Chorioretinal venous anastomoses: effect of different laser methods and energy in human eyes without vein occlusion

BACKGROUND

This study was performed to determine the laser energy required to rupture both Bruch's membrane and retinal veins reliably in order to create a venous chorioretinal anastomosis.

METHODS

A histological examination was conducted of argon green and YAG laser applications to the retina made prior to enucleation in eight eyes with large intraocular melanomas.

RESULTS

Argon laser application of 50 microns in size and 0.1 s duration to intervascular areas of the retina will reliably rupture Bruch's membrane at a power level of at least 1.5 W. If the argon laser spot is placed overlying a retinal vein, a power level of up to 2.5-3.0 W will rupture Bruch's membrane in 60%, with only 34% of the retinal veins showing evidence of rupture. The YAG laser with power levels of 3-4 mJ will reliably rupture the retinal vein in cases where it has not previously been ruptured by the argon laser.

CONCLUSION

When attempting to create a chorioretinal venous anastomosis in an eye with a non-ischaemic central retinal vein occlusion, Bruch's membrane should be ruptured first by placing the argon laser application at the side of the retinal vein before an attempt to rupture the retinal vein itself is made in case haemorrhage from the ruptured vein obscures the view. A power level of at least 2.5 W should be used. If the argon laser is unsuccessful in rupturing the retinal vein, a YAG laser (3-4 mJ) is effective.

The distribution of angioarchitectural changes within the vicinity of the arteriovenous crossing in branch retinal vein occlusion

OBJECTIVE

Branch retinal vein occlusions (BRVOs) are known to occur most commonly in the vicinity of arteriovenous (A/V) crossings. The authors aimed to identify types of venous wall abnormalities in BRVO and document their position in relation to the A/V crossing.

DESIGN

A retrospective review of the color photographs and fluorescein angiograms from the most recent 110 patients with first- or second-order BRVO was performed.

MAIN OUTCOME MEASURES

The films were examined for the presence of angioarchitectural changes of specified type within one-quarter disk diameter of the A/V crossing involved in the BRVO. The specific changes noted were fluorescein leakage, presumed thrombi, and flow abnormalities, which were recorded along with their position in relation to the A/V crossing.

RESULTS

Of the 110 patients diagnosed with BRVO, 59 had photography of satisfactory quality. Forty-one (70%) of these 59 patients had venous lesions, of which significantly more (chi-square -5.74, P < 0.02) were downstream (56%) than upstream (12%) from the A/V crossing. Thirty-two percent were upstream and downstream. Of the hemodynamic changes seen, 49% had late venous phase leakage of fluorescein, 85% had abnormal flow, and 7% had presumed thrombi. All thrombi seen were downstream.

CONCLUSIONS

Venous lesions in the vicinity of the A/V crossing commonly are seen in BRVO, most of which occur downstream. This suggests that the venous narrowing at the crossing may induce downstream hemodynamic changes predisposing to endothelial damage and thrombus generation.

Pretreatment with glucocorticoid is essential for lactogenic induction of the bovine beta-casein/CAT expression in HC11 cells

Hormonal regulation of the bovine beta-casein gene expression was studied in a murine mammary epithelial HC11 cells and compared with that of the rat beta-casein gene expression. CAT expression vectors driven by their promoter sequences were transfected into HC11 cells. Stable transfectents were treated with lactogenic hormones, dexamethasone and prolactin for 2 days in confluent cultures. While the lactogenic hormones synergistically induced a strong activation of the rat beta-casein/CAT expression, neither a single or combined treatment of dexamethasone and prolactin induced the bovine beta-casein/CAT expression. To test a sequential treatment effect of lactogenic hormones on the bovine beta-casein/CAT expression, cells were first treated with either dexamethasone or prolactin for various days and then subjected to the second treatment with both hormones for 2 days. Only dexamethasone-, but not prolactin-pretreated cells showed a strong lactogenic induction. Moreover, the fold induction of dexamethasone-pretreated cells increased gradually as a function of duration of dexamethasone pretreatment. A series of the bovine beta-casein/CAT constructs with different length of the bovine beta-casein 5' flanking region ranged from 0.3 kb to about 15 kb was analyzed in 12-days dexamethasone-pretreated cultures. CAT expression was increased even in 0.3 kb-containing construct, but prominent induction was seen in more than 1.8 kb-containing constructs. Therefore, it could be concluded that a long-term dexamethasone pretreatment is essential for lactogenic induction of the bovine beta-casein expression and the 0.3 kb proximal promoter region is important, but more distal promoter element(s) is necessary for mediating the coordinated action of lactogenic hormones to the bovine beta-casein expression.

