Timolol decreases aqueous humor flow but not Na+ movement from plasma to aqueous.

PURPOSE: To determine whether the well-known effect of timolol in reducing ocular pressure and aqueous humor (AH) flow is a function of reduced Na+ movement from plasma to aqueous. Previously, the authors have shown this to be the case for carbonic anhydrase inhibitors. METHODS: The rate of appearance of 22Na in rabbit posterior aqueous was measured 1 to 3 minutes after the intravenous injection (time T) of the isotope. One hour before this, the animals received one of the following: two drops of 0.5% timolol, two drops of 3.5% pilocarpine, or 25 mg/kg intravenous methazolamide. At 1 minute (T + 1), a posterior chamber sample was taken; 2 minutes later (T + 3) a second sample was removed from the fellow eye. The rate constant of sodium accession is simply the difference between the two counts/2 minutes. Aqueous flow was measured by dilution of sulfacetamide marker as described previously. RESULTS: The rate constant (k(in)) for sodium entering the posterior chamber was 0.036 +/- 0.004 minute-1 (n = 17). Corresponding to previous findings, methazolamide (25 mg/kg intravenous) reduced this to 0.023 +/- 0.003 minute-1 (n = 14). Conversely, timolol (two drops of 0.5% solution) had no effect on kin, which measured 0.037 +/- 0.004 minute-1 (n = 12). Similarly, as expected, pilocarpine had no effect on k(in) (0.035 +/- 0.003 minute-1). Control flow was 3.9 microliters/minute +/- 0.4; after timolol, 2.5 microliters/minute +/-0.1; after methazolamide, 2.4 microliters/minute +/-0.2; after pilocarpine, 3.6 microliters/minute +/- 0.2. These are converted to rate constants by dividing by volume of posterior aqueous (60 microliters). The control rate constant for fluid entry was 0.065 minute-1, 1.8-fold higher than for sodium. CONCLUSIONS: A central dogma of the formation of AH (and cerebrospinal fluid) is that fluid moves isotonically from plasma to AH or cerebrospinal fluid and, therefore, that rate constant k(in) for fluid and for sodium are approximately the same. In the authors' hands, the fluid constant was modestly higher than for sodium. This holds for normal function and also for the reduced k(in) for fluid and sodium after carbonic anhydrase inhibition. The k(in) for neither flow nor sodium was affected by pilocarpine. Surprisingly, however timolol, which reduces flow, had no effect on Na+ entry.

Renal and cerebrospinal fluid formation pharmacology of a high molecular weight carbonic anhydrase inhibitor.

To achieve selective inhibition of cytosolic and membrane-bound carbonic anhydrase (CA II and CA IV, respectively), we synthesized a polymer of molecular weight 3500 from polyoxyethylene bis acetic acid and aminobenzolamide. The new compound, designated F (for Florida) 3500, is stable, water soluble and nontoxic. It is excreted largely unchanged by glomerular filtration. The Ki at 37 degrees C against CA II is 0.14 microM and against CA IV 4.0 microM, some 20 times more than the parent aminobenzolamide and about three times more than acetazolamide or methazolamide. F 3500 does not penetrate red cells and is confined to extracellular fluid. Relations of dose to renal HCO3-excretion was studied by i.v. injection into rats. Peak effect was reached with 100 mg/kg, eliciting 40 mM HCO3- in urine; this is taken to be the effect of inhibiting CA IV. This compares to a peak of 103 mM HCO3- after 3 mg/kg aminobenzolamide, which agrees with previous data on other low molecular weight sulfonamides and defines the effect on CA II and CA IV. We conclude that both isozymes are necessary for normal full renal reabsorption of HCO3-. We studied the effect of perfusing F 3500 (100-4000 microM) through the ventriculo-cisternal system of rats on cerebrospinal fluid (CSF) formation. F 3500 was also given intravenously at 100 mg/kg. CSF formation was unaffected. Low MW sulfonamides by either route lowered CSF formation 30 to 50%. It appears either that membrane-bound enzyme (CA IV) is not accessible from the CSF or blood side or that it plays little or no role in CSF formation.

