A,kCetazolamide lowers intraocular pressure

Transcription

1 Ocular and systemic effects of acetazolamide in nephrectomized rabbits Zvi Friedman,* Theodore Krupin, and Bernard Becker The effects of acetazolamide on intraocular pressure (IOP) were studied on rabbits previously nephrectomized to eliminate the renal effects of the drug. Administration of acetazolamide ( mg/kg i.v.) reduced IOP from a baseline of.2 to 12.2 mm Hg 2 hr later. This dose was found not to alter arterial blood ph, pco 2, bicarbonate, or base excess. However, hr after drug administration anterior chamber aqueous humor showed significant reductions in bicarbonate, ph, and base excess, whereas aqueous humor ascorbate was significantly elevated. Administration of acetazolamide ( to mg/kg i.v.) to nephrectomized rabbits caused significant acidosis and pco 2 retention, presumably related to red blood cell carbonic anhydrase inhibition. IOP reduction at these higher doses was greater than that which followed the mg/kg administration. (INVEST OPHTHALMOL VIS Sci 23: , 1982.) Key words: intravenous acetazolamide, nephrectomy, rabbits, intraocular pressure, aqueous humor dynamics A,kCetazolamide lowers intraocular pressure (IOP) in normal and glaucomatous subjects 1 as well as in experimental animals. 2 * 3 Although the decrease in IOP is associated with a reduction in the rate of aqueous humor formation, 2 the mechanism of action of acetazolamide is still debatable. Acetazolamide is a potent carbonic anhydrase inhibitor. Since carbonic anhydrase is present in the ciliary epithelium, the direct inhibition of this enzyme is assumed to be the site of the ocular effect. However, carbonic anhydrase is also From *the Eye Department, Rothschild Hospital, Haifa, Israel, and the Department of Ophthalmology and the Oscar Johnson Institute, Washington University School of Medicine, St. Louis, Mo. This study was supported in part by research grants EY 0000 and EY 0192, and a Research Career Development Award EY (Dr. Krupin) from the National Eye Institute, Bethesda, Md. Submitted for publication April 9, Reprint requests: Theodore Krupin, M.D., Department of Ophthalmology, Washington University School of Medicine, 0 South Euclid Ave., St. Louis, Mo present in other tissues, including the red blood cell and the kidney. Renal carbonic anhydrase inhibition results in systemic metabolic acidosis, which may play a role in lowering IOP, since acidosis by itself decreases aqueous humor production and IOP. 3 ' > 9 We have studied the dose response of acetazolamide on IOP and on ocular and blood chemistries in rabbits nephrectomized to eliminate the renal effects of this agent. Materials and methods Albino rabbits weighing 2 to 3 kg were used. IOP was measured using a manometrically calibrated pneumotonograph and topical proparacaine hydrochloride (0.%) anesthesia. Arterial blood samples were collected in a heparinized syringe. Blood chemistries, including ph, pco 2, bicarbonate, total CO 2, and base excess were determined immediately after collection using a Corning Analyzer (Model No. /2). Plasma osmolality was measured with a vapor pressure osmometer (Model 100; Wescor Osmometer). Anterior chamber paracentesis was performed with similar topical anesthesia. Aqueous humor chemistries were determined on the Corning Analyzer. In addition, 01-00/82/ $00./ Assoc. for Res. in Vis. and Ophthal., Inc. 209 Downloaded From: on 11/23/2018

2 210 Friedman et al. Invest. Ophthalmol. Vis. Sci. August 1982 Table I. Effect of intravenous acetazolamide on IOP and blood chemistry in nephrectomized rabbits A Acetazolamide Parameter Dose (mg/kg) No. Baseline IOP (mm Hg) Blood ph Blood pco 2 (mm Hg) Blood bicarbonate (meq/l) Base excess (meq/l) A Bilateral nephrectomy was performed 18 hr before baseline measurements. Significant difference from baseline paired t test, p < 0.0. Significant difference from baseline paired t test, p < dt 0..2 dt db 0..1 dt dt dt dt dt dt dt dt dt dt dt dt dt dt dt ± ± 1. aqueous humor ascorbate was measured by microtitration. 7 Bilateral nephrectomy was performed with pentobarbital sodium (30 mg/kg i.v.). Eighteen hours later, one femoral artery was cannulated under local xylocaine hydrochloride (1%) anesthesia. The unanesthetized rabbit was placed in a box holder. Baseline IOP was obtained 1 hr later and an arterial blood sample was obtained. Acetazolamide was administered via the marginal ear vein in doses from 1 to mg/kg. IOP and arterial blood samples were remeasured, 30, 0, and 120 min later. In six rabbits baseline measurements were followed by anterior chamber paracentesis on one eye for aqueous humor chemistries. Acetazolamide ( mg/kg i.v.) was then administered, and IOP, arterial blood chemistries, and paracentesis of the remaining eye for chemistries were performed hr later. The paired t test was used for statistical analysis. Results Baseline IOPs and blood chemistries were within normal limits for the rabbit 18 hr after nephrectomy. The administration of acetazolamide (1 mg/kg) did not alter either IOP or arterial blood chemistries in four rabbits. The responses to higher doses of acetazolamide are shown in Table I. IOP was reduced after 2. mg/kg acetazolamide and was markedly lowered after higher doses. Percent decrease in IOP from baseline value 2 hr after acetazolamide administration demonstrated a dose-dependent reduction, which was maximal after the mg/kg dose (Fig. 1). Blood ph was not altered (p > 0.2) at any time after the 2. or mg/kg doses. Systemic acidosis occurred after larger doses of acetazolamide (Table I). Minor decreases in blood pco 2 occurred after 2. mg/kg, but no significant changes in blood pco 2 followed doses of or mg/kg. A dose of mg/kg caused a significant and persistent elevation of arterial pco 2. There was a tendency toward reduction of blood bicarbonate levels and base excess; however, changes were small and variable. Plasma osmolality was unchanged at all times after any dose of acetazolamide (p > 0.2). Anterior chamber aqueous humor chemistries were studied in six nephrectomized rabbits before and hr after administration of Downloaded From: on 11/23/2018