Polymorphic sequence of Korean Native goat lactoferrin exhibiting greater antibacterial activity

Lactoferrin, which exhibits antibacterial activity to protect infants from infectious disease, is a major component of colostrum and milk. Lactoferrin was purified from the colostrum of Korean Native goat, and the cDNA from the mammary gland mRNA of the animal was cloned and sequenced. The nucleotide sequence of the lactoferrin gene of Korean Native goat was found to differ in 15 sites from that of the goat lactoferrin gene reported earlier. This difference in nucleotide sequence resulted in six amino acid substitutions: five in the N-lobe and one in the C-lobe. The antibacterial activity of Korean Native goat lactoferrin was found to be greater than that of Sannen goat lactoferrin.

Diabetic retinopathy: early functional changes

1. The present review reports some of the earliest physiological changes that occur in the diabetic retina prior to any clinical or anatomical changes in an animal model of diabetes. 2. Using chemically induced diabetes (by streptozotocin) in rats, retinal blood flow and vitreal and retinal oxygen tension were determined after 5 weeks of sustained hyperglycaemia. Blood flow was greater and was also redistributed in the diabetic group compared with values for the control group. At the same time, oxygen tension distribution was altered around retinal arterioles, implying an increase in retinal oxygen consumption in these early diabetic retinas. 3. The possibility that the blood flow changes could be due to altered control mechanisms in the retinal vasculature was confirmed using an isolated, perfused eye preparation. In diabetic eyes an altered reactivity to test pharmacological agents was demonstrated after 4 weeks of diabetes. 4. To further explore these vascular response changes we developed an isolated, perfused retinal arteriolar preparation in which individual segments of the vasculature can be tested. The possibility that insulin has a direct vasodilator effect on retinal arterioles was confirmed and was demonstrated to act via nitric oxide released from the vascular endothelial cells. These data may implicate the diabetic-induced insulin changes in early retinal changes. 5. Evidence is presented that although early glucose control may be vital in stopping the onset of diabetic retinopathy, there comes a stage in the induced diabetic cascade where if the retinopathy has commenced, good glucose control cannot stop the further progression of the retinopathy.

Hepatitis C virus envelope proteins bind lactoferrin

Hepatitis C virus (HCV) has two envelope proteins, E1 and E2, which form a heterooligomer. During dissection of interacting regions of HCV E1 and E2, we found the presence of an interfering compound or compounds in skim milk. Here we report that human as well as bovine lactoferrin, a multifunctional immunomodulator, binds two HCV envelope proteins. As determined by far-Western blotting, the bacterially expressed E1 and E2 could bind lactoferrin in human milk directly separated or immunopurified and separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The bindings of lactoferrin and HCV envelope proteins in vitro were confirmed by another method, the pull-down assay, with immunoprecipitated lactoferrin-bound protein A resin. By the same assay, mammal-expressed recombinant E1 and E2 were also demonstrated to bind human lactoferrin efficiently in vitro. Direct interaction between E2 and lactoferrin was proved in vivo, since anti-human lactoferrin antibody efficiently coimmunoprecipitated with secreted and intracellular forms of the E2 protein, but not glutathione S-transferase (GST), from lysates of HepG2 cells transiently cotransfected with the expression plasmids of human lactoferrin and gE2t-GST (the N-terminal two-thirds of E2 fused to GST) or GST. The N-terminal loop of lactoferrin, the region important for the antibacterial activity, has only a little role in the binding ability to HCV E2 but affected the secretion or stability of lactoferrin. Taken together, these results indicate the specific interaction between lactoferrin and HCV envelope proteins in vivo and in vitro.

