Vasopressin receptors in ocular tissues and their impact on ocular hydrodynamics

Purpose To investigate the effect of intravenous applied vasopressin on aqueous flow and to localize involved receptor types in ocular tissues. Methods In anesthetized rabbits mean arterial pressure (MAP), intraocular pressure (IOP) and orbital venous pressure (OVP) were measured by direct cannulation of the central ear artery, the vitreous, and the orbital venous sinus, respectively. Laser Doppler flowmetry was used to record CilBF continuously. Aqueous flow (AF) was measured simultaneously by fluorophotometry. After baseline measurements arginine-vasopressin (AVP) was applied intravenously (infusion rate: 0.08 ng/kg/min). Immunostaining (SABC-method, immunofluorescence) was performed to localize potential vasopressin receptors in ocular tissues. Results AVP caused a considerable increase of MAP (+6,90±1,21%; p < 0,001), a significant decrease of IOP (-9,56±2,35%; p = 0,003), a highly significant reduction of AF (-21.34±4.08%; p < 0,001) and no significant change of CilBF (-2.37±3.38%; n.s.). Immunofluorescent stainings show a characteristic pattern of V1- and V2-receptor distribution. Both, V1 and V2 receptors are detectable in choroidal, ciliary and iridal vessels. Moreover, they are localized in the ciliary epithelium, whereas the labeling is more intensive in the non-pigmented epithelium than in the pigmented epithelium. Conclusion Although the applied dose of AVP did not changed CilBF considerably, a highly significant reduction of AF was observed. This suggests that the reduction of AF is predominantly caused by affecting the secretory mechanisms in the ciliary epithelium. The localization of vasopressin receptors in ciliary epithelium supports this assumption.