3 Volume 23 Number 2 Ocular and systemic effects of acetazolamide 211 Time after acetazolamide (min) : t 0. B 12.2 : t 0. c 12.0 : t 0.3 c 10.9 dt 0.3 c 7. dt dt dt 0.0 B 7.27 dt 0.03 c 23.9 : b 1.0" 28. ± ± ± dt dt dt d: d: d: d: d: ± ± 0.2 c 10.8 ± : t 0. c 7. dt db : b 0.0 B 7.2 : b 0.02 c 2.2 : b dt dt : b 1. C 18.7 db db 2..9 ± ± ± ± ± ± ± ± 0.8 c 10. ± 0.2 c 9. ± 0.3 c 7. ± ± ± 0.03 c 7.2 ± 0.01 c 22. ± 1.1 B 27. ± ± ± ± 1.8 C 22.8 ± 2.9 s 1. ± ± ± ± ± ± d: d: 0. c 10.8 d: 0.3 c 9. i: 0.3 c 7.2 : b : b : b 0.0 c 7.2: b 0.02 c 2. : b : b : b : b 1.2 B 17. : b 1.8 B 22. : b : b : b : b 2.2 B -1.8 : b : b 1. B -9.8 : b 1.2 acetazolamide ( mg/kg i.v.) (Table II). The time of hr was chosen to ensure anterior chamber steady-state conditions. This dose was selected because it lowered IOP significantly without altering blood chemistries (Table I). Four hours after drug administration IOP was significantly reduced, whereas blood ph, pco 2, base excess, and osmolality were unaltered (Table II). Blood bicarbonate was decreased slightly. Of great interest was the change in aqueous humor chemistries; anterior chamber aqueous humor ph, bicarbonate, and base excess were lowered significantly, whereas pco 2 was unchanged. Anterior chamber aqueous humor ascorbate was elevated significantly. Discussion Carbonic anhydrase catalyzes the reversible hydration and dehydration of CO 2 by the following reaction: CO 2 + H 2 O ^ H 2 CO 3 ^ H + + HCO 3 ". Carbonic anhydrase is present in many tissues, including the renal cortex, gastric mucosa, red blood cell, lung, pancreas, central nervous system, and eye. 8 Inhibition of the enzyme lowers IOP and also produces systemic acidosis. The mechanism by which acetazolamide lowers IOP is confused by the associated systemic acidosis, which is capable 3> > 9 by itself of lowering IOP. Nephrectomy avoids much of the systemic acidosis and fall in serum bicarbonate. However, nephrectomy does not eliminate the inhibition of red blood cell carbonic anhydrase, with resultant impairment of CO 2 elimination from the pulmonary capillary blood. Our results in nephrectomized rabbits show that acetazolamide in a dose of mg/kg can lower IOP without an accompanying alteration in arterial blood ph, bicarbonate, or pco 2. These findings are in contrast to those of Bietti et al., 10 who studied acetazolamide in normal rabbits. They report that a dose of 2. mg/kg decreases IOP as well as blood ph, bicarbonate, and base excess, and their conclusion is that the effect of acetazolamide on IOP is only due to the metabolic acidosis it produces. Our findings are consistent with those of Benedikt et al., u who show that IOP is lowered even when acetazolamide is given after the administration of sodium bicarbonate to avoid systemic acidosis. Higher doses of acetazolamide in our nephrectomized animals produced a systemic acidosis ( or mg/kg) and an increase in pco 2 ( mg/kg). These effects are presum- Downloaded From: on 11/23/2018

