoxazolidinedione act differently on Ca(2+) and K(+) channels in vascular smooth muscle. The action of thiazolidinediones on I(Ca) could be attributed to specific regions of the molecules and not to activation of PPAR gamma. Involvement of PPAR gamma activation in the stimulation of I(Kca) is possible but should be tested further.

Betaxolol, a beta(1)-adrenoceptor antagonist, reduces Na(+) influx into cortical synaptosomes by direct interaction with Na(+) channels: comparison with other beta-adrenoceptor antagonists

Betaxolol, a beta(1)-adrenoceptor antagonist used for the treatment of glaucoma, is known to be neuroprotective in paradigms of ischaemia/excitotoxicity. In this study, we examined whether betaxolol and other beta-adrenoceptor antagonists interact directly with neurotoxin binding to sites 1 and 2 of the voltage-sensitive sodium channel (Na(+) channel) in rat cerebrocortical synaptosomes. Betaxolol inhibited specific [(3)H]-batrachotoxinin-A 20-alpha-benzoate ([(3)H]-BTX-B) binding to neurotoxin site 2 in a concentration-dependent manner with an IC(50) value of 9.8 microM. Comparison of all the beta-adrenoceptor antagonists tested revealed a potency order of propranolol>betaxolol approximately levobetaxolol>levobunolol approximately carteolol>/=timolol>atenolol. None of the drugs caused a significant inhibition of [(3)H]-saxitoxin binding to neurotoxin receptor site 1, even at concentrations as high as 250 microM. Saturation experiments showed that betaxolol increased the K(D) of [(3)H]-BTX-B binding but had no effect on the B(max). The association kinetics of [(3)H]-BTX-B were unaffected by betaxolol, but the drug significantly accelerated the dissociation rate of the radioligand. These findings argue for a competitive, indirect, allosteric mode of inhibition of [(3)H]-BTX-B binding by betaxolol. Betaxolol inhibited veratridine-stimulated Na(+) influx in rat cortical synaptosomes with an IC(50) value of 28. 3 microM. Carteolol, levobunolol, timolol and atenolol were significantly less effective than betaxolol at reducing veratridine-evoked Na(+) influx. The ability of betaxolol to interact with neurotoxin site 2 of the Na(+) channel and inhibit Na(+) influx may have a role in its neuroprotective action in paradigms of excitotoxicity/ischaemia and in its therapeutic effect in glaucoma.

Suppressive actions of betaxolol on ionic currents in retinal ganglion cells may explain its neuroprotective effects

Betaxolol, a beta 1-selective adrenoceptor antagonist, is widely used in the treatment of glaucoma. In addition to its ocular hypotensive effects, betaxolol has been suggested to act as a retinal neuroprotective agent (Osborne et al., 1997). To investigate possible mechanisms underlying the neuroprotective effects, we tested the actions of betaxolol on ion channels and calcium signaling in isolated retinal ganglion cells. Betaxolol (50 microM) reduced by about 20% the high-voltage-activated (HVA) Ca channel currents in ganglion cells isolated from tiger salamander retina. In contrast, the beta 1-adrenoceptor antagonists propranolol (10 microM) and timolol (50 microM) had no inhibitory actions on HVA Ca channel currents. The L-type Ca channel antagonist, nisoldipine, blocked the HVA Ca channel current partially and the remaining current was not inhibited by betaxolol. Outward current was inhibited in the presence of betaxolol. Both iberiotoxin (IBTX; 10 nM), a selective inhibitor of large-conductance Ca-activated K channels, and Cd2+ (100 microM), which suppresses Ca-activated K channels subsequent to its block of Ca channels, reduced outward current and the remaining current was not blocked significantly with betaxolol. In the presence of betaxolol, Na channel currents were reduced by about 20%, as were currents evoked by glutamate (10 mM) and GABA (1 mM). Current clamp recordings from isolated ganglion cells showed that betaxolol had several effects on excitability: spike height decreased, repetitive spike activity was suppressed, spike width increased and hyperpolarization following spikes was reduced. Calcium imaging in isolated rat retinal ganglion cells revealed that betaxolol inhibited glutamate-induced increases in [Ca2+]i. These results suggest that betaxolol has a diversity of suppressive actions on ganglion cell ion channels and that, as a consequence, one of the net actions of the drug is to reduce Ca2+ influx. The subsequent reduction in [Ca2+]i may contribute to the apparent neuroprotective actions of betaxolol in promoting ganglion cell survival following ischemic insult, as may occur in glaucoma and retinal disease.

