An in vivo and in vitro comparison of the effects of vasoactive mediators on pulpal blood vessels in rat incisors.

The effects of endogenous vasoactive substances were evaluated in anaesthetized rats using a laser Doppler flowmeter to monitor changes in pulpal blood flow, as well as directly in isolated pulpal arteriole preparations utilising a microperfusion and monitoring system to observe changes in vessel diameter. In anaesthetized rats, while systemic arterial blood pressure remained relatively stable, intra-arterial delivery of adrenaline (epinephrine) (A), noradrenaline (norepinephrine) (NA), phenylephrine (PHE), dopamine (DOPA), 5-hydroxytryptamine (5-HT), or endothelin-1 (ET-1) produced a dose-dependent reduction in pulpal blood flow (order of potency: ET-1>>A=NA>PHE=DOPA=5-HT); acetylcholine induced a dose-dependent increase in pulpal blood flow; histamine, isoproterenol and adenosine produced no significant changes. In isolated arteriole preparations, intraluminal delivery of A, NA, PHE, DOPA or 5-HT produced dose-dependent vasoconstriction (A=NA>PHE=DOPA=5-HT). Acetylcholine relaxed NA-precontracted vessels dose-dependently. Histamine and isoproterenol produced a small vasodilatation. Intraluminal ET-1 produced a small vasoconstriction at 10(-8)M, whereas extraluminal ET-1 produced a dose-dependent vasoconstriction from 10(-10)M and above. Intraluminal adenosine failed to dilate vessels precontracted with ET-1, whereas extraluminal adenosine caused a complete relaxation. These combined in vivo and in vitro data suggest that, in the rat incisor, the pulpal microcirculation is capable of functional regulation and that pulpal blood flow may be modulated by endothelium-related factors, metabolic (tissue-related) factors, as well as humoral (blood-borne) factors.

Oxygen distribution and consumption in rat lower incisor pulp.

The aim was to determine the oxygen tension (P(O(2))) and rate of oxygen consumption in the pulp. Twelve rats were anaesthetised and artificially ventilated. Under an operating microscope, a recessed oxygen-sensitive microelectrode was inserted into the pulp through a small saline-covered cavity on the labial surface of the lower incisor. P(O(2)) was measured as a function of the transverse distance from the saline medium through to the middle of the pulp. Oxygen profiles were characterised by a decline of oxygen tension outside the pulp in the saline medium and a steeper gradient across the interface, before a localised oxygen consuming region corresponding to the odontoblasts. A plateau with some localised fluctuations was then followed by an increase in oxygen tension in the middle of the pulp. The average oxygen tension in the plateau region was 23.2 mmHg+/-2.1 mmHg (n=12). A mathematical model was used to extract oxygen consumption data from P(O(2)) profiles recorded from non-perfused pulp (created by reducing systemic blood pressure). The analysis revealed that there was a distinct oxygen consumption zone in the outer pulp, which anatomically corresponded to the odontoblast layer. The average oxygen consumption rate of the odontoblasts was 3.2+/-0.2 ml O(2)/min per 100g pulp tissue. The zone of high oxygen consumption was 68.7 micro m+/-6.9 micro m (n=24) thick. It is concluded that pulpal oxygen distribution is heterogeneous and that the odontoblast could be a major oxygen consumer within the rat incisor pulp.

Agonist-induced vasoactive responses in isolated perfused porcine dental pulpal arterioles.

A novel isolated perfused pulpal arteriole preparation and microperfusion system was used to evaluate the direct vasoactive responses of pulpal arterioles to selected agonists. Short lengths of porcine pulpal arterioles (101.7+/-2.2 microm o.d., n=105) were dissected out and placed in an environment-controlled bath on the stage of an inverted microscope. Both ends of the vessel were cannulated and perfused at a controlled rate through the lumen. The diameter of the vessel was measured online. Following equilibration, the vessel was challenged with various agonists: adrenaline (epinephrine), noradrenaline (norepinephrine), phenylephrine, dopamine, isoproterenol, 5-hydroxytryptamine, histamine and adenosine. The endothelium-dependent vasodilator acetylcholine was used to evaluate endothelial cell function. Adrenaline, noradrenaline, phenylephrine, 5-hydroxytryptamine and dopamine caused dose-dependent contractions (adrenaline=noradrenaline>phenylephrine>dopamine>5-hydroxytryptamine). Isoproterenol and histamine provoked a dose-dependent dilation. Adenosine produced pronounced vasodilatation in vessels precontracted with 10(-8)M endothelin-1. Functional adrenergic, histamine, 5-hydroxytryptamine and adenosine receptors are, therefore, present in porcine pulpal arterioles. The isolated perfused pulpal arteriole preparation may prove valuable in understanding local control mechanisms of pulpal microcirculation.

