Analysis of blood flow in the long posterior ciliary artery of the cat.
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PURPOSE: Experiments were undertaken to use a new technique for direct on-line measurement of blood flow in the long posterior ciliary artery (LPCA) in cats and to evaluate possible physiological mechanisms controlling blood flow in the vascular beds perfused by this artery. METHODS: Blood flow in the temporal LPCA was measured on a continuous basis using ultrasonic flowmetry in anesthetized cats. Effects of acute sectioning of the sympathetic nerve and changes in LPCA and cerebral blood flows in response to altered levels of inspired CO2 and O2 were tested in some animals. In others, the presence of vascular autoregulatory mechanisms in response to stepwise elevations of intraocular pressure was studied. RESULTS: Blood flow in the temporal LPCA averaged 0.58+/-0.03 ml/min in 45 cats anesthetized with pentobarbital. Basal LPCA blood flow was not altered by acute sectioning of the sympathetic nerve or by changes in low levels of inspired CO2 and O2, although 10% CO2 caused a modest increase. Stepwise elevations of intraocular pressure resulted in comparable stepwise decreases of LPCA blood flow, with perfusion pressure declining in a linear manner throughout the perfusion-pressure range. CONCLUSIONS: Ultrasonic flowmetry seems to be a useful tool for continuous on-line measurement of LPCA blood flow in the cat eye. Blood flow to vascular beds perfused by this artery does not seem to be under sympathetic neural control and is refractory to modest alterations of blood gas levels of CO2 and O2. Blood vessels perfused by the LPCA show no clear autoregulatory mechanisms. (+info)
Effect of Ox-LDL on endothelium-dependent response in pig ciliary artery: prevention by an ET(A) antagonist.
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PURPOSE: To investigate whether oxidized low-density lipoprotein (Ox-LDL) affects endothelium-dependent responses in isolated porcine ciliary arteries. METHODS: In a myograph system for isometric force measurements, quiescent vessels were incubated with 50 microg/ml, 100 microg/ml, or 200 microg/ml Ox-LDL; 100 microg/ml native LDL (n-LDL); 1 microM of the ET(A)- endothelin receptor antagonist BQ 123; 100 microg/ml Ox-LDL coadministered with 1 microM BQ 123; or 100 microg/ml Ox-LDL coadministered with 50 microM of the protein synthesis inhibitor cycloheximide. Vessels with nonfunctional endothelium (intentionally and mechanically damaged) were also exposed to 100 microg/ml Ox-LDL. Two hours later, vessels were washed, precontracted with the thromboxane A2 analog U 46619 (approximately 0.1 microM), and exposed to bradykinin (0.1 nM to 3 microM), an endothelium-dependent relaxing agent. RESULTS: In quiescent vessels, Ox-LDL evoked delayed contractions. In contrast, no contractions were observed after exposure to n-LDL, BQ 123, Ox-LDL with BQ 123, or Ox-LDL with cycloheximide. In vessels with nonfunctional endothelium, Ox-LDL did not evoke contraction. Bradykinin-induced relaxations were inhibited in a dose-dependent manner by Ox-LDL, but not by n-LDL, BQ 123 alone, Ox-LDL with BQ 123, or Ox-LDL with cycloheximide. CONCLUSIONS: In porcine ciliary arteries, Ox-LDL affects endothelium-dependent responses through the activation of ET(A)- endothelin receptors. As Ox-LDL can accumulate in atherosclerotic plaques, such a mechanism might be involved in the occlusion of the ophthalmic circulation observed in patients with hypercholesterolemia and atherosclerosis. (+info)
Effect of acute intraocular pressure changes on short posterior ciliary artery haemodynamics.
