S Panchal, A Mehta, D Santani
iop, loop diuretics, prostaglandins, torasamide
S Panchal, A Mehta, D Santani. Occulohypotensive Effect Of Torasamide In Experimental Glaucoma. The Internet Journal of Pharmacology. 2007 Volume 5 Number 2.
Glaucoma is a disease of eye that is characterized by an increase in intraocular pressure (IOP). We have studied the effect of torasamide (1%) on intraocular pressure (IOP) in experimentally induced acute and chronic models of glaucoma in rabbits. Some of the possibilities regarding mechanism of action involved were also studied.
Acute glaucoma was induced by intravenous administration of 5% dextrose. Pretreatment with topical torasemide (1%) prevented acute rise in IOP induced by intravenous administration of 5% dextrose infusion. Chronic glaucoma model was produced by injection of freshly prepared 50 unit of α-chymotrypsin in 0.1 ml of sterile saline in the posterior chamber of the eye. Torasemide (1%) (From 33.06±0.73 to 19.96±0.1 mmHg) and pilocarpine (1%) (From 30.13±0.40 to 20.8±0.04 mmHg) produced a significant fall in intraocular pressure in rabbits with α-chymotrypsin induced ocular hypertension. Pretreatment with indomethacin (1%) (A prostaglandin synthesis inhibitor) blocked the IOP lowering effect of torasemide (1%). Pretreatment with pilocarpine (1%) did not produce any significant change in IOP lowering action of torasemide (1%).
Our data suggest that torasemide showed oculohypotensive effect probably by enhancing aqueous humor outflow.
Glaucoma is a multifactorial disease with a number of elements contributing to its development. An elevation of intraocular pressure (IOP) is a prominent component in optic nerve damage, which is the hallmark of glaucoma. If the elevated IOP is inadequately treated, progressive blindness may result.
Various drugs used in treatment of glaucoma 1 are parasympathomimetics,β-adrenoceptor blockers, carbonic anhydrase inhibitors, α2-adrenoceptor agonists, prostaglandin analogues and angiotensin converting enzyme inhibitors 2 (ACE inhibitors). Timolol eye drops are a golden standard in the treatment of glaucoma. However, timolol is known to get into systemic circulation and causes various systemic effects. 3 Although glaucoma is known to be a serious chronic eye disease, an ideal agent to be used in this disease is still not available and there has been a constant urge for the discovery of newer drugs.
Tingey et al. (1992) observed that topical application of a loop diuretic, ethacrynic acid in the eyes of rabbits and monkeys, reduced intraocular pressure. 4 Neumann et al. (1992) reported that a single intracameral injection of ethacrynic acid reduced IOP in patients with glaucoma. 5
In the present study, we investigated the effect of torasemide on IOP in rabbits and possible mechanism of action of this agent.
Materials And Methods
New Zealand white rabbits of either sex weighing 1.5 to 2.5 kg, (Zydus Research Center, Ahmedabad, India) housed under well controlled conditions of temperature (22±2°C), humidity (55±5%) and 12/12-h light dark cycle were given access to food and water ad libitum. The protocol of the experiment was approved by the Institutional Animal Ethical Committee as per the guidance of the Committee for the Purpose of Control and Supervision of Experiments on Animals (CPCSEA), Ministry of Social Justice and Empowerment, Government of India.
Acute glaucoma model
Rabbits weighing 1.5 to 2.5 kg were used for the study. The basal intraocular pressure was measured by tonometer (Schiotz type indentation tonometer) and the drug solutions prepared in suitable solvents were instilled topically into the left eye. The drug solutions used in the study were torasemide (1% in PEG 400) and pilocarpine (1%, FDC Ltd., India). The right eye received the vehicle, which served as control. After 15 minutes of drug administration, 5% dextrose solution (15 ml/kg) was intravenously infused through marginal ear vein. The Intra ocular pressure (IOP) changes were recorded every 15 minutes till the pressure became normal.
Chronic glaucoma model
Rabbits weighing of 1.5 to 2.5 kg (either sex) were sedated with diazepam (1 mg/kg i.v.) and anaesthetized with ketamine (25 mg/kg i.v.). Freshly prepared α-chymotrypsin (50 units) solution prepared in 0.1 ml of sterile saline was irrigated through the cannula into the posterior chamber. The debris of tissue blocked the pathway of aqueous humor outflow in the trabecular meshwork to induce ocular hypertension. 6 Rabbits with glaucoma rabbits were selected and used for determining the effect of drugs on IOP. After achieving a steady elevated IOP, similar drug solutions as in acute glaucoma model were administered topically into the left eye whereas, right eye served as control. The IOPs were measured at time zero (just before eye drop instillation) and suitable time intervals.
