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William J. Lavery, Jeffrey W. Kiel; Effects of Atropine and Timolol on Rabbit Episcleral Venous Pressure during Head-Down Tilt. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1986.
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In humans, cranial venous pressure increases when going from an upright to supine position due to the hydrostatic column effect, but usually there is only a small change in episcleral venous pressure (EVP) and IOP. This suggests a regulatory mechanism may be isolating EVP and IOP from the effect of gravity during posture change. Because the episcleral circulation is innervated by both parasympathetic and sympathetic nerves, the goal of the present study was to test the hypothesis that EVP during head-down tilt is under muscarinic cholinergic or beta adrenergic neural control.
In anesthetized rabbits (n=19), we measured mean arterial pressure (MAP), IOP, and orbital venous pressure (OVP) by direct cannulation; carotid blood flow (BFcar) by transit time ultrasound, heart rate (HR) by a digital cardiotachometer, and EVP with a servo-null micropressure system. Measurements were made initially with the animals in the supine position, and then with the animals tilted in a head down position after which the animals were given either topical atropine (0.5 mg/kg, iv bolus, n=8) or timolol (5 mg/ml, 50 microliters topical, n=11) while remaining in the head down position. The data (mean +/- standard error) were analyzed by repeated measures ANOVA.
In both groups, head-down tilt caused significant (p<0.05) increases in OVP (0.9 ± 0.1 to 3.1 ± 0.3 mmHg), EVP (10.4 ± 0.4 to 12.4 ± 0.5 mmHg) and IOP (13.8 ± 0.8 to 16.2 ± 0.8 mmHg) while MAP (72.3 ± 0.9 to 71.6 ± 1.0 mmHg), BFcar (30.5 ± 2.2 to 29.6 ± 1.8 ml/min) and HR (293 ± 6 to 290 ± 6 bpm) were not significantly different. Timolol cause significant decreases in BFcar (11% ± 3%), HR (11% ± 2%) and IOP (17% ± 5%) but did not alter MAP or EVP. Atropine did not alter any of the measured parameters.
Since neither atropine nor timolol affected EVP during head-down tilt, we conclude that EVP during posture change is not regulated by cholinergic or beta adrenergic neural control.
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