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J. W. Kiel, D. O. Zamora; Aqueous Flow Is Unchanged in Response to an Ocular Hypertensive Dose of Sodium Nitroprusside. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2032.
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Nitric oxide donors like sodium nitroprusside (SNP) are potent vasodilators; however, their effect on intraocular pressure (IOP) is controversial, with some studies finding a hypertensive effect, and others reporting a hypotensive effect. In our rabbit model, topical application of SNP rapidly increases episcleral venous pressure (EVP) and IOP, but the increase in IOP is greater than the EVP response would suggest based on the Goldmann equation. Because the nitric oxide synthase inhibitor, L-NAME, decreases aqueous production, we hypothesized that SNP would increase aqueous production and perhaps account for this observed discrepancy between EVP and IOP.
In anesthetized rabbits (n=13), we measured arterial pressure, IOP, and orbital venous pressure by direct cannulation; carotid blood flow by transit time ultrasound, heart rate by a digital cardiotachometer, ciliary blood flow by laser Doppler flowmetry, and aqueous flow by fluorophotometry. The protocol entailed 60 min of baseline measurements, followed by topical application of SNP (10 mg/ml, 100 µl) to the eye. Measurements were then recorded for another 90 min. The data were recorded and analyzed using ADInstument’s Chart Software and Graphpad Prism.
IOP increased (p<0.01) from 16.6 ±0.7 mmHg to 20.0 ± 0.9 mmHg, but aqueous flow was unchanged (2.65 ± 0.31 µl/min versus 2.99 ± 0.31 µl/min) before and after SNP application. Blood pressure, carotid blood flow, heart rate and orbital venous pressure were also unchanged.
Topical SNP does not increase aqueous production and so does not account for the larger increase in IOP than EVP. Alternative candidates are decreases in outflow facility or uveoscleral outflow, or some combination of both.
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