Acetylcholine-induced vasodilation of isolated pulpal arterioles.

The presence of cholinergic mechanisms in the control of pulpal microcirculation has been a controversial issue. In this study, we aimed to determine the direct vasoactive responses of isolated pulpal arterioles to acetylcholine, and to investigate whether such responses are endothelium-dependent. Using an in vitro micro-perfusion system, we isolated pig pulpal arterioles, cannulated and perfused them intraluminally, and monitored the diameter. Following equilibration, the vessels were contracted with 10(-5) M noradrenaline, and the effect of increasing doses of acetylcholine was determined. The influence of the muscarinic antagonist, atropine, or the loss of endothelial cell function following saponin treatment was also determined. Acetylcholine induced a dose-dependent vasodilation, reaching 94.6+/-1.4% (n = 22) of the uncontracted diameter at 10(-4) M. The vascular relaxation effect of acetylcholine was abolished in the presence of atropine, and by saponin treatment. Analysis of these data suggests that, in the pig, the acetylcholine-induced vasodilation of incisor pulpal arterioles is endothelium-dependent and mediated by muscarinic receptors.

The effects of induced acute hyperglycemia in the cat on the retinal capillary blood flow.

PURPOSE: To study the effect of acute hyperglycemia on the erythrocyte flow in specific retinal capillary paths. METHODS: A technique for fluorescent labeling of a known fraction of the erythrocyte moiety of systemic blood was combined with fluorescence viewing of the retinal capillary network in live cats. This technique was developed to enable visualization of the erythrocyte flow in the retinal capillary network and used to acquire video recording of the retinal capillary erythrocyte flow in normal feline eyes. The pattern of capillary erythrocyte flow under normal blood glucose levels and normal systemic blood pressure served as baseline. Acute hyperglycemia was induced by intravenous injection of glucose while monitoring the systemic blood pressure. Two subsets of functional capillary pathways previously defined as S (simple) and C (complex) were identified in the recorded data. The relationship between erythrocyte flux in random selections of these two pathways and the level of hyperglycemia and systemic blood pressure was determined. RESULTS: Induction of acute hyperglycemia led to acute elevation of the systemic blood pressure that returned to baseline levels within few minutes, while blood glucose remained high. The capillary erythrocyte flux in S paths was significantly higher than in C paths at all values of systemic blood pressure. The capillary erythrocyte flux in the S paths was directly proportional to the systemic blood pressure whereas the flux in C paths was minimally affected by acute hyperglycemia for the systemic blood pressure range between 110 and 160 mm Hg. CONCLUSIONS: The erythrocyte flux in S paths is affected by the changes of the systemic blood pressure and these paths act as 'shunt vessels' when acute elevation of the systemic blood pressure occurs. C paths maintain stable perfusion under changing conditions, most probably in an effort to minimally alter the basic metabolic needs of the retinal tissue. Hyperglycemia per se was not responsible for changes of the cell flux in these capillary paths.

Continued progression of retinopathy despite spontaneous recovery to normoglycemia in a long-term study of streptozotocin-induced diabetes in rats.

