Oxygen distribution and consumption within the retina in vascularised and avascular retinas and in animal models of retinal disease.

Maintenance of an adequate oxygen supply to the retina is critical for retinal function. In species with vascularised retinas, such as man, oxygen is delivered to the retina via a combination of the choroidal vascular bed, which lies immediately behind the retina, and the retinal vasculature, which lies within the inner retina. The high-oxygen demands of the retina, and the relatively sparse nature of the retinal vasculature, are thought to contribute to the particular vulnerability of the retina to vascular disease. A large proportion of retinal blindness is associated with diseases having a vascular component, and disrupted oxygen supply to the retina is likely to be a critical factor. Much attention has therefore been directed at determining the intraretinal oxygen environment in healthy and diseased eyes. Measurements of oxygen levels within the retina have largely been restricted to animal studies in which oxygen sensitive microelectrodes can be used to obtain high-resolution measurements of oxygen tension as a function of retinal depth. Such measurements can immediately identify which retinal layers are supplied with oxygen from the different vascular elements. Additionally, in the outer retinal layers, which do not have any intrinsic oxygen sources, the oxygen distribution can be analysed mathematically to quantify the oxygen consumption rate of specific retinal layers. This has revealed a remarkable heterogeneity of oxygen requirements of different components of the outer retina, with the inner segments of the photoreceptors being the dominant oxygen consumers. Since the presence of the retinal vasculature precludes such a simple quantitative analysis of local oxygen consumption within the inner retina, our understanding of the oxygen needs of the inner retinal components is much less complete. Although several lines of evidence suggest that in the more commonly studied species such as cat, pig, and rat, the oxygen demands of the inner retina as a whole is broadly comparable to that of the outer retina, exactly which cell layers within the inner retina have the most stringent oxygen demands is not known. This may be a critical issue if the cell types most at risk from disrupted oxygen supply are to be identified. This paper reviews our current understanding of the oxygen requirements of the inner and outer retina and presents new data and mathematical models which identify three dominant oxygen-consuming layers in the rat retina. These are the inner segments of the photoreceptors, the outer plexiform layer, and the deeper region of the inner plexiform layer. We also address the intriguing question of how the oxygen requirements of the inner retina are met in those species which naturally have a poorly vascularised, or even totally avascular retina. We present measurements of the intraretinal oxygen distribution in two species of laboratory animal possessing such retinas, the rabbit and the guinea pig. The rabbit has a predominantly avascular retina, with only a narrow band of retinal vasculature, and the guinea pig retina is completely avascular. Both these animals demonstrate species adaptations in which the oxygen requirement of their inner retinas are extremely low when compared to that of their outer retinas. This finding both uncovers a remarkable ability of the inner retina in avascular species to function in a low-oxygen environment, and also highlights the dangers of extrapolating findings from avascular retinas to infer metabolic requirements of vascularised retinas. Different species also demonstrate a marked diversity in the manner in which intraretinal oxygen distribution is influenced by increases in systemic oxygen level. In the vascularised rat retina, the inner retinal oxygen increase is muted by a combination of increased oxygen consumption and a reduction of net oxygen delivery from the retinal circulation. The avascular retina of the guinea pig demonstrated a novel and powerful regulatory mechanism that prevents any dramatic rise in choroidal oxygen levels and keeps retinal oxygen levels within the normal physiological range. In contrast, in the avascular regions of the rabbit retina the choroidal oxygen level passively follows the increase in systemic oxygenation, and there is a dramatic rise in oxygen level in all retinal layers. The presence or absence of oxygen-regulating mechanisms may well reflect important survival strategies for the retina which are not yet understood. Intraretinal oxygen measurements in rat models of retinal disease are also presented. We describe how oxygen distribution across the rat retina is influenced by manipulation of systemic blood pressure. We examine the effect of acute and chronic occlusion of the retinal vasculature, and explore the feasibility of meeting the oxygen needs of the ischemic retina from the choroid. (ABSTRACT TRUNCATED)

An explanation of the "supernormal" b-wave in vitro.