Retinal artery and vein pressures in the dog and their relationship to aortic, intraocular, and cerebrospinal fluid pressures

The relationship between retinal arterial (Pra) and aortic (Pa) pressures is unknown, and the relationship between retinal vein (Prv) pressure and intraocular pressure (IOP) is not clear. Also unclear is the effect of cerebrospinal fluid pressure (CSFp) upon retinal venous pressure. We aimed to measure the relationships among Pra, Prv, Pa, IOP, and CSFp. Dogs were anesthetized while IOP, CSFp, and Pa were monitored. Pipettes with 2.5-micron diameter tips, connected to a servonulling pressure transducer, were used to record pressures from the retinal arteries and veins. Across a range of IOP (16-22 mmHg), CSFp (0-21 mmHg), and Pa (23-195 mmHg) the Pra = 0.72 Pa + 4.3 (r = 0.99, n = 61, P < 0.01), which suggests that the relationship between Pra and Pa is linear over a broad range of systemic blood pressures. The correlation coefficient between Prv and IOP was greater than 0.96 (P < 0.01) at all venous sites and whether IOP was greater than or less than CSFp. The transmural pressure varied along the retinal vein from 1.3 +/- 0.3 mmHg (+/-95% CI, n = 30) at 1 disk diameter from the optic disk rim to 0.3 +/- 0.2 mmHg (n = 66) at the optic disk, with a 0.9-mmHg/mm pressure gradient. These are the first measurements demonstrating a retinal vein transmural pressure close to zero.

Measurement of vasoactivity in the guinea-pig choroid

PURPOSE

A perfusion system for studying the vasoactive properties of the guinea-pig choroid is described.

METHODS

The principle of operation is that the vascular resistance of the entire vascular network of an isolated, perfused eye can be monitored by recording the pressure required to deliver a constant flow of perfusate through the network. Delivery of the pharmacological agent of interest into the perfusate stream and the subsequent determination of the magnitude of any induced pressure changes allows the vasoactive potency of various agonists to be assessed.

RESULTS

The baseline vascular resistance was 1.35 +/- 0.16 mmHg min/microL (mean +/- SEM; n = 10) and the mean response to intraluminal delivery of 124 mmol/L K+ Krebs was an increase in resistance of 297 +/- 67%. Vasoactive responses were sustainable for more than 8 h.

CONCLUSIONS

This system will now be used to study the vasoactive properties of the guinea-pig choroid in greater detail.

Direct vasodilatory effect of insulin on isolated retinal arterioles

PURPOSE

To test the hypothesis that insulin has a direct vasodilatory effect on retinal arteries and their branches and to investigate the mechanisms involved.

METHODS

Segments of porcine retinal arteries were dissected, cannulated, and perfused. Vessel diameter was measured continuously on-line. Vessels were precontracted to 66% +/- 0.9% (SEM, n = 148) of their original diameter by perfusing with 124 mM K(+)-Krebs solution. Dose-response curves to insulin (2 to 2000 microU/ml) were compared for extraluminal (EL), intraluminal (IL), and combined IL-EL application. The effect of cyclooxygenase and nitric oxide synthase inhibition on the insulin response was determined, as was Ca2+ channel involvement.

RESULTS

EL insulin alone had no significant effect on vessel diameter. IL insulin produced a dose-dependent dilatation of 5.6% +/- 2.9% (n = 22) of the K+ contracted diameter at 200 microU/ml and up to 12.4% +/- 3.6% (n = 22) by 2000 microU/ml, whereas combined IL-EL insulin application caused dilatation at all concentrations, rising to 15.1% +/- 2.9% (n = 44) at 200 microU/ml and 19.7% +/- 3% (n = 44) at 2000 microU/ml. IL indomethacin (5 x 10(-5) M) had no significant effect on the insulin-induced dilatation, whereas IL L-NAME (10(-4) M) inhibited insulin dilatation completely. The addition of EL verapamil (10(-6) M) during insulin-induced dilatation resulted in further dilatation to 37.8% +/- 4.2% (n = 18). However, the addition of insulin to verapamil-dilated vessels caused no further dilatation. Exposure to EL insulin while the IL K+ contraction dose-response curve was measured had no effect. Results in main arteries and branches did not differ.

CONCLUSIONS

The IL application of insulin dilates potassium-contracted pig retinal arteries. This effect was enhanced by the EL presence of insulin, which did not result in dilatation when it was administered alone. The dilatation response was mediated by nitric oxide but not by prostaglandins. There was some evidence for the involvement of Ca2+ channels in insulin-induced dilatation. These results imply that insulin is a vascular regulator in normal conditions and may have relevance to the vascular changes occurring in diabetes and hypertension in the retina.