BACKGROUND: This long-term (2.3 years) study determines the temporal relationship between systemic glucose levels and the progression of diabetic retinopathy during the natural course of streptozotocin-induced diabetes in rats. METHODS: Of 367 rats, 200 were randomly assigned into a group injected with streptozotocin (50 mg.kg(-1)) and 167 into a control group. Subsets of the rats were killed at 6, 28, 40, 65, 90 and >100 weeks post induction to allow the severity of retinopathy to be assessed quantitatively and qualitatively by trypsin digests of the retinal vasculature. Concurrently blood glucose, body weight and death rate were monitored. RESULTS: Three glycemic phases were observed in the streptozotocin rats. In phase 1 (0 to 36-40 weeks) hyperglycemia was established and maintained. In phase 2 (36-40 to 84-90 weeks) normoglycemia was restored, and maintained during phase 3 (84-90 to 120 weeks). Control rats were normoglycemic throughout. The retinal microangiopathy was marked at 28 weeks during phase 1, developed more slowly in phase 2 and continued to worsen with loss of capillaries in all retinas and saccular microaneurysms present in 50% of retinas in phase 3. Cumulative death rate in streptozotocin rats also followed three phases, with maximum vulnerability occurring between 28 and 40 weeks. Body weight was significantly lower in streptozotocin rats throughout, increasing slowly in phase 1, then more rapidly during and after spontaneous glycemic recovery. CONCLUSION: The worsening retinopathy, despite sustained recovery to normoglycemia, implies that good glucose control alone does not stop the progression of the retinal microangiopathy at this late stage.

Systemic and ocular vascular roles of the antiglaucoma agents beta-adrenergic antagonists and Ca2+ entry blockers.

This review addresses whether the antiglaucoma agents beta-adrenergic antagonists and Ca2+ entry blockers cause vasoactive effects in the retinal and other ocular vasculatures, as they do in other tissues. The potent vasodilating effects of Ca2+ entry blockers on ocular vessels have recently been demonstrated in in vivo and in vitro studies, implying that the maintenance of ocular vascular tone relies almost exclusively on extracellular Ca2+. Ca2+ entry blockers may potentially play a role in relaxing the retinal, long posterior ciliary, and ophthalmociliary arteries to improve the ocular circulation in vascular diseases in which there is considerable vascular tone present. The beta-adrenergic antagonists are discussed with reference to their antihypertensive role, their effect on other vascular beds, and finally what is known of their effect in the ocular vasculature. The emerging evidence that particular selective beta-adrenergic antagonists, such as betaxolol, are also potent Ca2+ channel entry blockers in other vascular beds is presented. Betaxolol has been shown to induce vasodilatation in the retinal and other ocular vascular beds, although studies have shown that beta1-adrenergic receptors are sparse in these vascular beds. This implies that an alternative mechanism must be responsible for betaxolol-induced vasodilatation. Evidence is presented that betaxolol vasodilates via its potent Ca2+ channel entry blocking properties, and its potency and ability to vasodilate are compared with those of nimodipine and timolol, as well as with those of other Ca2+ channel entry blockers. Important areas for future research in this area are discussed.

Localization of IgG in the normal and dystrophic rat retina after laser lesions.

PURPOSE: To test the hypothesis that access to extravasated plasma protein IgG may influence photoreceptor survival following laser photocoagulation and to determine whether this correlates with the retinal glial reaction. METHODS: A total of 45 rats (18 Royal College of Surgeons (RCS) dystrophic and 18 RCS-rdy+ congenic control) were used for this experiment. Nine non-lasered littermates of same age were used as controls. The superior retinas of postnatal day 23 rats were irradiated with a grid pattern of 40 argon green laser lesions of 50 microm in diameter and two powers (150 and 300 mW) for 0.2 s. At various times after laser lesions (up to 14 days), animals were perfused, the retinas snap frozen and sectioned on a cryostat. A one-step immunohistochemical technique was used by incubating with rabbit anti-rat IgG conjugated directly to horseradish peroxidase. Adjacent sections were processed using an antibody to glial fibrillary acidic protein (GFAP) by the standard avidin-biotin complex method. RESULTS: The labelling pattern for extravasated IgG after laser lesion was very similar in both RCS and RCS-rdy+ rat retinas. At 6, 12 and 24 h after lesions, IgG immunoreactivity (IR) was very intense in the lesion core and flanks. The outer plexiform layer (OPL) and photoreceptor inner segments provided a ready pathway for lateral spread of IgG. However, in the outer nuclear layer (ONL), IgG localization was much more restricted. Despite very intense IgG IR in the ONL of the coagulated lesion core, there was always a very sharply delineated boundary where the label abruptly halted. The GFAP labelling in both RCS dystrophic and RCS-rdy+ congenic control rat retinas showed that this boundary was between normal and necrotic cells because there was a core where GFAP was not produced by Müller cells. By 2 days after lesions, the coagulated cells in the lesion core were being removed by phagocytic cells that were IgG IR. Labelled phagocytic cells were also found among the inner and outer segment region on the lesion flanks. There was still IgG IR in the lesion, but the label was faint. No IgG IR was found in the retina at 3, 4, 7 and 14 days after lesions. Absorption control with pure rat IgG showed the label to be specific. CONCLUSIONS: The extravasated IgG was derived from the choroidal circulation because at no stage was IgG localized around the retinal vasculature. The IgG labelling was surprisingly widespread and, therefore, did not correlate with photoreceptor sparing, although it preceded the widespread Müller cell expression of GFAP and may, therefore, trigger glial reaction.