The isolated arterially perfused eye preparation has proven to be comparable to the in vivo model in many respects. However, the existence of "supernormal" b-wave amplitudes in the perfused eyes has remained an unexplained functional difference between the two preparations. The term "supernormal" reflected the observation that at high perfusate flow rates the amplitude of the b-wave from the perfused eyes was frequently larger than that recorded in vivo under the same stimulus and adaptation conditions. Recent investigations in this laboratory have demonstrated that the position of the scleral electrode on the isolated eye greatly influences the amplitude of the b-wave obtained. The simple comparison of b-wave amplitudes in vivo and in vitro was therefore not appropriate, due to the different electrode locations used in the two situations. In addition, the relationship between perfusate flow rate and b-wave amplitude at a fixed location has been reinvestigated. In our perfusion system the b-wave amplitude has been shown to saturate at moderate flow rates (1.5 ml/min), considerably lower than those required to maximize the b-wave amplitude in earlier studies. This difference is due to the higher oxygen tension of our perfusate at the entry point to the eye. It is concluded that b-wave stability with increasing perfusate flow can be achieved in vitro, and that the apparently supernormal b-wave amplitudes observed under these conditions can be explained in terms of the different electrode environment in the in vivo and in vitro preparations. The implications of these findings with regard to autoregulation of the retinal circulation are discussed.

The effect of regional retinal photocoagulation on vitreal oxygen tension.

PO2 measurements have been made for the first time within the retina and vitreous of cat eyes to compare normal and photocoagulated areas. This was done to test the hypothesis that the observed beneficial effects of pan-retinal photocoagulation therapy in the treatment of retinal vascular diseases with an ischemic origin, may be due to more oxygen becoming available to the remaining functioning retina. A xenon arc photocoagulator was used to photocoagulate large areas of cat retinas served by one major set of vessels while leaving the remaining retina untouched. After 6 months an acute experiment was performed in which retinal and vitreal oxygen tension profiles were measured using oxygen-sensitive microelectrodes to compare PO2 profiles in normal and photocoagulated regions, for two ventilation conditions: air and 100% O2. The only differences in PO2 values were found for the 100% O2 breathing condition, where values within the retina and in the overlying vitreous were larger in photocoagulated areas. It is proposed that any differences in PO2 distribution which occur for air breathing are masked by the autoregulatory capacity of the retinal circulation and the PO2 buffering capacity of hemoglobin.

The retinal oxygen profile in cats.

This study records for the first time the retinal tissue oxygen partial pressure as a function of location within the retina of the domestic cat. Tissue pO2 was recorded with oxygen sensitive microelectrodes that use the polarographic principle. The mean vitreal pO2 close to the internal limiting membrane was 20.2 +/- 2.3 mmHg. The internal limiting membrane does not act as a diffusion barrier for oxygen. As the electrode was advanced into the inner retina, the tissue pO2 rose gradually to a value of 24.6 +/- 2.3 mmHg and then fell to a minimum of 12.0 +/- 5.5 mmHg before rising again to a value of 29.2 +/- 2.5 mmHg. Further insertion resulted in a sudden steep rise of tissue pO2 values to 72.0 +/- 5.1 mmHg, after which there was no further alteration in measured values. Although the exact location within the retina of the recording electrode was not known, it is probable that the tissue pO2 minimum occurs at about the level of the inner nuclear layer. Therefore, it is probable that the retinal avascular layers receive their oxygen supply primarily from the choroidal circulation in the cat.

PO2 profiles and oxygen consumption in cat retina with an occluded retinal circulation.

