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J. W. Kiel, A. Maldonado; Effects of Prazosin and Yohimbine on Aqueous Flow. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3706. doi: https://doi.org/.
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We previously reported that phentolamine, a non-selective alpha adrenergic antagonist, decreases IOP initially by lowering orbital venous pressure followed by a sustained decrease in aqueous production. In this follow-up study, we determined the effects of prazosin, an alpha-1 antagonist, and yohimbine, an alpha-2 antagonist, in the same rabbit model.
In two groups of pentobarbital anesthetized rabbits (n=10 per group, 2.3 ± 0.4 kg), mean arterial pressure (MAP), intraocular pressure (IOP), and orbital venous pressure (OVP) were measured by direct cannulation. Carotid blood flow was measured by transit time ultrasound (BFcar, Transonic Systems TS420), heart rate (HR) by a digital cardiotachometer, ciliary blood flow by laser Doppler flowmetry (BFcil, Perimed PF 5000), and aqueous flow by fluorophotometry (Flow, Ocumetrics FM2). The protocol entailed 60 min of baseline measurements, then an infusion of prazosin (1 µg/kg/min, iv) or yohimbine (10 µg/kg/min, iv) was started and the measurements continued for another 90 min.
Both drugs significantly decreased MAP, IOP and carotid resistance (Rcar). Also, prazosin significantly decreased Flow and yohimbine significantly decreased ciliary resistance (Rcil). No other significant effects were noted.
In contrast to phentolamine, neither drug altered OVP or caused a rapid fall in IOP. Prazosin decreased Flow consistent with the gradual decline in IOP. Yohimbine tended to decrease Flow, but its hypotensive effect is more likely due to an increase in outflow facility or uveoscleral outflow.
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