Effect of betaxolol, timolol and nimodipine on human and pig retinal arterioles.

This study tested the hypothesis that the beta-adrenergic antagonists betaxolol and timolol, cause retinal arteriolar vasodilatation in addition to their ability to reduce intraocular pressure (IOP), and compared their vasodilatory ability with that of a known Ca2+ channel entry blocker nimodipine in donor human and pig isolated perfused retinal arterioles. This study was performed using a microperfusion technique specifically established to allow investigations in arterioles as small as the first order human and pig retinal arterioles (approximately 100 microns diameter). The scarcity of viable human tissue was overcome by the successful development of controlled rate freezing and cryopreservation techniques which were able to preserve the vascular responsiveness of the retinal arterioles, thus enabling multiple experiments to be performed on segments of retinal arterioles from each individual donor eye. Furthermore, relaxation by acetylcholine in noradrenaline contracted pig retinal arterioles showed that endothelial cell function was well maintained after cryopreservation (n = 8). Baseline diameters of retinal arterioles used in the main studies were: cryopreserved human 92.3 +/- 3.4 microns (n = 44), fresh pig 94.7 +/- 2.2 microns (n = 42), and cryopreserved pig 94.3 +/- 2.3 microns (n = 30). Precontraction with extraluminal endothelin-1 (ET-1) 10(-9) M reduced the diameters to 74.3 +/- 0.9%, 71.6 +/- 1.6% and 72.5 +/- 0.9% respectively. Intraluminally applied nimodipine and betaxolol caused a significant dose dependent dilatation (P < 0.001) in human retinal arterioles with a threshold of 10(-12) M. Timolol did not produce a significant dilatation in human arterioles. Timolol produced a small but significant dilatation in fresh and cryopreserved pig arterioles but the dilatation with betaxolol and nimodipine was significantly larger. The nimodipine and betaxolol dose response curves were not significantly different in human arterioles, but nimodipine produced significantly greater dilatation than betaxolol (P < 0.001) in fresh and cryopreserved pig arterioles. Both nimodipine and betaxolol were significantly more effective vasodilators than timolol (P < 0.001) in human and pig retinal arterioles.

Intracellular structures of retinal vascular endothelium in normal and early diabetic rats.

PURPOSE: To determine whether the arrangement of microfilaments in retinal vascular endothelium is altered in early diabetes. METHODS: F-actin and nuclei of retinal vascular endothelial cells of normal and diabetic (28 weeks postinduction) rats were examined using confocal microscopy. RESULTS: An increase in the incidence of aberrant F-actin bundle arrangement in the diabetic retinal arteriole was observed, but there was no disruption of peripheral border F-actin. The diameter of superficial capillaries was increased in diabetic rats. CONCLUSION: These changes may be related to the endothelial dysfunction demonstrated in a previous pharmacological study.

The correlation between cerebrospinal fluid pressure and retrolaminar tissue pressure.

