Interaction between head-down tilt and anterior chamber infusions on intraocular pressure of anesthetized rats.

Head-down tilt or infusions of a balanced salt solution into the anterior chamber of the eye raise intraocular pressure. We measured intraocular pressure directly in adult male Sprague-Dawley rats, anesthetized with pentobarbital, and subjected to 45 degrees head-down tilt alone, tilt with an anterior chamber infusion (0.087 microliter min-1), or tilt with an infusion containing arginine vasopressin. The intraocular pressure of the three groups differed during the 1 hr tilt and recovery periods. In the case of tilt alone, intraocular pressure quickly reached a peak after tilting, partially decreased during the tilt period, recovered to baseline immediately after tilt, then a secondary rise occurred. Combined infusion and tilt caused a slower rise to peak intraocular pressure, and only a partial recovery occurred during the 1 hr recovery period. Combined vasopressin infusion and tilt caused a gradual rise in intraocular pressure of a lesser magnitude than the other groups, followed by a rapid recovery to baseline pressure and no secondary rise. Systemic arterial pressure was stable within and between the groups. The underlying mechanism for these differing response patterns is unknown. However, some evidence indicates that infusions are independent of aqueous synthesis rate, and that vasopressin, acting on a V1 receptor subtype reached from the anterior chamber, exerts a vascular effect.