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Ocular effects of clonidine in cats and monkeys (Macaca irus)
Exp. Eye Res. (1975) 21, 481-488
Ocular Effects of Clonidine in Cats and Monkeys (Macaca irus)
(Received 12 Mn,y 197??. Hostott ) In monkeys administration of about 0.01 pg clonidine into t’he ant,erior chamber caused no significant change in the facility of outflow. At 10 pg/kg body weight given intravenously from a level of O-51 10.05 ~1. the facility decreased by 0.09*.0.01@ . min- 1 mmHg-’ min-l * mmHg-l. The same dose had no significant effect’ on the rate of aqueous humor formation determined with an indicator dilution technique. Before the administration of t,he drug the formation of aqueous humor was 1.4350.13 $/min, and 30 min later it was 1.32&0.15~l/min. The fall in eye pressure was 1.0-i-O-3 mmHg. In cats Z-lOpg/kg body weight clonidine given intravenously reduced the intrascleral venous pressure and the eye pressure in a way which indicated marked va.soconstriction in the eye. Unilateral carotid injection of the drug at a dose reducing the homolateral eye pressure by P5 mmHg caused a in the blood flow through the 19% reduction in the choroidal blood flow, a 34O, reduction iris and a 4276 reduction in the blood flow in the ciliary body when t*he injected side a-as compared to t.he other.
1. Introduction Clonidine is a drug widely usedin the treatment of arterial hypertension. The pressure lowering effect of the drug is due to activation of adrenergic receptors in the central nervous system which causesreduced activity in the sympathetic nervous system (Kobinger and Walland, 1967). The drug also has effects on peripheral adrenerpie mechanisms: it inhibits adrenergic transmission by a specific interference with the norepinephrine release (Starke, Wagner and Schiimann, 1972) and it also stimulates the post-ganglionic alphareceptors (Kobinger and Walland, 1967). After intravenous administration of the drug the latter effect causesa transient rise in the blood pressnrc which then falls as a result of the other effects. Makabe (1966) reported that intravenous injection of clonidine causesa slight fall in the eye pressurein individuals with normal eyes but a more marked fall in the eye pressurein patients with glaucoma. Tonography indicated that the reduction in eye pressurewas due to a reduced formation of the aqueoushumor. A study on the effects of clonidine given as eye drops (Hasslinger, 1969) indicated that the drug also had an effect on the eye pressurewhen given this way. Tonography indicated that the reduction in eye pressure was due to an increase in the facility of outtlow. Further studies on the effects of clonidine given as eye drops (Heilmann: 1970) indicated that there was a very rapid absorption of the drug from the conjunctiva and the nasal mucosa. Clinically useful reductions in eye pressure were obtained only at doses which also had effects on the blood pressure. At a recent symposium Sayegh and Weigelin (1974) concluded that the effects of clonidine could not be explained only by a reduction in the blood pressurein the ophthalmic artery. What other mechanismsmight be involved was not clear. Clonidine hasbeen reported to causea redistribution of the cardiac output and even low dosestend to causevasoconstriction in some vascular beds such as the skeletal muscle and the skin and gastro-intestinal organs. However, the blood flow to other
4st’
organs
.\.
such as the kidneys
lSlI,I.
.\Sl)
and the liver
K.
HEILMASn-
via the hepatic
artery
is increased
(Rolnlc.
Tshizaki, Forsyth and Melmon, 1973). A possible explanation for the eye JWSWIY~ lowering effect of the drug thus could be vasoconstriction in the eye (.Jiinemann. JYX). The purpose of the experiment,s reported here wrasto study the effect! of cloni(lint~ in experimental animals. Effects on the rate of aqueous formation and facility of outflow were studied in monkeys. Effects on the intrascleral venous pressureand t&t> regional
blood
flow in the rye were studied
in cats.
