The retina receives its nutrients from two separate circulations: retinal and choroidal circulation. This short overview describes the determinants in the regulation of these circulations. Retinal circulation is characterized by a low blood flow while flow in the choroid is high. The choroidal circulation is mainly controlled by sympathetic innervation and is not autoregulated. Retinal circulation lacks autonomic innervation, shows an efficient autoregulation and is mainly influenced by local factors. Local mediators released by endothelial cells and surrounding retinal tissue also have a substantial role in the regulation of retinal circulation.

Abstract-The present study provides evidence that retinal tissue may profoundly influence the retinal arterial smooth muscle cell tone by releasing an unknown retinal relaxing factor. Isolated bovine retinal arteries with and without adhering retinal tissue were mounted in a wire myograph for isometric tension recordings. The maximal contraction induced by prostaglandin F 2␣ was 0.95Ϯ0.7 mN (nϭ6) in the presence and 5.15Ϯ0.76 mN (nϭ6) in the absence of adhering retinal tissue. The contractions induced by U-46619, serotonin, and endothelin-1 were similarly blocked in the presence of retinal tissue. The K ϩ 120 mmol/L-induced contraction was not significantly affected (2.8Ϯ0.7 mN, nϭ6, in the presence and 3.6Ϯ0.7 mN, nϭ6, in the absence of retinal tissue). Placing a piece of bovine retinal tissue in the proximity of a contracted (ie, with prostaglandin F 2␣ ) retinal artery induced a complete relaxation of the retinal vessel, suggesting the involvement of a diffusible chemical vasorelaxant. Also porcine, canine, and ovine retinal tissue completely relaxed the contracted (with prostaglandin F 2␣ ) bovine retinal artery. Other smooth muscle preparations, including rat mesenteric and renal arteries and rat main bronchi, also relaxed with the application of a piece of bovine retinal tissue. Incubation of bovine retinas in a Krebs-Ringer bicarbonate solution yielded a solution that relaxed isolated precontracted bovine retinal arteries, confirming the involvement of a diffusible chemical messenger. Hexane extraction, heating the solution to 70°C, or treatment with trypsin did not alter the relaxing properties of the incubation solution. The characteristics of the retinal relaxing factor do not correspond with those of nitric oxide, prostanoids, adenosine, acetylcholine, or any other of the known vasoactive neurotransmitters released from the retina. Our results suggest that retinal arterial tone is controlled by a diffusible, hydrophilic, and heat-stable relaxing factor that does not correspond with a known vasoactive molecule formed within the retina. (Circ Res. 1998;83:714-720.) Key Words: retina Ⅲ bovine Ⅲ arteries Ⅲ vasodilation Ⅲ relaxation V ascular smooth muscle tone is the result of myogenic tone modulated by circulating humoral factors and by neurotransmitters released from the vascular nerve endings. The original observations of Furchgott and Zawadzki 1 make it obvious that vascular smooth muscle tone is also modulated by adjacent cells. In a study on isolated bovine retinal arteries, we accidentally observed that retinal arteries incompletely cleaned of retinal tissue contracted strikingly weaker in response to prostaglandin F 2␣ than retinal arteries without adhering retinal tissue. This suggests that retinal tissue might profoundly influence vascular smooth muscle tone in retinal arteries. The regulation of retinal circulation by local factors might be very important considering the absence in the retinal vessels of sympathetic innervation that predominantly controls the arterial tone in most other parts o...

The aim of this study was to investigate the contribution of stereo vision to the acquisition of a natural interception task. Poor catchers with good (N = 8; Stereo+) and weak (N = 6; Stereo¡) stereo vision participated in an intensive training program spread over 2 weeks, during which they caught over 1,400 tennis balls in a pre-postretention design. While the Stereo+ group improved from a catching percentage of 18% to 59%, catchers in the Stereo¡ group did not signiWcantly improve (from 10 to 31%), this progress being indiVerent from a control group (N = 9) that did not practice at all. These results indicate that the development and use of of compensatory cues for depth perception in people with weak stereopsis is insuYcient to successfully deal with interceptions under high temporal constraints, and that this disadvantage cannot be fully attenuated by speciWc and intensive training.

These results provide support for the potential of microplasmin as a nonsurgical treatment for vitreomacular adhesion.

Retinal blood flow is regulated by local factors. In vitro bioassay experiments give evidence that retinal tissue from different species (dogs, pigs, sheep, cows, rats, and mice) continuously releases a factor lowering tone of isolated retinal arteries. This factor is a general relaxant as it was effective in relaxing different types of vascular as well as nonvascular smooth muscle preparations. This factor is called the retinal relaxing factor (RRF) and its characteristics do not correspond with those of the many well-known vasorelaxants found in retina (i.e., NO, prostanoids, adenosine, ADP, ATP, lactate, glutamate, GABA, taurine, adrenomedullin, CGRP, ANP, BNP, and CNP). This unknown RRF is transferable, hydrophilic, and heat-stable. Its relaxing effect is independent of the presence of the vascular endothelium and of NO-synthase, adenylyl cyclase, guanylyl cyclase, and cyclooxygenase activity. RRF might have a role in hypoxic vasodilation in retinal arteries since hypoxia induces relaxation only when retinal tissue is present. Thus, the RRF pathway is sensitive to changes in oxygen tension and might be a sensitive mechanism for adjusting vascular diameter to retinal oxygen levels. Diminished RRF release might explain the decreased retinal circulation observed in disease with atrophic retina.