Asymmetrical response of the intraluminal and extraluminal surfaces of the porcine retinal artery to exogenous adenosine

The relative effects of exogenous adenosine applied intraluminally or extraluminally were compared on first-order pig retinal arteries in an isolated perfused artery preparation. First-order retinal arteries with at least one side branch were cannulated and perfused at a constant flow in an environmentally-controlled organ bath on the stage of an inverted microscope. Vessels were precontracted with 10(-4) methoxamine applied extraluminally, which produced a sustained contraction. Then, either extraluminal or intraluminal adenosine was added in increasing concentrations from 10(-9) to 10(-3) M. During these procedures continuous measurements of external vessel diameter were made. The average external diameter of the retinal arterial segments used was 127.6 +/- 2.3 microns (n = 13). Extraluminal methoxamine (10(-4) M) constricted the vessels to 77.9 +/- 2.0% (n = 9) and 78.8 +/- 0.8% (n = 4) of the control value for the vessels later exposed to extraluminal and intraluminal adenosine respectively. Extraluminal adenosine caused a dose-dependent dilatation which commenced between 10(-7) M and 10(-6) M, and reached a percentage dilatation of 22.6 +/- 1.8% (n = 9) at 10(-3) M. For concentrations of 10(-4) M and above, spontaneous oscillations in diameter were observed for extraluminally-applied adenosine with an average period of 0.46 +/- 0.02 (n = 9) cycles per minute. The average percentage diameter oscillation was +/- 7.1% of the mean diameter. In contrast, intraluminal adenosine failed to cause dilatation or spontaneous oscillations at all concentration values, although the dilatory ability of these vessels was confirmed by intraluminal application of the Ca2+ channel blocker verapamil. In conclusion this study has demonstrated that the two sides of the retinal artery wall are differentially sensitive to adenosine, with the intraluminal route being ineffective. In vivo, in hypoxic or ischemic situations, adenosine is released by extraluminal neural tissue and minimizes tissue damage, partially by acting as a signaller of metabolic status to the vasculature leading to vasodilatation and hence increased local blood flow. This study shows that delivery of adenosine for therapeutic purposes through an intraluminal route is not a feasible proposition. This isolated, perfused artery technique has considerable potential to improve our understanding of uptake mechanisms, metabolism and vasoactivity of the retinal vessel wall.

Oxygen consumption in the avascular guinea pig retina

Oxygen consumption across the retina of a mammal with a naturally avascular retina has not previously been investigated. The oxygen consumption across the avascular retina of the guinea pig was measured in vivo by spatial analysis of the intraretinal oxygen profile. The avascular nature of the guinea pig retina allows the inner retina to be included in the analysis without disrupting the normal physiological state of the retina. Oxygen-sensitive microelectrodes (1-micron tip) were used to make high-resolution measurements of oxygen tension through the retina of anesthetized, mechanically ventilated guinea pigs (n = 10). Oxygen profiles were then analyzed in terms of oxygen tension as a function of distance from the choriocapillaris/Bruch's membrane, and the data were fitted to an established mathematical model of retinal oxygen consumption. The average oxygen consumption of the full thickness of the guinea pig retina was 1.1 +/- 0.09 ml O2.min-1 x 100 g-1 (n = 10). The average oxygen consumption of the outer half of the retina was 2.07 +/- 0.17 ml O2.min-1 x 100 g-1, while that of the inner half was only 0.12 +/- 0.04 ml O2.min-1 x 100 g-1. A localized region of high oxygen consumption was identified in the outer retina in every case, and this region accounted for an average of 93.9 +/- 2.0% of the total retinal oxygen consumption. Only 5.2 +/- 1.4% of the total oxygen consumption was attributable to the inner half of the retina. When choroidal oxygen tension was increased via a combination of systemic hyperoxia and hypercapnia, high oxygen levels could be sustained in all retinal layers. Under these conditions of an excess oxygen supply, the inner retina still consumed only 0.45 +/- 0.11 ml O2.min-1 x 100 g-1, which was 13.8 +/- 2.5% of the total retinal oxygen consumed. The relatively low oxygen consumption in the inner retina of the guinea pig may reflect an interesting adaptation to the constraints imposed by the absence of a retinal circulation in this species.

