Paradoxical effect of phentolamine on aqueous flow in the rabbit

The aim of this study was to assess the effects of acute systemic, nonselective alpha-adrenergic blockade on aqueous flow.

This study used pentobarbital-anesthetized rabbits (n=7), in which the following parameters were measured: mean arterial pressure, carotid blood flow, heart rate, intraocular pressure (IOP), orbital venous pressure (OVP), ciliary blood flow, and aqueous flow (AqFlow). Measurements were made before and after an intravenous administration of phentolamine (0.1 mg/kg).

Phentolamine caused significant decreases in IOP -23%+/-2%; P<0.01), OVP (-28%+/-12%; P<0.05), and AqFlow (-33%+/-6%; P<0.01). The other parameters were not significantly altered. The rapidity of the OVP and IOP responses were noteworthy, being essentially complete 60 s after the phentolamine injection.

A subpressor dose of phentolamine has complex effects on ocular hydrodynamics. The initial IOP decrease is too fast to be explained by aqueous dynamics or ocular rigidity, and so is most likely a result of the disgorgement of choroidal blood volume caused by decreased venous pressure outside the eye. The more prolonged ocular hypotensive effect is explained by the decrease in AqFlow, and perhaps a decrease in episcleral venous pressure or increase in uveoscleral outflow. However, the inhibition of aqueous production is odd, as lost prejunctional inhibition of norepinephrine release and unopposed beta-receptor activation should have increased aqueous production.