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BACKGROUND/AIMS: Vascular insufficiency due to abnormal autoregulation has been proposed as a major factor in the development of glaucoma. The anterior optic nerve is primarily perfused by the short posterior ciliary arteries. The autoregulatory capacity of these vessels in response to acutely elevated intraocular pressure (IOP) was examined in normal human subjects. METHODS: Colour Doppler imaging was performed on the short posterior ciliary arteries of 10 normal subjects at baseline and during four incremental IOP elevations. Using a scleral suction cup placed temporally, IOP was elevated to approximately 25, 30, 40, and 50 mm Hg. Additional measurements were performed immediately after pressure release. Systolic and diastolic flow velocities were measured and Pourcelot's resistivity index was calculated. RESULTS: Systolic and diastolic flow velocities decreased linearly with each incremental increase in IOP (p < 0.001). Pourcelot's resistivity index increased linearly with each incremental increase in IOP (p < 0.001). Changes in end diastolic velocity, peak systolic velocity, and Pourcelot's resistivity index were linearly related to changes in IOP. CONCLUSION: The normal healthy eye is not able to autoregulate to maintain PCA blood flow velocities in response to acute large elevations in IOP. (+info)
Morphological variations of the peripapillary circle of Zinn-Haller by flat section.
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AIMS: To evaluate the morphometric and morphological variations of the circle of Zinn-Haller (CZH) in the human eye. METHODS: 42 human enucleated eyes were used in this study. After transverse flat thick sections were cut through the optic nerve and adjacent sclera, tissue sections were stained with haematoxylin and eosin or examined immediately by wet preparation under a light microscope. The average vessel diameter of the arterial circle and the average distance between the optic nerve head (ONH) and the arterial circle were determined. Various branching patterns of the CZH were also evaluated. RESULTS: The vessel diameter of the arterial circle was 123 (SD 75) microm (range 20-230 microm). The distance of the CZH from the ONH margin was 403 (352) microm (0-1050 microm). The CZH gave off branches to the optic nerve and to the peripapillary choroid (PPC) with various branching patterns especially at the entry point of paraoptic short posterior ciliary artery. CONCLUSIONS: The CZH exists within a variable distance from the ONH and its average diameter is similar to that of the central retinal vessels though it shows marked variation even in the same circle. The CZH also shows variable configurations in branching patterns. These variations may act as contributing factors that are responsible for the individual susceptibility of the anterior optic nerve and the PPC to circulatory disturbances. (+info)
Microvasculature of the rat optic nerve head.
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PURPOSE: To describe the arterial blood supply, capillary bed, and venous drainage of the rat optic nerve head. METHODS: Ocular microvascular castings from 6 Wistar rats were prepared by injection of epoxy resin through the common carotid arteries. After polymerization, tissues were digested with 6 M KOH, and the castings washed, dried, and coated for scanning electron microscopy. RESULTS: Immediately posterior to the globe, the ophthalmic artery trifurcates into the central retinal artery and two posterior ciliary arteries. The central retinal artery directly provides capillaries to the nerve fiber layer and only contributes to capillary beds in the neck of the nerve head. The remainder is supplied by branches of the posterior ciliary arteries that are analogous to the primate circle of Zinn-Haller. Arterioles arising from these branches supply the capillaries of the transitional, or laminar, region of the optic nerve head. These capillaries are continuous with those of the neck and retrobulbar optic nerve head. All optic nerve head capillaries drain into the central retinal vein and veins of the optic nerve sheath. A flat choroidal sinus communicates with the central retinal vein, the choriocapillaris, and with large veins of the optic nerve sheath. CONCLUSIONS: The microvasculature of the rat optic nerve head bears several similarities to that of the primate, with a centripetal blood supply from posterior ciliary arteries and drainage into the central retinal and optic nerve sheath veins. Association of nerve sheath veins with the choroid represents an important difference from the primate. (+info)
Neurogenic vasoconstriction as affected by cholinergic and nitroxidergic nerves in dog ciliary and ophthalmic arteries.