Topically applied betaxolol attenuates NMDA-induced toxicity to ganglion cells and the effects of ischaemia to the retina

The present results show that topically applied Betoptic(R)(0.5% betaxolol) to the rabbit or rat eye reaches the retina and can counteract the detrimental effects caused by ischaemia/reperfusion or N -methyl- d -aspartate (NMDA)-induced insults to the retina. Betaxolol is a beta(1)-adrenergic blocker but its neuroprotective action is generally thought to be due to its calcium channel blocking properties. Support for this view comes from studies on cultures of cortical neurones where it was found that betaxolol attenuated the NMDA-induced influx of(45)Ca(2+)while beta-adrenoreceptor agonists were ineffective. Topically applied Betoptic(R)to the rabbit eye was observed to reach the retina in maximal amounts within 60 min. Some of the substance was also found in the contralateral retina of the untreated eye suggesting that the agent reaches the retina by local systemic and retinal circulation. Concurrent treatment with Latanoprost(R)did not result in a greater amount of betaxolol reaching the retina. An ophthalmodynamometric procedure, which raises the intraocular pressure, was used to apply an ischaemic insult to the rabbit retina. After three days of reperfusion the b-wave of the electroretinogram was reduced by an average of 59% and the choline acetyltransferase immunoreactivity in the retina was almost obliterated. However, when experiments were carried out on animals which had been treated with one drop of Betoptic(R) twice daily for 4 weeks before ischaemia and also during the reperfusion phase, the reductions in both the b-wave of the electroretinogram and retinal choline acetyltransferase immunoreactivity due to ischaemia/reperfusion were greatly attenuated. Intravitreal injection of NMDA into the rat eye caused a decrease in the immunostaining for Thy-1 antigen which is associated with ganglion cells. The Thy-1 mRNA level was also reduced as was the mRNA for the common subunit of the NMDA receptor, the NR1 subunit. However, in animals subjected to a topical Betoptic(R)regime, before and after intravitreal injection of NMDA, the decreases in the mRNA levels of Thy-1 and NR1 were significantly attenuated.

Betaxolol, a beta1-adrenoceptor antagonist, has an affinity for L-type Ca2+ channels

The effect of betaxolol on the specific binding of [3H]diltiazem and [3H]nitrendipine to rat cortical membranes was examined. Betaxolol inhibited specific [3H]diltiazem and [3H]nitrendipine binding with IC50 values of 19.7 and 46.3 microM, respectively. The effect of betaxolol on L-type Ca2+ channels showed little stereospecificity, since similar inhibitions of radioligand binding were observed with both racemic betaxolol and L-betaxolol. The dissociation kinetics of [3H]diltiazem were unaffected by 30 microM betaxolol, whereas it increased the [3H]nitrendipine dissociation rate, thus suggesting that betaxolol directly interacts with the benzothiazepine binding site and allosterically modulates the dihydropyridine binding site. Carteolol, propranolol and timolol were also found to inhibit both specific [3H]diltiazem and [3H]nitrendipine binding to rat cortical membranes, but with less potency than betaxolol. The ability of betaxolol to interact with L-type Ca2+ channels may have a role in its therapeutic effects in the management of systemic hypertension and in reducing neuronal death as occurring in glaucoma.