If the retinal circulation is occluded, the retina is forced to rely on the choroidal circulation for its oxygen supply. We have measured intraretinal PO2 profiles before, during, and after such an occlusion in cat. Oxygen-sensitive microelectrodes were used to measure intraretinal PO2, and the retinal circulation was occluded by means of a glass probe placed on the retinal vessels at the optic disk. Both air and 100% O2 breathing conditions were investigated. With the retinal circulation occluded, intra-retinal PO2 fell to zero within 60% of the distance through the retina, measured from the choriocapillaris to the internal limiting membrane. With the circulation occluded, but with breathing of 100% O2, PO2 rose throughout the retina so that values within the inner retina were as high or higher than for air breathing with the retinal circulation present. This meant that the whole retina could be supplied with adequate oxygen by breathing with 100% O2 in cat. From these PO2 profiles, oxygen flux and consumption were calculated as a function of distance through the retina. These calculations showed that the outer 20% of the retina had a consumption of 5.45 +/- 2.46 (SD) ml.min-1.100 ml-1 compared with a mean value for the remaining retina of 1.47 +/- 2.66 ml.min-1.100 ml-1. This difference was statistically significant (P less than 0.001) which indicates that there are at least two regions in the retina with different oxygen consumption.

The response of rat vitreal oxygen tension to stepwise increases in inspired percentage oxygen.

The effect of graded systemic hyperoxia on vitreal PO2 distribution has been determined for the rat eye. Oxygen tension profiles were measured, using oxygen-sensitive microelectrodes, as a function of distance from the internal limiting membrane as the inspired oxygen percentage was increased in 10% steps from 20-100%. Depending on the original touching location of the microelectrode on the retina, there could be substantial PO2 gradients within 500 microns of the retina; at greater distances vitreal PO2 was constant and a function of the inspired oxygen percentage. Whatever the location of the microelectrode in the vitreous, PO2 rose with increasing hyperoxia. The relationship between vitreal PO2 and inspired oxygen was nonlinear with a central relatively flat region between 50-80% inspired oxygen. The ratio between vitreal PO2 during 100% O2 breathing and air breathing was 3.42 +/- 1.08 (standard deviation, n = 7). Possible explanations for the plateau region are the maintenance of a relatively constant PO2 by vascular autoregulation and/or the buffering of capillary PO2 by hemoglobin. The rat eye, therefore, responds to hyperoxia similarly to that of the cat and monkey but differs from that of the miniature pig where there is no rise in preretinal PO2 during hyperoxia.

Intraretinal oxygen distribution in the rat with graded systemic hyperoxia and hypercapnia.

PURPOSE: To describe the nature of oxygen level changes in specific layers in the rat retina under graded levels of systemic hyperoxia, with and without hypercapnia. METHODS: Oxygen-sensitive microelectrodes were used to measure oxygen tension as a function of depth through the retina of anesthetized, mechanically ventilated rats. Breathing mixtures were manipulated to produce stepwise increments in systemic oxygen levels, with or without 5% CO2. Retinal arteriovenous oxygen differences were also measured as an indicator of oxygen delivery through the retinal circulation. Systemic blood gas levels were measured under each condition. RESULTS: Hyperoxia increases PO2 throughout the retina to a varying extent in different retinal layers, with the increase more pronounced in the outer retina than in the inner retina. Simultaneous hypercapnia results in further increases in retinal oxygen levels. The lowest intraretinal oxygen level was consistently found in the inner plexiform layer (IPL), between the two capillary layers that support this region. There was a greater than fourfold increase in oxygen supply from the choroid with hyperoxia but, remarkably, the retinal circulation continued to provide a net delivery of oxygen to the retina. CONCLUSIONS: Hyperoxia results in a significant but nonuniform increase in oxygen level in all layers of the rat retina, which is augmented by hypercapnia. The persistence of a minimum oxygen level in the IPL, despite the dramatic increase in oxygen flux from the choroid, suggests that oxygen consumption increases significantly in the IPL under hyperoxic conditions.

Light and choroidal PO2 modulation of intraretinal oxygen levels in an avascular retina.