Intraretinal oxygen distribution and choroidal regulation in the avascular retina of guinea pigs

O2-sensitive microelectrodes were used to measure PO2 as a function of depth through the retina and choroid of anesthetized and artificially ventilated guinea pigs. The guinea pig retina is of particular interest, because it has a typically mammalian structure but no retinal circulation; it relies totally on choroidal delivery of O2 and other nutrients. Measurements of intraretinal O2 distribution in an avascular mammalian retina have not previously been reported. Under normal ventilation conditions, PO2 decreased monotonically from the choroid (33.6 +/- 2.9 mmHg, n = 11) to near zero (0.4 +/- 0.1 mmHg) at the retina-vitreous boundary. The inner half of the retina had an average PO2 of only 0.6 +/- 0.1 mmHg. Stepwise increases in inspired O2 (from 20 to 40 to 60 to 80 to 100%) had surprisingly little effect on choroidal PO2. Rapid changes (20-100%) produced overshoot-type responses in the choroid before recovery to levels only slightly above those found in normoxia. This indicates the presence of an active O2-regulatory mechanism in the guinea pig choroid. Addition of CO2 (5%) to O2 ventilation appeared to break down this control mechanism and led to dramatic and sustained increases in PO2 throughout the retina and choroid. The demonstration of an O2-regulating mechanism in the guinea pig choroid that maintains choroidal PO2 well below that in the systemic arterial blood, coupled with the observation of very low O2 levels throughout the inner retina, suggests that the O2 requirement of the inner retina in the guinea pig is small and that O2 levels in the choroid are deliberately constrained.

An efficient expression of human growth hormone (hGH) in the milk of transgenic mice using rat beta-casein/hGH fusion genes

In order to produce human growth hormone (hGH) in the milk of transgenic mice, two expression vectors for hGH differing in their 3' flanking sequences were constructed by placing the genomic sequences of hGH gene under the control of the rat beta-casein gene promotor. The 3' flanking sequences of the expression constructs were derived from either the hGH gene (pBCN1GH) or the rat beta-casein gene (pBCN2GH). Transgenic lines bearing pBCN1GH expressed hGH more efficiently than those bearing pBCN2GH in the milk (19-5500 micrograms/mL vs 0.7-2 micrograms/mL). In particular, one of the BCN1GH lines expressed hGH as much as 5500 +/- 620 micrograms/mL. Northern blot analysis showed that the transgene expression was specifically confined to the mammary gland and developmentally regulated like the endogenous mouse beta-casein gene in the mammary gland. However, a low level of nonmammary expression was also detected with more sensitive assay methods. In conclusion, the rat beta-casein/hGH fusion gene could direct an efficient production of hGH in a highly tissue-and stage-specific manner in the transgenic mice and the 3' flanking sequences of hGH gene had an important role for the efficient expression.

Altered vasoactivity in the early diabetic eye: measured in the isolated perfused rat eye

The effect of 4 weeks streptozotocin-induced diabetes on ocular vascular resistance responses to noradrenalin (NA), adrenalin (A), phenylephrine (PHE), isoproterenol (ISOP), prostaglandin F2 alpha (PGF2 alpha). 5-hydroxytryptamine (5-HT) and angiotensin II (ANG II), was determined using a newly-developed, isolated, arterially-perfused rat eye preparation, by comparing responses from control and diabetic eyes. After extensive preliminary experiments to establish optimum parameters, the ophthalmic artery of enucleated control and diabetic rat eyes was cannulated and the retinal and uveal vasculature perfused at a constant flow with Na(+)-Krebs solution after streptozotocin-induced diabetes had been established for 4 weeks. The eyes were maintained in an environment-controlled organ bath. Perfusion pressure was monitored as increasing log M concentrations of agonists were added to the perfusate. Total ocular resistance could be calculated from knowledge of flow and pressure. In control eyes, NA, A, PHE, PGF2 alpha, and 5-HT all produced dose-dependent increases in total vascular resistance, with the following order of potency: NA = A > 5-HT > PHE = PGF2 alpha at 10(-4) M. The ocular circulation was not sensitive to isoproterenol and angiotensin II. In diabetic eyes responses to NA, A, PGF2 alpha and 5-HT were altered. Diabetic responses to NA and A had lower thresholds with larger resistance increases at low concentrations. However, the rate of increase in resistance with concentration was more gradual in diabetic eyes so that at 10(-4) M control responses were larger. Diabetic resistance responses to PGF2 alpha had the same threshold as in control eyes, but were greater in magnitude with an earlier peak at 10(-4) M. In contrast diabetic resistance responses to 5-HT were reduced, peaked at a lower resistance at 10(-4) M, but had the same threshold as those in the control eye. Basal vascular resistances in control: 3.14 +/- 0.32 mmHg min microliter-1 (n = 28), and diabetic eyes: 3.44 +/- 0.19, mmHg min microliter-1 (n = 36), were not significantly different. Vasoactivity in the early diabetic eye is disturbed with the effective balance between different agonists altered in favour of catecholamines at physiological concentrations. This may be related to the early changes in blood flow and oxygen distribution already reported in the rat eye, as well as changes to autonomic function. The isolated perfused rat eye is a valuable technique for investigating such vascular reactivity in animal models of retinal disease.