PURPOSE: To measure the effects of cerebrospinal fluid pressure (CSFp) on retrolaminar tissue pressure (RLTp) and the translaminar pressure gradient (TLPG), particularly at low CSFp, which is the normal situation in erect posture. METHODS: Micropipettes coupled to a servonull pressure system were passed into eyes of anesthetized dogs to the optic disc and advanced in steps through the lamina cribrosa to the optic nerve subarachnoid space (ONSAS), while pressure measurements were taken. Cerebrospinal fluid pressure and intraocular pressure (IOP) were monitored and controlled. The TLPG was measured at varying IOPs and CSFps. The RLTp and ONSAS pressure (ONSASp) were measured at varying CSFps. In separate experiments, the optic nerve dura was incised, and pressure measurements were taken across the pia mater. RESULTS: The TLPG was strongly correlated to the difference between IOP and CSFp (r=0.93; n=18) when CSFp was more than zero. Mean RLTp was 3.7+/-0.2 mm Hg (SEM; n=15) when CSFp was 0 mm Hg. The ONSASp and RLTp were largely dependent on the presence of CSFp higher than break point pressures of -0.5 mm Hg and 1.33 mm Hg, respectively. However, below these break points, RLTp (slope 0.07) and ONSASp (slope 0.18) were little influenced by CSFp. Separate measurements across the pia mater revealed that 95% of the pressure drop occurred within 100 microm of the pial surface. CONCLUSIONS: The TLPG and RLTp are dependent on CSFp when CSFp is more than -0.5 mm Hg. Below this level, there is no hydrostatic continuity between the intracranial and optic nerve subarachnoid space. In this range, RLTp is stable and is little influenced by CSFp changes.

Quantification of retinal oxygen consumption changes from preretinal oxygen transients.

BACKGROUND: In the avascular retina of the guinea-pig all the oxygen supply is from the choroid. Under these circumstances we had proposed, on theoretical grounds, that a change in retinal oxygen consumption would modulate the preretinal oxygen level. METHODS: In the present study in guinea-pigs,the oxygen consumption of the outer retina was modulated by light-dark exposure while preretinal oxygen tension was monitored. RESULTS: The results were in complete agreement with the proposed mathematical models. CONCLUSION: The present study demonstrates that it is feasible to quantify oxygen consumption changes in a known retinal layer from measurements of preretinal oxygen tension.

Culture and characterization of smooth muscle cells from porcine retinal microvessels.

PURPOSE: To establish long-term cultures of retinal vascular smooth muscle cells for future studies of their functions under normal and diseased conditions. METHODS: Smooth muscle cells (SMC) from porcine retinal microvessels were isolated and cultured. RESULTS: Cells in culture retained the common biochemical characteristics of SMC propagated from other sources. The cells were large, polygonal, spindle shaped and demonstrated the presence of smooth muscle-specific alpha-actin. Post-confluent cultures showed the 'hill and valley' multilayer growth characteristics. However, the growth rate was lower and the population doubling time was significantly larger than those reported for SMC propagated from big vessels. CONCLUSION: Retinal vascular SMC could be cultured separately from other cell types. The availability of this culture provides a unique model for functional and metabolic studies of the retinal vessels.

Intraretinal oxygen distribution in urethan-induced retinopathy in rats.

This study was performed to explore the interaction between chronic neural degeneration and the subsequent vascular remodeling. Weekly urethan administration in rats from birth produces a retinopathy model characterized by photoreceptor degeneration, retinal vascular regression, and retinal pigment epithelium (RPE) neovascularization. We investigated the hypothesis that altered oxygen distribution across the retina and choroid could be involved in the vascular changes seen in this retinopathy. We compared measurements of vitreal, intraretinal, and choroidal oxygen tension (PO2) distribution in anesthetized and ventilated control and urethan-treated rats at 8 and 16 wk of age with the use of oxygen-sensitive microelectrodes. Striking differences were observed in both choroidal and intraretinal PO2 distribution in urethan-treated rats compared with controls. At both ages, intrachoroidal PO2 was much lower in the urethan-treated rats. In addition, the intraretinal PO2 distribution across the inner retinal layers was more uniform than normal. A small elevation in PO2 was present at 8 wk in the inner nuclear layer where subsequent vascular regression occurred, and a small reduction in PO2 was present at the RPE, which recovered to normal values by 16 wk in regions where RPE vessel networks were first evident. Although the retinal arteries were considerably thinner at both ages in the urethan-treated rats, the vitreal PO2 profiles and superficial retinal PO2 remained normal. The unexpected and large reduction in the oxygen delivery from the choroid found in the urethan-treated rats may account for the lack of major hyperoxia within the pathological retina and the lower oxygen tension in the RPE before the vascular proliferation in this region. We propose that tissue PO2 is an important determinant of the vascular remodeling, which is seen in this model of neural degeneration and that the PO2 distribution changes described in this study help provide a new view of the pathogenesis of this model.