Systemic and ocular vascular roles of the antiglaucoma agents beta-adrenergic antagonists and Ca2+ entry blockers

This review addresses whether the antiglaucoma agents beta-adrenergic antagonists and Ca2+ entry blockers cause vasoactive effects in the retinal and other ocular vasculatures, as they do in other tissues. The potent vasodilating effects of Ca2+ entry blockers on ocular vessels have recently been demonstrated in in vivo and in vitro studies, implying that the maintenance of ocular vascular tone relies almost exclusively on extracellular Ca2+. Ca2+ entry blockers may potentially play a role in relaxing the retinal, long posterior ciliary, and ophthalmociliary arteries to improve the ocular circulation in vascular diseases in which there is considerable vascular tone present. The beta-adrenergic antagonists are discussed with reference to their antihypertensive role, their effect on other vascular beds, and finally what is known of their effect in the ocular vasculature. The emerging evidence that particular selective beta-adrenergic antagonists, such as betaxolol, are also potent Ca2+ channel entry blockers in other vascular beds is presented. Betaxolol has been shown to induce vasodilatation in the retinal and other ocular vascular beds, although studies have shown that beta1-adrenergic receptors are sparse in these vascular beds. This implies that an alternative mechanism must be responsible for betaxolol-induced vasodilatation. Evidence is presented that betaxolol vasodilates via its potent Ca2+ channel entry blocking properties, and its potency and ability to vasodilate are compared with those of nimodipine and timolol, as well as with those of other Ca2+ channel entry blockers. Important areas for future research in this area are discussed.

In vitro studies of the effects of beta-adrenergic drugs on retinal and posterior ciliary microarteries

The small-vessel myograph allows for precise measurements of physiopharmacologic responses of the ocular microarteries under controlled conditions. Studies using the myograph have shown that beta-adrenergic agonists are unable to induce significant relaxation in retinal and posterior ciliary microarteries, indicating that these microarteries have very few or no functional beta-adrenoceptors. Thus, beta-blockers would not be expected to have important adverse vasoconstrictory effects in the posterior part of the eye that are caused by their beta-adrenoceptor binding capacities. On the contrary, some beta-blockers, such as propranolol (the standard beta-blocker in pharmacology) and betaxolol (a beta-blocker used in ophthalmology), have vasorelaxant effects, probably a result of their Ca2+ channel-blocking activity. This activity shows no stereospecificity. Betaxolol could thus act as a vasodilator in glaucoma patients, on the condition that it penetrates in the posterior part of the eye after topical application. If so, it could also induce vasodilatation in circumstances of vascular endothelium injury, because this effect is endothelium-independent.

The potential of neuroprotection in glaucoma treatment

Visual field loss in glaucoma is due to death of retinal ganglion cells. Reducing or slowing down the loss of ganglion cells in glaucoma, a concept known as neuroprotection, would appear to be the only way forward. This does not imply that treatment of risk factors, such as elevated intraocular pressure, must not be continuously implemented. In this paper we point out that very little is known about the mechanisms of ganglion cell death in glaucoma and that data derived from studies on the "ideal animal model for glaucoma" must not be overemphasized. We also propose that the death processes of neurones in various diseases are fundamentally the same but vary in cause. Experimental data show that the death rate of neuronal populations is dependent on the impact of the insult and that neuroprotectants are more likely to benefit a patient in diseases in which the neurones die slowly, as in glaucoma, than in a disease in which the death of a set of neurones is rapid. We conclude that if a putative neuroprotectant can be administered in such a way that it reaches the retina in appropriate amounts and has insignificant side effects, it is likely to attenuate ganglion cell death and thus benefit the glaucoma patient.