2. Materials and Methods E.zperiments
in monkeys
Cynomolgus monkeys of both sexes and weighing between 1.8 and 3.3 kg were used. The monkeys were anesthetized with sodium methohexital (Brietal Sodium, Lilly) 5(~100 mg given intramuscularly. Anesthesia was maintained with small doses of sodium pentobarbital given every 20-30 min. The animals were placed prone and kept warm with a heating pad. The mean arterial blood pressure was measured in a femoral artery. Effects of intracameral clonic&e. Each eye was connected to a pair of push-pull coupled syringes through needles shot into the eye with a needle gun. A third cannula connected each eye to a reservoir the weight of which was determined continuously (BBrriny, 1964). A T-connection on one of the tubings from the eye to the push-pull syringes connected the anterior chamber to a pressure t,ransducer. On one side the syringes contained 0.1 tug clonidine/ml mock aqueous humor, and on the other side there was only mock aqueous humor (BBrliny, 1964). When the intraocular pressure had stabilized after the cannulntiou, gross outflow facilities (Bill and Bgr;inp, 1966) were det’ermined by measuring the rate of inflow from the reservoirs at two different heights and dividing the change in the inflow rate by the change in the intraocular pressure. Each pressure level was maintained for at least 7 min. Spontaneous pressures were then measured again for about 15 min and then t,he contents of the anterior chambers were mixed with those of the push--pull coupled syringes. Fifteen to 20 min later the facility of outflow was determined in both eyes. In three of these experiments facility measurements were continued for l-2 hr and then IO pg clonidine/kg body weight WAS given intravenously and a last facility determination was made 10-15 min later. Effects of clmidinr on aqueous Aurn.or fortnation. The rate of aqueous humor formation was determined in one eye with an indicator dilution technique described previously (Bill, 1971). The other eye was also cannulated and in some experiments its spontaueous pressure was determined with a pressure transducer; in other experiments the outflow fa.cility was determined. The mean arterial blood pressure was measured in a, cannulated femoral artery. When the rate of aqueous humor formation had been steady for ;It lea,xt 30 min, 10 &kg body weight clonidine was given intravenously over l--IO min. Experiments
in cats
Effects of clonidine OS the eye presswe and the ,intrascleral venous pressure, Cats of both sexes and weighing between 2.2 and 3.3 kg were used. Anesthesia was induced with chloroform and maintained with sodium pentobarbital or chloralose supplemented with urethane. The animal was placed prone and kept warm with a heating pad. The temporal part of the sclera was exposed and a small slit was made through the sclera into one of the branches of the intrascleral venous plexus. To measure the intrascleral venous pressure the tapered tip of a polyethylene tubing was introduced into the vein in such a way that the open end pointed towards the large superior anterior ciliary vein that drains the superior part of the venous plexus (Bill, 1962a). The pressure in the plexus was measured with a pressure transducer. A cannula connected to another pressure transducer was shot into the anterior chamber for measurements of the intraocular pressure. The arterial blood
EFFECTS
OF
CLONIDIXE
483
pressure was measured through a cannulated femoral artery. When the pressures had been stable for at least 5 min, 2-10 pg/kg body weight clonidine was injected intravenously. Effects of clonidineon thebloodjow in theeye.Cats anesthetized as mentioned above were tracheotomized and artificially ventilated. Ventilation was adjusted to give normal values for the oxygen tension, the pH and the P ~0~. A thyroidal artery was isolated and can-
nulated in such a way that injections could be madeinto the commoncarotid artery. The heart was exposed through a thoracotomy and a cannula connected to a polyethylene tubing was introduced into the left ventricle. The eye pressure in both eyes were measured with pressure transducers connected to the eyes by needles and polyethylene tubings. A O+)Olq/, solution of clonidine was given intra-arterially at a rate of about 0.1 ml/l0 set-. When a fall in eye pressure was observed on the side of the injection, the injection was stopped and l-2 ml of a l”/b suspension of 15 pm *5Sr labelled microspheres (3 M Company, St Paul, Minnesota) were injected into the left. heart. A reference flow (Alm and Bill, 1972) of about, l-2 ml/min was obtained from one femora,l artery. The blood pressure was monit,ored via the other. One min after the start of the inject’ion of the labelled spheres the animal was put to death with a sat,urated solution of KC’1 given into the heart. The eyes \\-ere dissected and regional blood flow rates were determined as described in a previous report (Altn and Bill, 19’72).
