Isolated preparations of ocular vasculature and their applications in ophthalmic research

The purpose of this review is to outline the techniques and applications for isolated ocular vascular preparations and their significance to ophthalmic research. Various isolated ocular vascular preparations have been utilized in studies of ocular vascular biology, physiology and pharmacology, including work in both normal and diseased conditions. However, there is still significant potential for further studies to improve our understanding of the ocular circulation and its regulation. Experience has shown that there is no single preparation capable of addressing all of the questions that must be answered if a complete understanding of mechanisms of vascular regulation in the eye is to be achieved. Rather, it is necessary to select the appropriate preparation and techniques to address each individual question in the most appropriate manner. In this review, particular emphasis is placed on the applications for isolated ocular preparations and the relevance of such studies to our understanding of the pathogenesis of eye diseases involving the vasculature. Examples are given where therapeutic approaches in diabetes and glaucoma are assessed in terms of their impact on the vasoactive properties of the ocular vasculature.A significant heterogeneity is present in the different parts of the ocular vasculature, not only in the structural and functional properties of vessel itself, but also in terms of the tissue environment and innervation. A single vasoactive agent may also have different effects when applied to the inside or the outside of the same region of a vessel. The vasoactive response of the vascular system as a whole is what determines the rate of blood flow through the system, but this is regulated by a multitude of factors in different regions of the vascular network. Isolating individual components of the ocular vasculature is readily achievable for the extraocular vessels such as the ophthalmic or ophthalmocilliary arteries, which can be studied in myograph type systems measuring the mechanical vasoactive force generated by the vessel. Retinal vessels from very large animals can also be studied in this way, but the small diameter of the retinal vessels in most species requires a perfusion rather than myograph based technique. Perfusion based studies of vessel diameter in response to vasoactive stimuli can be applied to individual retinal arteries and their branches. Perfusion of more complex elements of the ocular vasculature such as isolated segments of the retina or ciliary body, or whole isolated perfused eyes may use the perfusate pressure as the determinant of vasoactive state. However, when several components of the ocular vasculature are being perfused simultaneously it may be difficult to separate out the contribution from the different vascular elements. The advantage of isolated preparations is that systemic influences can be eliminated, and vascular components can be studied that are inaccessible in vivo. The disadvantage is that no matter how well controlled the in vitro environment may be, it will always be a relatively poor mimic of the in vivo conditions. However, such in vitro work has certainly improved our understanding of the vasoactive properties of different regions of the ocular vasculature in both health and disease.