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E N Su

Publications and source records attributed to E N Su.

At least 37 records · Page 2Linked to original sources
PubMed1998

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.

Animals↗
PubMed1997

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.

Animals↗
PubMed1996

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.

Adenosine↗
PubMed1996

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.

Animals↗
PubMed1996

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.

Animals↗
PubMed1996

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.

Animals↗
PubMed1995

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.

Angiotensin II↗
PubMed1994

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.

Animals↗
PubMed1994

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.

Acetylcholine↗
PubMed1994

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.

Adrenergic alpha-Agonists↗
PubMed1994

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.

Animals↗
PubMed1993

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.

Animals↗
PubMed1993

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.

Animals↗
PubMed1992

Oxygen tension and blood flow in the retina of normal and diabetic rats.

The use of microelectrode techniques for studying oxygen distribution and blood flow in the eye of a physiologically well maintained rat provides a very convenient model in which to study oxygen supply to the retina. The availability of rat models of vascular disease such as diabetes and hypertension, and the existence of several models of retinal degeneration, make studies of oxygen supply in the rat eye of particular relevance. The experiments reported in this paper demonstrate changes in oxygen distribution and blood flow very early in STZ induced diabetes. Thus, we have established a preparation in which the role of changes in oxygen supply can be correlated with the pathological events that are apparent later in the disease.

Animals↗
PubMed1992

Pharmacological and mechanical heterogeneity of cat isolated ophthalmociliary artery.

The mechanical, histological and pharmacological properties of the isolated cat ophthalmociliary artery are reported and compared for passively stretched ring segments from three locations on the artery. Ring segments were mounted in a modified myograph system after the method of Hogestatt, Andersson and Edvinsson (1983, Acta Physiol Scand. 117, 49-61) and comparative length tension curves were measured. The most distal segment (DOA), nearest the eye, developed by far the smallest maximal tension compared to the middle segment (MOA) and the most proximal segment (POA), despite the fact that lumen diameters were similar. All segments displayed a phasic and tonic component in response to passive stretching and during activation with K(+)-Krebs. Cumulative contractile dose-response curves were measured for nine agonists: histamine (HIS), 5-hydroxytryptamine (5-HT), dopamine (DOPA), adrenaline (A), noradrenaline (NA), tyramine (TYR), phenylephrine (PHE), isoproterenol (ISOP) and xylazine (XYL) and the maximum active tension developed at a concentration of 10(-3) M was compared with that for 0.124 M K(+)-Krebs. The order of contraction magnitude in the three segments was: POA, HIS much greater than Kmax much greater than NA greater than A greater than DOPA greater than PH = TYR greater than 5-HT = ISOP greater than XYL; MOA, Kmax much greater than NA much greater than A = HIS much greater than DOPA greater than ISOP = PHE greater than TYR = 5-HT greater than XYL; DOA, Kmax much greater than NA greater than A greater than HIS = DOPA greater than ISOP greater than PHE = 5-HT greater than XYL = TYR. Pre-activation did not produce relaxation in response to histamine and isoproterenol. Testing with specific alpha 1 and beta antagonists supported the presence of alpha 1 receptors but not beta receptors. It is concluded that functional HIS, alpha 1 and 5-HT receptors are present and that the ophthalmociliary artery is heterogeneous in its response to HIS, with the POA segment producing the largest responses. The magnitude of the contractile responses to A and NA grew with increasing passive tension, whereas the response to histamine was independent of passive tension.

Adrenergic Agonists↗