3. Results E$ects on facility of outjiow
The effects of intracameral administration of about 0.01 pg clonidine on the faciiity of outflow were determined in 10 eyes. The facility of outflow before the administration of clonidine was 0.35f0.05 $*min-l*mmHg-l, after clonidine it was 0.38&0.03 ~1. minel*mmHg-l . In the control eyes the facility values were 0*37&0*3 $*rnin-l* mmHg-l and 0.39-&0*03$*min-l*mmHg-l, respectively. The spontaneous change in the facility of outflow in the control eyes t,hus was almost the same as the change in the treated eyes.
0
o-2 Facility
0.4
0.6
O-8
I,pl.min-‘.mmHg-’
PIG. 1. Facility I was determined before the intravenous injection of 10 pg/kg body weight facilit,y II was determined lo-15 min after t,he injection. The line shown is the identity line.
The effect of intravenous
administration
of clonidine
in eyes not previously
clonidinr.
injected
with the drug was also determined in 10 animals. Figure 1 shows a plot of facilities before and after the administration of the drug. Ten pg/kg body weight reduced the mean facility of outflow from 0*51+0*05 ~l*min-l*mmHg-l mmHg-l. The fall in facility was 0*09+0*01 $min-l*mmHg-l. significant at the P < 0.001 level.
to 0.43 *O-O5 dmin+ The change was
The effects of int.rHvr>nousinjection of 10 pg/kg l~orty weight cionitlinc~ was iii vestigatetl in 17 aninlals. There was a biphasic efiect, on thr I~lootl prrlssurcl -first ;i
transient rise over a few lllinut,es and then a.long-lasting fall in the pressure. The +J!.ti pressure fell l*O+W3 mmHg after the injection of the clonidine (11= 11). Thr fall watt statistically significant. P < O-01. There was no con&em? change in the rate ot aqueous formation. The mean flon just I)eforc t,htx inject,ion of the clonidint~ \~a\ 1.4310.13 #min. The mean arterial blood pressure at this time was X8+:7 rnrnHg. Thirty min later when t,he blood pressure was about, 20 mmHg lower. tht IIICHII rates of aqueous hnnlor forniation was 1-Z +0.15 ~l/iiiin.
?‘r IOP
15 ,o T P w5 8 - Ok---
5m1n
Effects of dondine
ON pye pressure
md ~ntmsclernl
veytous pressure
In cats intravenous injection of 2-10 tug clonidine per kg body weight causeda longlasting fall both in the eye pressure and the intrascleral venous pressure. Figure 2 shows the effect of 2 pg/kg body weight’ in one animal. The fall in eye pressure and venous pressure had already started during the phase of increased arterial blood pressure and lasted for the rest of the experiments~--that is for more than 60 min. The fall in IOP was faster than the fall in the intrascleral venous pressure. At, 15-30 min after the injection of the drug both the eye pressuresand the intrascleral venous pressureswere relatively constant. In three of five casesthe two pressure> were reduced by about the sameamount,. In two eyes the fall in the eye pressurewalmore marked than t,he fall in the intrascleral venous pressure(Table I).