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PURPOSE: To determine the involvement of noradrenergic and other vasoconstrictor nerves in the contraction of ocular arteries and the modification by cholinergic and nitroxidergic nerves of vasoconstrictor nerve function. METHODS: Changes in isometric tension were recorded in helical strips of the canine posterior ciliary and external ophthalmic arteries denuded of the endothelium, which were stimulated by transmurally applied electrical pulses (5 Hz). Vasoconstrictor mediators were analyzed by pharmacological antagonists, such as prazosin, alpha,beta-methylene ATP, a P2alpha-purinoceptor antagonist, and BIBP3226, a neuropeptide Y receptor antagonist. RESULTS: Transmural electrical stimulation produced contractions that were potentiated by N(G)-nitro-L-arginine (L-NA), a nitric oxide (NO) synthase inhibitor. The contraction was partially inhibited by prazosin and abolished by combined treatment with alpha,beta-methylene ATP but was not influenced by BIBP3226. Stimulation-induced contraction was attenuated by physostigmine and potentiated by atropine. Contractions induced by exogenous ATP were reversed to relaxations by alpha,beta-methylene ATP. In the strips treated with L-NA, prazosin, and alpha,beta-methylene ATP, the addition of L-arginine elicited relaxations by nerve stimulation. The ATP-induced relaxation was attenuated by aminophylline, whereas neurogenic relaxation was unaffected. CONCLUSIONS: Ciliary and ophthalmic arterial contractions by nerve stimulation are mediated by norepinephrine and ATP, which stimulate alpha1-adrenoceptor and P2X purinoceptor, respectively. ATP from the nerve is unlikely involved in vasodilatation. Acetylcholine derived from the nerve impairs the neurogenic contraction, possibly by interfering with the release of vasoconstrictor transmitters, and neurogenic NO also inhibits the contraction postjunctionally by physiological antagonism. (+info)
Specialised sympathetic neuroeffector associations in immature rat iris arterioles.
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Sympathetic nerve-mediated vasoconstriction in iris arterioles of mature rats occurs via the activation of alpha(1B)-adrenoceptors alone, while in immature rat iris arterioles, vasoconstriction occurs via activation of both alpha1- and alpha2-adrenoceptors. In mature rats the vast majority of sympathetic varicosities form close neuroeffector junctions. Serial section electron microscopy of 14 d iris arterioles has been used to determine whether restriction in physiological receptor types with age may result from the establishment of these close neuroeffector junctions. Ninety varicosities which lay within 4 microm of arteriolar smooth muscle were followed for their entire length. Varicosities rarely contained dense cored vesicles even after treatment with 5-hydroxydopamine. 47 % of varicosities formed close associations with muscle cells and 88 % formed close associations with muscle cells or melanocytes. Varicosities in bundles were as likely as single varicosities to form close associations with vascular smooth muscle cells, although the distribution of synaptic vesicles in single varicosities did not show the asymmetric accumulation towards the smooth muscle cells seen in the varicosities in bundles which were frequently clustered together. We conclude that restriction of physiological receptor types during development does not appear to correlate with the establishment of close neuroeffector junctions, although changes in presynaptic structures may contribute to the refinement of postsynaptic responses. (+info)
Optic nerve and peripapillary choroidal microvasculature of the rat eye.
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PURPOSE: To investigate the three-dimensional microvascular anatomy of the optic nerve and peripapillary choroid in the rat eye. METHODS: Gross vascular anatomy of the posterior eye segment of Wistar rats was studied in serial microsections with a light microscope. The optic nerve and peripapillary choroidal vessels were sequentially microdissected, using methylmethacrylate corrosion microvascular castings, and were examined with a scanning electron microscope to determine the three-dimensional relationships of the vessels. RESULTS: The posterior ciliary artery traveled along the inferior side of the optic nerve sheath, directly entered the optic nerve head, and divided into three branches: the central retinal artery and medial and lateral long posterior ciliary arteries, which provided several short branches to the choroid. The optic nerve head vasculature was consistently nourished by a recurrent arteriole from the central retinal artery and an arteriole from the choroidal artery at the peripapillary choroid. The central retinal vein flowed into a venous anastomosis along the optic disc border of the peripapillary choroid. Capillaries within the optic nerve drained into the central retinal vein, the marginal venous anastomosis of the peripapillary choroid, and the pial veins, all of which flowed into the posterior ciliary veins along the optic nerve sheath. CONCLUSIONS: The findings illustrate vascular anatomic differences in optic nerve and peripapillary choroidal microcirculation between rat and human. In rats, the peripapillary choroid plays a significant role in both blood supply and venous drainage of the optic nerve head. The central retinal artery also contributes to the optic nerve head circulation. (+info)