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W. W. OPPELT; Measurement of Aqueous Humor Formation Rates by Posterior-Anterior Chamber Perfusion with Inulin: Normal Values and the Effect of Carbonic Anhydrase Inhibition. Invest. Ophthalmol. Vis. Sci. 1967;6(1):76-83.
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Aqueous humor (AH) formation rates in anesthetized cats were measured by continuous posterior-anterior chamber perfusion with an inulin-containing, AH-like buffer. AH formation rates were calculated, with the knowledge of inulin concentration in inflow, outflow, and rate of infusion. They averaged about 14 µL per minute, which represents a turnover rate of about 1.4 per cent per minute. Insignificant inulin left the system either by diffusion into blood, or by uptake by ocular tissues. Moderate elevations of outflow protein concentrations occurred; however, there was no correlation between protein concentrations and measured AH formation rates, unless protein concentrations rose above 600 mg. per cent. AH formation rates were stable over a six hour experimental period. There was adequate mixing of the isotope in the anterior chamber fluids, as measured by changing the position of the outflow needle during the experiment. Acetazolamide, 0.1 to 20 mg. per kilogram, was given in a single intravenous injection after a three hour control period. Maximum reduction in AH formation rates of about 45 per cent occurred when either 10 or 20 mg. per kilogram acetazolamide was used, while smaller decreases were noted with smaller doses. There was no physiologically significant effect at the lowest dose. It is concluded that posterior-anterior chamber perfusion with inulincontaining buffer is a valid, direct method of measuring AH formation rates which allows one to use each eye as its own control, and which can be used to measure AH formation rates after a great variety of pharmacological or physiological manipulations. The carbonic anhydrase inhibitor, acetazolamide, at doses which lower intraocular pressures (IOP), acts by directly reducing AH formation rates.
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