PURPOSE: To determine the influence that choroidal oxygen level and outer retinal oxygen demand have on oxygen availability to the inner retina when the choroid is the only source of retinal oxygenation. This condition prevails in avascular retinas and in vascularized retinas suffering vascular occlusion. METHODS: Oxygen-sensitive microelectrodes were used to measure the oxygen tension as a function of depth in the naturally avascular retina of anesthetized and mechanically ventilated guinea pigs (n = 6). Choroidal PO2 was manipulated by varying the ventilation gas mixture, and outer retinal oxygen consumption was modulated by light-dark adaptation. Individual PO2 profiles were fitted to a multilayer mathematical model of PO2 distribution, and pairs of profiles at different choroidal PO2 levels, or under light and dark conditions, were fitted to an intraretinal PO2 difference model. Both models reflect the purely choroidal supply of retinal oxygenation. RESULTS: An increase in choroidal PO2 produced an equivalent increase in all retinal layers. Light induced a decreased oxygen consumption in the region of the inner segments of the photoreceptors, which resulted in a significant increase in PO2 in this layer, flowing on unattenuated to all inner retinal layers. The intraretinal PO2 distribution and the light- and ventilatory-induced changes in PO2 were consistent with theoretical predictions of the mathematical models. CONCLUSIONS: The present experimental studies confirm that when the choroid is the only source of retinal oxygenation, the full effect of increased choroidal oxygen level or reduced uptake in the outer retina passes through to the inner retinal layers if the oxygen utilization by the inner retina remains constant.

A new method for oxygen supply to acute ischemic retina.

This paper introduces a new method for supplying oxygen directly to ischemic inner retina, using an oxygen source in the vitreous. Acute retinal vascular occlusion was created in cat eyes by direct pressure on the optic disk and its margins with a glass probe. The satisfactory occlusion of the retinal vessels was documented by direct observation, and functionally by recording the ERG. The vascular occlusion caused a large decrease in the size of the ERG b wave, with no change in the a wave amplitude. The oxygen source was a catheter made of strands of an oxygen-permeable membrane which was inserted into the vitreal cavity. After successful vascular occlusion was documented, 100% gaseous oxygen was perfused through the catheter while recording the ERG. In response to the perfused oxygen the b wave partially recovered. Ventilating the animal with 100% oxygen when the retinal vessels were occluded also caused recovery of the b wave amplitude. Termination of the vitreal oxygen source caused a decrease in b wave amplitude to the level previously observed after the occlusion of the retinal vessels. When the retinal circulation was restored by removal of the glass probe the b wave recovered. The results show that it is possible to supply adequate oxygen to the inner retina via the vitreous to replace the oxygen normally supplied by the retinal circulation. Modification of this method may be useful for the treatment of recent and incomplete retinal vascular occlusion.

Intraretinal oxygen levels before and after photoreceptor loss in the RCS rat.

PURPOSE: To measure the intraretinal oxygen environment at different stages in the Royal College of Surgeons (RCS) rat model of retinal degeneration to determine whether changes in oxygen level are an important aspect of the disease. METHODS: Oxygen-sensitive microelectrodes were used to measure oxygen tension as a function of depth through the retina of anesthetized, mechanically ventilated RCS rats at ages ranging from postnatal day (P)20 to P104. The oxygen profiles were correlated with histologic observations of the cellular changes within the dystrophic retinas and compared with those in RCS-rdy(+) control animals and published values in normal mature rats. RESULTS: Although the youngest rats studied exhibited some differences in intraretinal oxygen distribution compared with mature animals, the distribution in dystrophic RCS rats at P20 was not significantly different from that in age-matched control subjects. However, the intraretinal oxygen distribution in dystrophic RCS rats was clearly affected after approximately P30, reflecting a loss of photoreceptor oxygen consumption consistent with histologic observations. In contrast, oxygen uptake by the inner retina was still evident long after the loss of photoreceptors was essentially complete. CONCLUSIONS: There was no significant tissue hypoxia during photoreceptor degeneration in the dystrophic RCS rat. The changes in intraretinal oxygen distribution are consistent with the loss of outer retinal oxygen uptake but the preservation of inner retinal oxygen metabolism.