4 212 Friedman et at. Invest. Ophthalmol. Vis. Sci. August 1982 IJ a: q LxJ CO < a: o LU Q O cr UJ -0 < ACETAZOLAMIDE (mg/kg I.V.) Fig. 1. IOP response to intravenous acetazolamide in nephrectomized rabbits. Each point represents the mean percent decrease in IOP (±S.E.M.) for six to eight animals. Table II. Effect of acetazolamide ( mg/kg i.v.) on aqueous humor and blood chemistries (mean ± S.E.M.) in six nephrectomized rabbits A Parameter IOP (mm Hg) Arterial blood ph pco 2 (mm Hg) Bicarbonate (meq/l) Base excess (meq/l) Anterior chamber aqueous humor ph pco 2 Bicarbonate Base excess Ascorbate (mg/dl) Baseline 1.2 ± ± ± ± ± ± ± ± ± ± 1.9 hr after acetazolamide 10.0 ± 0.3 c 7.38 ± ± ± ± ± 0.0 c 27.3 ± ±3.1 C ± 3.8 C 2.2 ± 3. B A Bilateral nephrectomy was performed 18 hr before baseline measurements. B Significant difference from baseline paired t test, p < 0.0. c Significant difference from baseline paired t test, p < ably related to the acetazolamide effect on red blood cell and pulmonary carbonic anhydrase. The IOP reduction is greater with this associated addition of systemic acidosis (Fig. 1). Recent studies demonstrate that increasing pco 2 by primary respiratory methods elevate IOP. 12 This effect of acetazolamide after the mg/kg dose did not increase IOP in our animals. Rabbit aqueous humor has an excess bicarbonate when compared with blood levels. The decrease in anterior chamber aqueous humor bicarbonate after mg/kg acetazolamide is similar to that reported by Becker 7 Downloaded From: on 11/23/2018

5 Volume 23 Number 2 Ocular and systemic effects of acetazolamide 213 and Zimmerman et al. 13 Since this occurs in the absence of alteration of blood bicarbonate, it suggests a direct inhibition of ocular enzyme, with resulting decrease of aqueous humor bicarbonate, ph, and base excess. The increase in anterior chamber ascorbate is consistent with a decreased entry of water into the eye. 7 Our results clearly demonstrate that in nephrectomized rabbits, acetazolamide in a dose of mg/kg can lower IOP and decrease aqueous humor bicarbonate and ph in the face of unaltered blood acid-base balance. However, reduction in IOP after higher doses of acetazolamide in this animal model and after administration of the larger doses in animals and humans with normal renal function may well be a combined effect of carbonic anhydrase inhibition in the eye and systemic acidosis. Both of these actions can reduce aqueous humor production and together may result in a greater decrease of IOP (Fig. 1). In human subjects acetazolamide in a dose of 0 mg twice a day results in a greater decrease of IOP than that produced by methazolamide in doses of 2 or mg twice a day. 1 The increased effectiveness of acetazolamide is associated with a metabolic acidosis and significant decreases in plasma ph and bicarbonate. REFERENCES 1. Becker B: Decrease in intraocular pressure in man by a carbonic anhydrase inhibitor, Diamox. Am J Ophthalmol 37:13, Becker B: The mechanisms of the fall in intraocular pressure induced by the carbonic anhydrase inhibitor, Diamox. Am J Ophthalmol 39:178, Langham ME and Lee PM: Action of diamox and ammonium chloride on formation of aqueous humour. Br J Ophthalmol 1:, Maren TH: Carbonic anhydrase: chemistry, physiology and inhibition. Physiol Rev 7:9, Wistrand PJ: Carbonic anhydrase in the anterior uvea of the rabbit. Acta Physiol Scand 2:1, Krupin T, Oestrich CJ, Bass J, Podos SM, and Becker B: Acidosis, alkalosis, and aqueous humor dynamics in rabbits. INVEST OPHTHALMOL VIS SCI 1:997, Becker B: The effect of the carbonic anhydrase inhibitor, acetazolamide, on the composition of the aqueous humor. Am J Ophthalmol 0(part 2): 129, Ashby W: Carbonic anhydrase in mammalian tissue. J Biol Chem 1:21, Wistrand P and Maren TH: The effect of carbonic anhydrase inhibition on intraocular pressure of rabbits with different blood CO 2 equilibria. Am J Ophthalmol :291, Bietti G, Virno M, Pecori-Giraldi J, and Pellegrino N: Acetazolamide, metabolic acidosis and intraocular pressure. Am J Ophthalmol 80:30, Benedikt O, Zirm M, and Harnnoncourt K: Relation between metabolic acidosis and intraocular pressure after inhibition of carbonic anhydrase with acetazolamide. Albrecht von Graefes Arch Klin Exp Ophthalmol 190:27, Kielar RA, Teraslinna P, Kearney JT, and Barker D: Effect of changes in pco 2 and intraocular pressure. INVEST OPHTHALMOL VIS SCI 1:3, Zimmerman TJ, Garg LC, Vogh BP, and Maren TH: The effect of acetazolamide on the movements of anions into the posterior chamber of the dog eye. J Pharmacol Exp Ther 19:10, Stone RA, Zimmerman TJ, Shin DH, Becker B, and Kass MA: Low-dose methazolamide and intraocular pressure. Am J Ophthalmol 83:7, Downloaded From: on 11/23/2018