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J. W. Kiel, A. Maldonado, D. O. Zamora; Effect of Verapamil on Ciliary Blood Flow and Aqueous Production in Rabbits. Invest. Ophthalmol. Vis. Sci. 2010;51(13):160.
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We have previously found that aqueous production is stable when ciliary blood flow is increased by raising arterial pressure. Under those conditions, neural or local vascular control mechanisms may have protected the ciliary epithelium from over-perfusion. Verapamil is a vasodilator that works by blocking calcium influx thereby overriding neural and local control mechanisms. This study sought to determine whether verapamil-induced vasodilation would increase ciliary blood flow and cause an increase in aqueous production.
In anesthetized rabbits (n=10), 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 verapamil (10 mg/ml, 100 µl). Measurements were then continued for another 135 min. Aqueous flow was calculated with Brubaker’s equation at hourly intervals, as were the mean values for the other measured variables. The data were analyzed by repeated measures ANOVA and Dunnett’s multiple comparisons test (Graphpad Prism).
Blood pressure, carotid blood flow, heart rate and orbital venous pressure were unchanged. During the first hour after verapamil, IOP, aqueous flow and ciliary blood flow were also unchanged. However, during the second hour after verapamil, IOP decreased (p<0.01) from 13.4 ±0.9 mmHg to 10.7 ± 0.7 mmHg, aqueous flow decreased (p<0.05) from 2.66 ± 0.37 µl/mmin to 1.87 ± 0.3 µl/min and ciliary blood flow increased (p<0.05) from 52 ± 5 P.U. to 62 ± 8 P.U.
Topical verapamil decreases aqueous production and increases ciliary blood flow. This uncoupling of perfusion from metabolic activity indicates a direct inhibitory effect of calcium channel blockade on ciliary ionic transport.
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