Adrenergic modulation of choroidal blood flow in the rabbit

Purpose. To determine whether the choroidal pressure-flow relationship is altered by the α-adrenergic antagonist, phentolamine, or the β-adrenergic antagonist, propranolol. Methods. In two groups of pentobarbital-anesthetized rabbits, the choroidal pressure-flow relationships were determined by raising the intraocular pressure (IOP) at mean arterial pressure (MAP) of 50, 60, 70, and 80 mm Hg before and after phentolamine (0.5 mg/kg, intravenously, n = 7) and propranolol (0.25 mg/kg, intravenously, n = 7) administration. Hydraulic occluders on the thoracic aorta and inferior vena cava were used to control MAP, which was measured in the central ear artery. The eye was cannulated with two 23-gauge needles, one to manipulate the ocular volume and the other to measure the IOP. Choroidal blood flow was measured by laser Doppler flowmetry with a probe positioned over the posterior retina. The protocol consisted of setting the MAP, then infusing saline into the eye at 30 μl/minute until the IOP increased from baseline to 100 mm Hg. Results. At the MAP of 70 mm Hg, α-adrenergic blockade caused an upward shift in the choroidal pressure-flow relationship; β-blockade shifted the relationship downward. Conclusions. In the pentobarbital-anesthetized rabbit, α- and β- adrenergic blockade cause choroidal vasodilation and vasoconstriction, respectively. Results indicate the presence of α- and β-adrenergic receptors in the rabbit choroid and a tonic level of adrenergic vascular tone in this preparation.