Studies on interaction of indomethacin and pilocarpine with torasemide in chronic glaucoma model in rabbits.
Rabbits with glaucoma induced by α-chymotrypsin were selected for the study. The intraocular pressure was initially recorded with the help of tonometer. Indomethacin (1%, Sterfil Laboratories Pvt Ltd., India) prostaglandin inhibitor was topically administered to the left eye. Right eye served as control. After 45 minutes of administration of indomethacin, torasemide (1%) was instilled topically. The changes in IOPs were recorded at suitable time interval using tonometer. Indomethacin was replaced by pilocarpine to study the interaction of pilocarpine with torasemide.
Torasemide solution was first tested in rabbit's eyes for ocular safety. No ill effects were observed.
Paired student t-test was employed for determining the statistical significance of most of the data at the probability level of 95%. A split-plot ANOVA analysis was carried out for studying the time dependent interaction between the drugs under study and other drugs.
An acute elevation in intraocular pressure (IOP) up to 35-40 mmHg was observed when 5% dextrose (15ml/kg) was administered intravenously. Pretreatment with torasemide (1%) and pilocarpine (1%) prevented this rise in IOP (From 32.13 ± 0.35 to 22.53 ± 0.14 mmHg and from 28.07 ± 0.62 to 17.10 ± 0.70 mmHg respectively) (fig. 1).
In α-chymotrypsin induced glaucoma, topical administration of torasemide and pilocarpine produced a significant fall in IOP (From 33.06 ± 0.73 to 19.96 ± 0.1 mmHg and from 33.07 ± 0.04 to 20.80 ± 0.02 respectively) (fig. 2). In case of interaction of indomethacin with torasemide, indomethacin (1%) has reversed IOP lowering effect of torasemide (fig. 3) but pretreatment with pilocarpine followed by torasemide did not further lower IOP in chronic glaucoma.
Diuretics like ethacrynic acid 4 , acetazolamide 7 and dorzalamide 8 and are well known for their IOP lowering effect. Torasemide belongs to the same class. We have studied the oculohypotensive action of a loop diuretic, torasemide in experimentally induced acute and chronic models of glaucoma in rabbits. Oral water loading and a more advantageous alternative 5% glucose infusion is one of the easiest, fastest and reliable techniques to screen antiglaucoma agents 9 . Oral water loading or 5% glucose infusion leads to reduction in blood osmolality, which leads to transfer of water into the eye causing elevation of IOP. In our study, torasemide and pilocarpine prevented the acute rise in IOP due to 5% glucose infusion. Chronic and stable elevation of IOP was achieved by introducing α-chymotrypsin into the posterior chamber of the rabbit eye. The sustained increase in IOP is caused by an inflammatory reaction in the trabecular meshwork 10 . Both pilocarpine and torasemide produced significant reduction in IOP in α-chymotrypsin induced chronic glaucoma model.
Prostaglandin analogs are known to reduce IOP by increasing the uveoscleral outflow. 11 Pilocarpine reduces uveoscleral outflow by providing a stretching of the scleral spur through which PGF2α reduces IOP. 12 To explore IOP lowering mechanism of torasemide, we have studied interaction of indomethacin and pilocarpine with torasemide. Pilocarpine did not alter IOP lowering effect of torasemide. We studied the indomethacin-torasemide interaction in chronic glaucoma model in rabbits. Pretreatment with prostaglandin synthesis inhibitor indomethacin, reversed the IOP lowering effect of torasemide in the concentration used in the present study. The natriuretic effect of torasemide is reported to be partially inhibited by the concomitant administration of indomethacin. 13 Prostaglandins produce oculohypotensive action by increasing uveoscleral outflow, which may be due to enzymatic lyses of the connective tissue of ciliary muscle 14 . This suggests that one of the possible mechanisms of torasemide may be through improvement in uveoscleral outflow.
In conclusion, our data suggest that torasemide possesses oculohypotensive effect probably by enhancing aqueous humor outflow.
We thank Cipla Ltd., India, for gifting sample of torasemide and animals for this research work.
Shital J Panchal, Ph. D. Assistant Professor, Department of pharmacology, K. B. Raval College of Pharmacy, At & Po: Shertha, Kasturinagar. Dist: Gandhinager-382423, Gujarat, India. E-mail: firstname.lastname@example.org