Effects of inhibition of angiotensin converting enzyme and carbonic anhydrase on fluid production by ciliary process, choroid plexus, and pancreas.

Inhibitors of angiotensin converting enzyme (ACE) (1) and of carbonic anhydrase (CA) (2,3) decrease intraocular pressure (IOP) in conscious rabbits. We asked whether ACE inhibition decreases IOP through effects on CA-dependent flow of aqueous humor (AH) and whether ACE inhibitors decrease other CA-dependent secretions. We show in anesthetized rabbits (a) that topical inhibitors of ACE decrease both IOP and AH flow as much as systemic inhibitors of CA; (b) that the maximal effects of ACE and CA inhibition are not additive, therefore these treatments may affect one or more components of a single system for fluid production; and that (c) ACE inhibitors do not work through inhibition of CA. Looking at other fluid production systems, we find (d) that cerebrospinal fluid (CSF) production is increased after ventriculocisternal perfusion with a potent ACE inhibitor and (e) that flow of pancreatic juice (PJ) is increased after systemic ACE inhibition.

Measurement of aqueous humor flow following scleral injection of sulfacetamide as marker: effect of methazolamide, timolol, and pilocarpine.

A simple technique for rapid determination of the flow of aqueous humor (AH) in rabbits is described. Data from control eyes and eyes treated with known inhibitors of aqueous humor production are presented as evidence of the suitability of the technique for comparing the influence of other drugs on aqueous flow. The flow measurement depends on marker dilution by fluid turnover during the first 30 min after 5 microliters marker solution is delivered to the anterior chamber. This is done by passing a needle through the sclera and behind the iris. The advantage of this route is that it permits withdrawal of the needle after delivery without leakage of marker or aqueous humor. Sulfacetamide (SAC), a sulfonamide with low lipid solubility and pKa 5.4, is the marker of flow used here. Control flow data from SAC, fluorescein (FL), reactive blue-2, and 14C-inulin as markers were compared. The control flow measured with SAC in two independent groups is 3.91 and 3.40 microliters/min or 3.69 +/- 0.37 S.E. overall. The data show that inhibition of carbonic anhydrase (CA) in ciliary processes by systemic methazolamide reduces flow 38%. Topical timolol reduces flow 35%. Pilocarpine, which is known to decrease IOP largely by increasing outflow facility, did not significantly reduce flow. This system for measuring AH flow allows rapid accumulation of data for topical, intracameral or systemic treatments.

Effect of AlCl3 and other acids on cerebrospinal fluid production: a correction.

Our earlier report that perfusion of Al and Ga salts through rat brain ventricles abolished cerebrospinal fluid (CSF) production is incorrect. We now realize there are three conditions which, when simultaneously present, will give a false reading of the events taking place. These are: metal salts, low pH with subsequent rise in pH during perfusion and use of blue dextran as the marker. When we perfused 10 mM AlCl3 or GaCl3 in vivo, all three conditions were provided, and led to the misinterpretation that all CSF production had been eliminated. In doing experiments that prevented the subsequent rise in pH during perfusion, or in using [14C]dextran as the marker in vivo, we found that Al and Ga (acting as Lewis acids at pH 4.7 and below), and also hydrochloric, phosphoric and acetic acids of the same pH, reduce CSF production by approximately 33%. We attribute this to the lowering of pH eliminating the uncatalyzed reaction OH- + CO2----HCO3- in secretory cells of choroid plexus. Acetazolamide blocks the HCO3- formation catalyzed by carbonic anhydrase, and reduces CSF production 42%. These mechanisms are additive in that, when acetazolamide and either Al or acetic acid are given together, there is a total reduction of 64%. Therefore, at least 64% of CSF production is dependent on formation of HCO3-. The mechanisms accounting for the balance of CSF production are not yet defined clearly. A significant portion of this production is expected to relate to transport of Cl-.

Aluminum and gallium arrest formation of cerebrospinal fluid by the mechanism of OH- depletion.