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.

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.

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.

Pharmacokinetics of intravitreal injection. Assessment of a gentamicin model by ocular dialysis.

Intravitreal drug administration is the treatment of choice for bacterial endophtalmitis, but improved knowledge of vitreal pharmacokinetics is essential for the development of optimal antibiotic regimes. We used our recently developed sampling device to estimate vitreal gentamicin concentrations for up to 30 hr after an intravitreal bolus injection of gentamicin. The device is based on the principle of dialysis, whereby a constant flow rate of dialysate through a loop of dialysis fiber in the vitreous attains a gentamicin concentration proportional to the intravitreal gentamicin level around the fiber. The dialysate is continuously recovered and the collected samples then assayed for gentamicin. Normal cat eyes and those with induced bacterial endophthalmitis formed the two groups tested. Concentration-time data fitted well to an open single compartment pharmacokinetic model that incorporated the processes of transfer of drug from the injection site to the sampling site (a function of diffusion within the vitreous), and the elimination from the sampling site (a function of elimination from the vitreous). The initial phase of transfer between the injection and sampling site was rapid and rates were comparable in the two groups. Elimination rate constants were uniformly greater in infected eyes than in controls (0.107 hr-1 compared to 0.055 hr-1). Aqueous humor gentamicin concentrations in control eyes varied between 3 and 6 times those found in fellow infected eyes at the end of each experiment. Accelerated elimination of gentamicin from the vitreous body of eyes with endophthalmitis may be explained by increased permeability of the blood-retinal barrier.

Oxygen reactivity of the feline isolated ophthalmociliary artery.

PURPOSE: The goal of this study was to investigate the modulating role of oxygen tension on noradrenaline (NA) and KCl responses in the ophthalmociliary artery and to ascertain whether these effects are mediated by the endothelial cells. METHODS: The isometric tension generated by myograph ring segments activated by NA or KCl was measured as the PO2 of the bathing solution was decreased in discrete steps from 506.1 +/- 16.0 mmHg to 29.4 +/- 1.4 mmHg in preparations with and without endothelial cells. RESULTS: Vessels pre-activated with K+ Krebs solution were insensitive to oxygen tensions between 506.1 +/- 16.0 mmHg and 124.6 +/- 4.2 mmHg. Lower PO2s caused a graded and increasing contraction that reached 176 +/- 12% of the contraction at the highest PO2. Vessels pre-activated with NA had a dichotomous response to reductions in oxygen tension: 44% of vessels showed a graded contraction, whereas a graded relaxation was observed for the remaining 56% of vessels as bath PO2 was reduced. In all cases, a functional endothelium was demonstrated. However, deliberate disruption of the endothelium caused all vessels pre-activated with NA to exhibit a consistent graded contraction for PO2s below 124.6 +/- 4.2 mmHg, similar to that observed for endothelial intact vessels pre-activated with K+ Krebs. The acetylcholine dose-response curve and passive tension were not affected by changes in oxygen tension. CONCLUSIONS: Endothelial cells modify the intrinsic smooth muscle response to a gradual reduction in PO2 by releasing relaxing and contracting factors, causing the observed dichotomous response in NA-activated vessels. However, the KCl-induced response is modulated only by low oxygen tensions.

Retinal blood flow by hydrogen clearance polarography in the streptozotocin-induced diabetic rat.