Comparison of growth rates of bovine retinal and brain microvascular pericytes in different oxygen concentrations in vitro

BACKGROUND

The hyperoxic injury of the microcirculation in the central nervous system appears to be specific to the retina in premature mammals. Oxygen tensions in normal adult mammalian retina and brain vary between nearly 0 and 90 mmHg. This study sought to compare the in vitro replication of retinal and brain microvascular pericytes in normal glucose medium and in 1%, 5% and 20% oxygen (equivalent to 15 mmHg, 35 mmHg and 150 mmHg, respectively).

METHODS

A preliminary study, using oxygen microelectrodes, confirmed that the pericellular oxygen tension of pericytes, cultured in medium under air, was within 13 mmHg of the tension of the gas phase above the media. Pericytes were highly enriched by magnetic antibody cell sorting with the anti-pericyte monoclonal antibody (3G5) to 95% to 99% purity, to remove cell contaminants which may have invalidated the mitogenic assay.

RESULTS

Mitogenic assays showed that brain pericytes replicated faster than their counterparts from retina (P < 0.0001, averaged for data from all culture conditions using three-way ANOVA). Reduction of oxygen tension from 150 to 15 mmHg led to significantly increased replication of retinal pericytes (P = 0.01), but an insignificant increase for brain pericytes.

CONCLUSIONS

We have found that pericytes from the brain and retina cultured conventionally in fetal calf serum consume a relatively low amount of oxygen. Decreasing the oxygen tension to 1% (15 to 20 mmHg) increased the replication of retinal pericytes but not brain pericytes in normal glucose concentrations and in fetal calf serum. That retinal pericyte replication is sensitive to variation in oxygen tensions, indicates that the retinal microvascular cells have a unique biological response. This growth sensitivity to oxygen may be important in the pathogenesis of retinopathy of prematurity.

The influence of cerebrospinal fluid pressure on the lamina cribrosa tissue pressure gradient

PURPOSE

To measure the tissue pressure gradient through the optic disk and to determine the relationship between intraocular, cerebrospinal fluid, and retrolaminar tissue pressures. The relationship of optic nerve subarachnoid space pressure to intracranial cerebrospinal fluid pressure also was explored.

METHODS

Micropipettes coupled to a pressure transducer were passed through pars plana and vitreous to enter the optic disk in the anesthetized dog. Using a micromanipulator, pipettes penetrated the optic disk in steps while pressure measurements were taken. In some animals, pipettes also were passed into the optic nerve subarachnoid space. Lateral ventricle cerebrospinal fluid pressure, intraocular pressure, and arterial blood pressure were measured concurrently, and the effect of raising CSF pressure was explored.

RESULTS

Retrolaminar tissue pressure was largely dependent on the surrounding cerebrospinal fluid pressure, which was on average 8.6 +/- 3.5 mm Hg (SD, n = 8) higher, and was independent of intraocular pressure. Most (85% +/- 15% [SD, n = 8]) of the pressure drop between intraocular pressure and retrolaminar pressure occurred across the anterior 400 microns of disk tissue. When the intraocular pressure was 21 mm Hg and the cerebrospinal fluid pressure was zero, retrolaminar tissue pressure averaged 7 mm Hg and the translaminar pressure gradient was 3.08 +/- 0.29 mm Hg/100 microns tissue (SD, n = 3). Optic nerve subarachnoid space pressure was equivalent to lateral ventricular pressure.

CONCLUSIONS

These results show that cerebrospinal fluid pressure largely determines retrolaminar tissue pressure; hence, along with intraocular pressure, it is of major importance in setting the translaminar tissue pressure gradient. Results also demonstrate hydrostatic continuity between the optic nerve subarachnoid space and the lateral ventricle. That the translaminar pressure gradient can vary independently of intraocular pressure may be of importance in understanding the pathophysiology of glaucoma.