AlCl3 or GaCl3 was added to artificial cerebrospinal fluid and perfused through the cerebral ventricles of the rat. Depending on the metal and its concentration (1-10 mM) the pH of the perfusate ranged from 7.2 to 3.5. At 10 mM metal chloride, yielding pH 4.7 (Al) or 3.5 (Ga), formation of cerebrospinal fluid was suppressed 100%. The effect was reversed as soon as control cerebrospinal fluid (pH 7.35) was introduced. There was no effect at pH greater than 6 in the presence of metal ions nor was the effect mimicked by cerebrospinal fluid acidified with HCl or phosphate buffer to pH 4.7. HCl and phosphate at pH 4.7 had partial effects (25-30%). Inhibition of carbonic anhydrase is well known to have a partial effect (approximately 50%), the remainder of normal flow being dependent on the uncatalyzed formation of HCO3- and the corresponding movement of Na+. The complete cessation of flow after exposure to Al or Ga ions appears to occur when these ions are perfused at pH where they are partly hydrolyzed. Under these circumstances they form very powerful proton generating systems (as shown by titration data), which could lower [OH-] at the secretory border of choroid plexus cells. As a result, both the catalyzed and the uncatalyzed processes for formation of HCO3- from CO2 are abolished and secretion stops. This mechanism may also account for the antiperspirant action of Al salts. Using metal ion hydrolysis to probe other secretory systems will also be of interest.

Timolol plus acetazolamide: effect on formation of cerebrospinal fluid in cats and rats.

Administration iv of 50 mg X kg-1 acetazolamide (A) and 3 mg X kg-1 timolol (T) causes the formation of cerebrospinal fluid (f-CSF) to be reduced to 43.7% of the control rate compared with a reduction to 82.5% of control by T alone and to 52.6% of control by A alone. The effect of combined drugs is the same when A is combined with T initially, when A is added to T after studying T alone, or when T is added to A after studying A alone. In contrast, in rats f-CSF is not influenced by T, either alone or when in combination with A. The rate in rats is reduced to 55% of control by treatments with A or A and T. Decrease in formation of cerebrospinal fluid by A occurs through inhibition of carbonic anhydrase, but the means whereby T (a known blocker of beta-adrenergic receptors) causes a reduction in f-CSF is not established; it is known that it does not inhibit carbonic anhydrase. Control of f-CSF by the sympathetic nervous system is discussed.

Addition of the effects of norepinephrine and acetazolamide to decrease formation of cerebrospinal fluid.

In a system for ventriculocisternal perfusion of the choroid plexus, the rate of formation of new cerebrospinal fluid was measured by changes in dilution of an impermeant dye in the perfusate. Norepinephrine added to the perfusate decreased formation of cerebrospinal fluid in rats as was previously demonstrated in rabbits. The dose-response relationship for rats was determined. The formation rate was decreased 42% by 10(-3) M norepinephrine. Acetazolamide, 50 mg/kg i.v., caused a decrease of 46%. Given together, these drugs decreased formation 79%, demonstrating essentially full addition between the regulatory mechanisms involved. Addition of equal magnitude occurred when intraventricular nialamide, an inhibitor of monoamine oxidase, and i.v. acetazolamide were given together. This demonstrates addition between acetazolamide and endogenous norepinephrine (or other catecholamines present) in which metabolic breakdown is prevented by the inhibitor. The degree of reduction in cerebrospinal fluid formation seen in these experiments exceeds that reported for numerous other trials of single drugs.

Cerebrospinal fluid formation in rats and cats during treatment with timolol.

The influence of timolol upon cerebrospinal fluid formation rate has been examined in rats by the measurement of 22Na+ entry into this fluid after 10, 100, or 1000 micrograms x kg-1 i.v. and in cats by the dye-dilution measurement of new fluid formation after 30, or 3000 microgram x kg-1 i.v., or 250 micrograms x mL-1 in ventricular perfusate. In rats no change from control rates occurred. In the cats there appeared to be no effect of intraventricular timolol; however, a significant decrease of approximately 25% in the mean flow rate was seen after 40 min when drug was given i.v. at either dose level. A time study showed that no further decrease occurred within 5 h and that the observed decrease continued for at least 3 h. These findings are of interest in view of the ability of topical, intraocular, and i.v. timolol to reduce aqueous humor formation rate.