PURPOSE: The authors compared retinal blood flow in rats after 5 weeks of streptozotocin (STZ)-induced diabetes with that in age-matched control animals. METHODS: The flow measurements were based on the hydrogen clearance technique and the intraocular placement microelectrodes at the surface of the retina. The hydrogen was delivered by bolus injection (100 microliters) of hydrogen-saturated saline into the ipsilateral carotid artery using a cannula through the lingual artery. The rats were anesthetized and artificially ventilated. Care was taken to match the systemic blood pressure and blood gases in the two groups. RESULTS: The mean retinal blood flow in the STZ group after 5-6 weeks duration of hyperglycemia was 487 +/- 59 ml/min/100g (standard error) compared with 330 +/- 16 ml/min/100 g in the age-matched controls. The variation in retinal blood flow was far more pronounced in the STZ group, even in different locations in the same eye. Changes in fundus appearance were also noted, with second-order arterioles being more apparent and the retina more "pinkish" in appearance in the STZ animals. CONCLUSIONS: The mean retinal blood flow in the region of retina studied in the two groups was significantly higher in the STZ animals than in age-matched controls. The increased heterogeneity of retinal blood flow may reflect a disruption to the normal blood flow control mechanisms in the retina after only 5 weeks of STZ-induced diabetes.

Agonist response of human isolated posterior ciliary artery.

The isometric responses of isolated human posterior ciliary artery to adrenergic agonists, histamine (HIS), and 5-hydroxytryptamine (5-HT) were studied in passively stretched ring segments mounted in a myograph bath. Cumulative dose response curves were measured for nine agonists: HIS, 5-HT, dopamine (DOPA), epinephrine (A), norepinephrine (NA), tyramine (TYR), phenylephrine (PHE), isoproterenol (ISOP), and xylazine (XYL), and the log(molar concentration) at which one half of the maximum active tension was developed (EC50) was estimated. The ring segments were unresponsive to DOPA and XYL; HIS and ISOP produced biphasic responses with a mild relaxation for low concentrations and small contractions for high concentrations of the agonist. The remaining agonists caused contractile responses of magnitude listed in the rank order following compared with the maximum active tension in response to 0.124 M K(+)-Krebs: Kmax much greater than A greater than 5-HT = PHE greater than NA greater than TYR It was concluded that functional HIS, alpha 1-adrenergic, and 5-HT receptors were present on human posterior ciliary artery but that there are no alpha 2-adrenergic receptors.

Comparison of the vasoactive effects of the docosanoid unoprostone and selected prostanoids on isolated perfused retinal arterioles.

PURPOSE: To compare the vasoactive properties of the docosanoid unoprostone, its free acid, and different members of the prostanoid family on isolated perfused pig retinal arterioles to assess their potential to modulate retinal blood flow. METHODS: Segments of porcine retinal arterioles were dissected, cannulated, and perfused, and their diameter monitored during either intraluminal or extraluminal application of increasing doses (10(-10)-10(-4) M) of either the docosanoid unoprostone isopropyl and its free acid or of selected prostanoids: prostaglandin (PG) F(2alpha) and thromboxane A(2) analogue (U46619). Studies were performed on arterioles in their uncontracted state, and also during precontraction with endothelin-1 (10(-9) M). The significance of any induced change in vessel diameter was assessed in relation to the initial vessel diameter or, in the case of endothelin-1 administration, to the contracted diameter with endothelin-1 alone. RESULTS: In normal-tone arterioles without endothelin-1 contraction, PGF(2alpha) and U46619 both produced a potent dose-dependent contraction, but neither unoprostone isopropyl nor unoprostone free acid had a significant vasoactive effect. In endothelin-1-contracted arterioles, U46619 produced further contraction, PGF(2alpha) produced a slight vasodilatation, and unoprostone isopropyl and its free acid produced a pronounced dilatation. CONCLUSIONS: Of the agents tested, unoprostone isopropyl and its free acid were the most potent vasodilators of endothelin-1-contracted pig retinal arterioles. Members of the prostanoid family demonstrated a different effect on the diameter of isolated retinal arterioles compared with the docosanoids. The potential therefore exists for the docosanoid unoprostone to have a beneficial effect on retinal blood flow in addition to any reduction in intraocular pressure.

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.

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.