Optimal conditions required for the creation of an iatrogenic chorioretinal venous anastomosis in the dog using argon green laser photocoagulation

We have previously reported iatrogenic retinal to choroidal vein anastomosis, developed as a potential method of by-passing the site of obstruction to venous outflow in retinal venous occlusion in dogs (1), and in rats (2). In order to minimise tissue damage to the retina and choroid and increase the rate of success in these experiments, we investigated in the dog model the factors that would promote an anastomosis and compare the effects of three different power levels. A small spot size (50 microns) argon green laser beam of 514 nm at power levels of 0.5w, 1.5w and 2.5w were used. Spaced serial sections from each lesion were examined by light and by transmission electron microscopy. Morphometric measurements of the corresponding retinal pigment epithelium (RPE)/glial scar was computed using IBMPC digitising pad and sigma scan software and the extent of tissue damage at the different power levels assessed. At the lowest power level of 0.5w the damage to the retina was mild and there was an absence of anastomosis formation. At the 1.5w power level an anastomosis formed in 40% of the lesions. At the highest power level of 2.5w a 71% rate of success was obtained however, the damage to the retina tended to be severe. The results of this study also indicate that disruption of Bruch's membrane and vein rupture at the time of irradiation are essential for anastomosis formation, which may be further enhanced by necrotic tissue, retinal pigment epithelial and glial scar formation and inflammation. These findings are useful in establishing optimal conditions for the creation of a chorioretinal venous anastomosis, for consideration in human trials.

Adrenergic and nitrergic neurotransmitters are released by the autonomic system of the pig long posterior ciliary artery

The role played by adrenergic, muscarinic and nitric oxide putative neurotransmitters released from autonomic nerve endings onto the pig proximal long posterior ciliary artery (LPCA) was determined. The proximal LPCA in the pig usually supplies both the uveal and retinal circulations. In this study, in vitro ring segments of the artery, passively stretched and with noradrenaline-induced tone, were neurogenically stimulated (NS) using electrical field stimulation with 5-sec trains of 0.2 msec pulses. NS produced a frequency dependent contraction in all vessels which was completely abolished by 10(-6) M tetrodotoxin. 40 Hz stimulation was used throughout the study as it produced a maximal NS contraction. 10(-5) M guanethidine abolished the NS-induced contraction and revealed a NS-induced relaxation, as did the alpha adrenergic blocker, phentolamine, in vessels passively stretched. The beta adrenergic blocker, propranolol, only slightly reduced the NS-induced constriction. In vessels pre-contracted with noradrenaline, NS produced a relaxation (D) which was proportional in magnitude to the tone (C) viz. D = (0.30 +/- 0.04).C + (0.24 +/- 0.06). The muscarinic blocker, atropine, had no effect on the NS-induced relaxation, implying that it is a non-adrenergic, non-cholinergic mediated system. Incubation with Nw-nitro-L-arginine methyl ester reduced the NS-induced relaxation to 48% of its control value, a reduction which was reversed in the presence of excess L-arginine. Damage to endothelial cell function did not reduce the NS-induced relaxation. It is concluded that the autonomic innervation of the proximal LPCA releases both contraction and relaxation neurotransmitters. Contraction is mediated by an alpha adrenergic neurotransmitter. At least two neurotransmitters mediate relaxation, one of which is probably nitric oxide. There is no functional evidence for the release of beta adrenergic neurotransmitter from the sympathetic system or acetylcholine from the parasympathetic system.

Intraretinal oxygen distribution in rats as a function of systemic blood pressure

Differential responses to induced changes in systemic blood pressure (BP) at different layers of both the retinal and choroidal vasculature were observed, by monitoring localized PO2 as a function of depth, in the retina and choroid of the rat eye using oxygen-sensitive recessed microelectrodes. Visual and electrophysiological localization of the microelectrode tip allowed the oxygen distribution to be related to the positions of the vascular beds of the retina and choroid. Highly reproducible intraretinal PO2 profiles were achieved. The relationship between PO2 and systemic BP was linear in the deep capillary layer of the retina (PO2 = 0.17 x BP - 2.63) and in the choriocapillaris (PO2 = 0.21 x BP + 2.95), whereas it was nonlinear in the superficial retinal capillary layer [PO2 = 40.01/[1 + (BP/66.22)-1.22]] and deep choroid [PO2 = 83.82/[1 + (BP/124.61)-0.87]]. The minimum PO2 occurred between the two retinal capillary beds, and a PO2 gradient was evident in the choroid. The contrasting responses of different layers of the two circulations reflect different blood flow control mechanisms not evident when studying the circulations as a whole.

Vasoactivity of intraluminal and extraluminal agonists in perfused retinal arteries

PURPOSE

To evaluate the vasoactive response of isolated perfused arteries of the pig to K+ and adrenergic agonists and to compare the effects of intraluminal (IL) and extraluminal (EL) drug delivery.