Overview of studies on metabolic and vascular regulatory changes in early diabetic retinopathy.

The present review provides an overview of recent research describing functional changes to the retinal vasculature in very early diabetes. The research focuses on the streptozotocin rat model after 4-6 weeks of induced diabetes and describes functional changes to retinal blood flow, vascular control and retinal oxygenation, as well as a strong vasodilatory response to insulin in the retinal vasculature. The review raises the question of whether choroid dysfunction is also a feature of diabetic retinopathy.

Relation between pressure determined by ophthalmodynamometry and aortic pressure in the dog.

AIMS: Ophthalmodynamometry has been used extensively since the last century; however, controversy surrounds what it actually measures. This study was set up to determine the relation between ophthalmodynamometric (ODP) and systemic blood pressures. METHODS: Aortic pressure was continuously monitored and altered by phlebotomy in six anaesthetised dogs, while ophthalmodynamometry was performed, by directly altering intraocular pressure. Maxillary artery pressure was monitored in two animals. All pressure transducers were zeroed at eye level. RESULTS: Mean ODP was 96.6% (1.6%) (95% confidence interval, n = 49) of aortic pressure. Mean maxillary artery pressure was 95.7% (5.5%) (95% CI, n = 16) of aortic pressure. ODP was 1.9 (0.6) mm Hg (95% CI, n = 33) higher than maxillary artery pressures. CONCLUSION: ODP was only slightly below aortic pressure and not significantly different from maxillary artery pressure, the analogue of the internal carotid artery in humans. These results also suggest a retinal artery collapse pressure of at least 1.9 mm Hg.

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.

Heterogeneous endothelial cell structure along the porcine retinal microvasculature.

The pivotal role of the endothelial cell in the regulation of vascular tone has been well demonstrated in many vascular beds, including the retina. However, in the retina, little is known about how the structural elements of the endothelial cells are arranged along the arborisation pathway from artery to vein, the nature of which has been linked to functional heterogeneity in other vascular beds. The relative vulnerability of the retina to vascular based diseases, and the heavy reliance on local regulation of the retinal vasculature makes an improved understanding of such local regulatory mechanisms of significant clinical importance. The present study focuses on identifying differences in endothelial cells along the arborisation pathway in the porcine retinal vasculature. Enucleated pig eyes were arterially cannulated and perfused with fixative followed by double staining for F-actin microfilaments (rhodamine phalloidin) and nucleic acid (YO-PRO-1). The intact retina was then viewed by confocal microscopy. The distribution of F-actin, vessel diameter, endothelial cell size and shape, nucleus size and shape, and position within the cell were determined as a function of location along the vascular tree. The main retinal arterioles (A1) contained full length F-actin internal stress fibers which lay parallel to the long axis of the endothelial cell. Subsequent branches from the A1 arteriole (A2 and A3) showed fewer, shorter fibers, with none visible in the A4 and A5 branches, the capillaries, or in the venous side of the vasculature. All endothelial cells showed peripheral border staining of F-actin microfilaments which allowed the shape of the cell to be determined. All endothelial cells were elongated with the long axis parallel to the vessel, but the mean aspect ratio decreased from 10.9+/-0.5, s.e.m. in the A1 arterioles to 3.2+/-0.2 in the major veins (V1). The position of the endothelial cell nucleus relative to the cell was eccentric in the downstream direction in the A2-A5 arterioles, whilst centrally placed in the A1 arterioles and veins. The structural heterogeneity of endothelial cells along the pig retinal circulation suggests that functional heterogeneity of the endothelium may be involved in regulation of retinal blood flow.

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.