Effects of vasodilatory beta-adrenoceptor antagonists on endothelium-derived nitric oxide release in rat kidney

The mechanisms for the vascular actions of vasodilatory beta-blockers remain undetermined. For some kinds of beta-blockers, the involvement of nitric oxide (NO) has been suggested. We studied the effects of vasodilatory beta-blockers on renal perfusion pressure (RPP) and NO release in the rat kidney. Infusion of bopindolol, celiprolol, and nebivolol caused a dose-dependent reduction in RPP and an increase in NO release (RPP: bopindolol 10(-6) mol/L, -23+/-2%; celiprolol 10(-4) mol/L, -27+/-2%; nebivolol 10(-5) mol/L, -35+/-3%; NO: bopindolol 10(-6) mol/L, +33+/-2; celiprolol 10(-4) mol/L, +41+/-2; nebivolol 10(-5) mol/L, +45+/-5 fmol. min-1. g kidney-1, mean+/-SEM). Metergoline (10(-6) mol/L), a 5-hydroxytryptamine (5-HT)1/2 antagonist, or NAN-190 (10(-6) mol/L), a 5-HT1A antagonist, almost completely abolished the vasorelaxation and NO release caused by bopindolol, celiprolol, and nebivolol. However, neither propranolol nor bisoprolol decreased RPP. Celiprolol and nebivolol caused vasodilation in the rat thoracic aorta, and it was markedly reduced by endothelial denudation, Nomega-nitro-L-arginine methyl ester (10(-4) mol/L), or NAN-190 (10(-6) mol/L). In deoxycorticosterone acetate-salt hypertensive rats, 4-week administration of celiprolol (50 mg. kg-1. d-1 IV) restored the responses regarding RPP and NO release to acetylcholine. These results suggest that several beta-blockers exert their vasodilatory action through the 5-HT1A receptor/NO pathway and that treatment with these beta-blockers may protect against endothelial injury in hypertension.

Ca2+ channel-blocking activity of propranolol and betaxolol in isolated bovine retinal microartery

The relaxant action of the standard beta-blocker propranolol was compared with betaxolol, a beta-blocker with established vasorelaxant properties. Ring segments of bovine retinal microartery (n=36, theta=237 microm), which lacks adrenergic nerves and beta-adrenoceptors, were mounted in an organ bath for isometric force recording. l-, d-, dl-Propranolol and betaxolol were equally effective in relaxing tonic K+-induced contractions. The median effective dose (ED50) value was approximately 10(-5) M for both beta-blockers. The relaxation by both beta-blockers was unaffected by endothelium removal. Like verapamil, both beta-blockers induced smaller relaxation of tonic prostaglandin F2alpha (PGF2alpha)-induced force, which depended less on Ca2+ influx than did K+-induced force: K+-, but not PGF2alpha-induced contractions were abolished in Ca2+-free medium. The minor betaxolol-induced relaxation of tonic PGF2alpha-induced force was blocked in Ca2+-free medium. With repeated exposures to PGF2alpha in Ca2+-free medium, initial phasic PGF2alpha-induced force declined less with every exposure than did subsequent tonic force. When the preparations were briefly equilibrated with K+- and Ca2+-rich solution before every exposure to PGF2alpha phasic force did not decline, indicating that phasic force primarily depended on Ca2+ released from intracellular stores. Both beta-blockers failed to relax phasic PGF2alpha-induced force. Thus propranolol and betaxolol are equipotent vasorelaxant drugs in retinal microartery, both probably acting via Ca2+ channel blockade. This activity (that shows no stereospecificity) thus appears to be a more general property of beta-blockers. Microarteries might be more sensitive to this activity than are conductance arteries.