METHODS

A new microperfusion system was developed, in which short lengths of porcine retinal arteries (outer diameter 90.4 +/- 2.7 microns) were cannulated at both ends and perfused at a controlled rate (5 microliters/min) with outflow through a single side branch. The diameter of the vessel and the intraluminal pressure were monitored, and the effect of intraluminally and extraluminally applied agonists was determined. Endothelial cell function and the integrity of the blood retinal barrier was verified.

RESULTS

Consistent vasoactive responses were obtained from most vessels. The resting diameter of the vessel was not greatly influenced by changes in flow rate or intraluminal pressure over the physiological range. Adrenaline and noradrenaline caused dose-dependent contractions, which were larger when applied intraluminally than they were when applied extraluminally. The largest contraction for adrenaline was 19.0% +/- 2.1% (n = 13) IL and 8.4% +/- 1.5% (n = 13) EL, and for noradrenaline, 17.8% +/- 1.9% (n = 13) IL and 6.8% +/- 1.1% (n = 13) EL. The IL contraction to 124-mM K+, 19.0% +/- 1.6% (n = 21), was also greater than that for EL application, 5.0% +/- 1.0% (n = 13). We found that the existence of myogenic contractions was restricted to the special case in which vessels with no branches were pressurized under zero flow conditions.

CONCLUSIONS

Pig retinal arteries exhibited asymmetry in their responses to adrenergic agonists and K+, with contractions significantly larger when the drug was applied to the intraluminal surface rather than the extraluminal surface. This asymmetry may reflect an important property of retinal vessels. Microperfusion systems of this type may prove valuable in developing a better understanding of control mechanisms in retinal circulations.

Effects of extracellular pH on agonist-induced vascular tone of the cat ophthalmociliary artery

PURPOSE

To test whether changes in extracellular pH (pHe) in an in vitro preparation of the cat ophthalmociliary artery affect passive tone and agonist responses and whether the endothelial cells are mediators of any pH-induced effect. This will determine the ability of the ophthalmociliary artery to influence retinal and choroidal blood flow in response to metabolic stimuli.

METHODS

The isometric tension generated by isolated ring segments preactivated by prostaglandin F2 alpha (PGF2 alpha), noradrenaline (NA), or 40 mM K+ was measured as the pHe of the bathing solution was changed stepwise from 6.0 to 8.0 by adjusting the bathing bicarbonate concentration in preparations with and without functioning endothelial cells.

RESULTS

PGF2 alpha produces a concentration-dependent contraction that is insensitive to an alkaline shift from control pHe (7.4) in the bathing medium. For acidic shifts to pHe 7.0, there is no significant change in the magnitude of the PGF2 alpha contraction, whereas at pHe 6.0, the PGF2 alpha contraction is reduced to 23 +/- 4% (n = 23) of its value at pHe 7.4. Threshold response concentration remains unaffected. Deliberate damage to the endothelial cells does not significantly affect the magnitude of the 10(-5) M PGF2 alpha response at pHe 7.4 nor the effect of acidic pH on this response. The 10(-5) M NA response is reduced in a graded fashion to acidic shifts to pH 7.0 and 6.0 (40 +/- 4% [n = 23]) and also to alkaline shift to pH 8.0 (22 +/- 5% [n = 23]) when compared to the induced tension at pHe 7.4. For the acidic shift only, endothelial cell damage causes a further significant reduction in the NA response to 20 +/- 3% (n = 5). For vessels contracted with K(+)-Krebs solution, there is a small but significant reduction in response at pHe 6.0 to 84 +/- 6% (n = 25), whereas for pHe 8.0 there is a much larger reduction to 45 +/- 5% (n = 24). All pHe-induced relaxations of K+ are endothelium independent. Passive tension is unaffected by all pHe manipulations.

CONCLUSIONS

Vessel responses to vasoactive agents are selectively mediated by pHe changes. Major acidic shifts cause reduced responses (relaxation) to NA, PGF2 alpha, or K+, whereas only vessels preactivated with NA and K+ relax to alkaline shifts. This implies that NA or K+ induced vascular responses are maximal close to neutral pHe with major shifts from neutrality in either the acidic or alkaline direction causing a reduced response. These results imply that the ophthalmociliary artery probably does not play a major role in controlling ocular blood flow in response to pHe changes within the normal metabolic range, but it may become important in ischemic conditions.