Control mechanisms of retinal vein pressure: evaluation in the cat using an in situ infusion micropipette.

OBJECTIVE

Previous work in our laboratory has shown that retinal vein pressure is significantly greater than intraocular pressure in the cat. Our purpose here is to evaluate the potential of an active mechanism being responsible for the control of vein pressure in the cat retina.

METHODS

A double-lumen, concentric barrel micropipette assembly designed for operation within the eye of a spontaneously breathing anesthetized cat was developed. The micropipette, initially filled with hypertonic saline for use with a servonull pressure measuring system, was used to determine intravascular pressure. With the tip of the micropipette still situated in the lumen of the vessel the solution in the micropipette was exchanged for papaverine, a potent smooth muscle dilator. Following vasodilation in the distal portion of a major retinal vein, the papaverine was exchanged for hypertonic saline and intravascular pressures were measured in the dilated vessel.

RESULTS

Microinjection of papaverine caused visible dilation of the major retinal vein downstream from the site of microinjection. This indicates a relaxation of the smooth muscle yet papaverine caused no change in measured retinal vein pressure.

CONCLUSIONS

Vein pressure in the cat retina is maintained above intraocular pressure by mechanisms other than active control. A passive high-resistance segment resides somewhere within the optic nerve possibly within the lamina cribosa or prelaminar region.