Inhibitory action of insulin-sensitizing agents on calcium channels in smooth muscle cells from resistance arteries of guinea-pig

1. The actions of troglitazone, pioglitazone, metformin and bezafibrate, agents that improve insulin-resistance, on voltage-dependent Ca2+ channels in arterial smooth muscle cells were examined by use of the conventional and nystatin-perforated whole-cell clamp methods. Single cells were freshly isolated from resistance mesenteric arteries of guinea-pigs. The actions of these agents on 77 mM K+-induced contraction of the isolated arteries were also examined with the use of isometric tension recording. 2. The thiazolidinedione derivatives, troglitazone and pioglitazone, inhibited whole-cell Ca2+ currents in a dose-dependent manner with dissociation constants of 3.0 microM and 44.9 microM and Hill coefficients of 0.61 and 0.68, respectively. These two agents inhibited the 77 mM K+-induced contraction with similar potencies as those inhibiting the Ca2+ currents. Metformin and bezafibrate had no apparent effects on the Ca2+ current or high K+-induced contraction. 3. The inhibitory action of troglitazone on Ca2+ currents was not affected by the command potential, the holding potential, or the stimulation frequency, suggesting that its mode of the action differs from that of known organic Ca2+ channel antagonists. 4. The inhibitory action of troglitazone on Ca2+ currents was not affected by the addition of insulin to, or the removal of glucose from, the solutions. 5. In conclusion, the thiazolidinedione derivatives directly inhibited the voltage-dependent Ca2+ channels in a different manner from that of organic Ca2+ channel antagonists. This inhibitory action on Ca2+ channels was not a common feature of insulin-sensitizing agents.

Effects of CP-060S on membrane channels in vascular smooth muscle cells from guinea pig

The newly developed cardioprotective drug, CP-060S, (-)-(S)-2-[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]-3-[3-[N-methyl-N- [2-(3,4-methylenedioxyphenoxy) ethyl] amino] propyl]-1,3-thiazolidin-4-one hydrogen fumarate, is reported to possess a vasodilating action. Our objective was to examine the effects of CP-060S on the membrane channels in mesenteric arterial cells from guinea pigs, using whole-cell patch-clamp techniques. CP-060S inhibited the Ca2+ channel current in a concentration-dependent manner (ED50 = 1.7 microM at a holding potential of -80 mV and a stimulation frequency of 0.1 Hz). The inhibition was potentiated by a more depolarized holding potential and a higher stimulation frequency. These effects of CP-060S resembled those of diltiazem and gallopamil more than to those of nifedipine; the inhibition was more frequency dependent and less holding-potential dependent than with nifedipine. Higher concentrations of CP-060S also inhibited the delayed K+ channel currents (ED50 = 18 microM). The present observations suggest that CP-060S exhibits the profile of a Ca2+ channel antagonist, similar to that of diltiazem and gallopamil.

Stretch-activated whole-cell currents in smooth muscle cells from mesenteric resistance artery of guinea-pig

1. Stretch-activated (SA) channels were studied in smooth muscle cells isolated from mesenteric resistance arteries using the whole-cell patch-clamp method. Membrane stretch was achieved by cell inflation after application of positive pressure through a patch electrode. 2. In the voltage-clamp configuration, cell inflation increased and cell deflation decreased the membrane conductance. Conductance of the evoked current depended on the increase in cross-sectional area of the cell. The current-voltage relationship was linear between -80 and 0 mV, while further hyperpolarization showed a slight inward rectification. 3. The reversal potential of the SA current depended on the extracellular Na+ concentration, suggesting that the inward SA current was carried predominantly by Na+. The SA current was also carried by other cations, suggesting that the channel responsible for this current is a non-selective cation channel. The permeability sequence of cations as assessed by reversal potential was as follows: K+ > or = CS+ > or = Na+ > Li+. The channel was also permeable to Ca2+. 4. Extracellular Ca2+ and Gd3+ inhibited the SA current carried by monovalent cations in a concentration-dependent manner with IC50 (concentration giving 50% of maximal inhibition) values of 0.9 mM and 14 microM, respectively. 5. In the current-clamp configuration, membrane stretch depolarized the cell, and 100 microM Gd3+ inhibited the stretch-induced depolarization. 6. The results suggest that SA cation channels exist in arterial smooth muscle cells. Activation of the channels may modify membrane potential and intracellular ionic environment, and promote stretch-mediated cell responses.