Active defence of retinal temperature during hypothermia of the eye in cats.

The first objective was to determine how large changes in externally applied cooling influence intraocular temperature. The second objective was to determine whether vasoactive changes in the choroidal circulation influence retinal temperature. Temperature in the conjunctiva was decreased down to 6 degrees C by direct cooling of the cat's eye and the temperatures along the visual axis were measured with thermocouple probes. Temperature in the retina was maintained fairly constant showing thus an active resistance to cooling of the eye, but temperatures in the anterior chamber and lens were dependent on external temperature changes. Administration of vasoconstrictor drugs in the retrobulbar space resulted in drop of retinal temperature presumably by reducing choroidal blood flow during eye cooling. The experiments showed that the eye is provided with vasoactive mechanisms that allow temperature preservation in critical areas, such as the retina, even in the presence of extreme temperature changes. However, other ocular regions show passive resistance to cooling of the eye.