Values of intraocular pressure, IOP, intrascleral 2‘emzi.s pressure, ISVP: and mewtt arterial blood pressure, MAP, before and k-30 W&Lafter the injection of 2-10 ~glk~q body weight clonidine in Jive cats ISVI’ mmHg
IOP
Before
21 40 26 32 19
mmHR
After
17 23 23 18 16
Before
12 18 14 23 12
MAP mmHg Aft,rr
Before
9 8 11 9 x
107 129 154 I69 184
Bftel
74 81 147 96 147
EFFECTS
Effects qf clom&ae
on the regional
OF
C’LONIDIRE
4x5
blood fEow in the eye
In cats t.he intra-arterial injection of l-3 pg clonidine over 15-30 set causeda fall in the homolateral eye pressure of 4-5 mmHg but little change in the pressure of the other eye and in the blood pressure.Figure 3 showsthat in the 10 animals investigated the blood flow in the eye on the t,reated side was reduced compared to that on the control side. The fall in blood flow was marked in the iris and the ciliary body. 33*8&6*5°h and 42.4f4.2% in the two tissues respectively. It was statistically significant in both tissues P < 0.001. In the choroid the fall in blood flow was 18.6+f*l”/,. P .’ 0.005. There was no significant effect on the ret,inal blood flow.
Ic’rc:. 3. The effect of clonidine on re,+xml blood H
via R side.
4. Discussion In cats a great part of the blood leaving the urea is drained into an intrascleral venous plexus (Bill, 1962a). The pressurein this plexus tends to fall if the blood flow through the uvea is reduced (Bill, 1962a,b). The aqueoushumor is also drained into the venous plexus (Bill, 1962a, 1966). Changesin t.he venous pressuretherefore cause parallel changesin the eye pressureif the aqueousflow into the plexus and t,he outflow facility remain constant. The experiments in cats with determination of the regional blood flow in both eyes after unilateral injection of clonidine demonstrated that the drug causesrapid and pronounced vaeoconstrict,ion in the iris and the ciliary body but a lesspronounced vasoconstriction in the choroid. Vasoconstriction can be assumedto affect the eye pressurein two different ways. One is by reducing the blood volume in the eye. The other is by reducing the intrascleral venous pressme. The former effect is transient since blood lost from t,he vascular compartment is soon compeasated by an increase in the volume of the aqueoushumor within the eye. The latter effect of clonidine will remain as long as the vasoconstrict,ion remains. The reasonis that aslong as the blood flow is reduced there is also a reduced intrasclelal venous pressure. In the cat experiments the rapid fall in eye pressurewasno doubt due t,o a reduction in the intraocular blood volume. The longstanding reduction in pressureoccurred very much in parallel with the reduction in the intrascleral venous pressure and can be assumedto be caused entirely by the latter in three of five animals. In two animals the fall in eye pressure was greater than the fall in venous pressure.The reason for this is not clear. One possibility is that the venous pressure measuredwas not representative for the average venous pressure in the plexus affecting the outflow of the aqueous humor. This is quite possiblesince there are considerable variations in pressurein the plexus from one part to anot,her (Bill, 1962a). Another possibility is that in two animals the F
4%
.i.
KILI,
:1X1)
K.
HEILMAKK
drug had an effect on the facility of outflow or the formation of aqueous humor. ‘I’hr combined effect of a vasoconstriction in the ciliary body and a fall iu the perfusion pressure caused by a marked fall in t’he blood pressure---not compensated 11ya sinlilal fall in the eye pressure---may he a reduced rate of aqueous format.ion. In monkeys the effects of clonidine on the eye pressure were much stnaller than iu cats-the fall being only about 1 mmHg. Determinations of the effects of the drug on the outflow facility and the rate of aqueous humor formation gave no explanabions for the fall in the eye pressure. It seems likely therefore that in monkeys the fall in eye pressure was also secondary to intraocular vasoconstriction and the retluced perfusion pressure, resulting in a fall in the pressure in the anterior ciliary veins. In monkeys intracameral clonidine had no significant effect on the facilit,y of outflow. Intravenous injection of the drug on the other hand seemed to cause a fall in the facility of outflow. Spont,aneous changes in the facility of outflow (luring experiments of the type conducted usually are in the opposite direction. i.cl. thr facility tends to increase. For that reason it seems unlikely that the chanail after clonidine was due to a spontaneous change. It thus seems that, if anvthing. the drug tends to decrease the facility of outflow. A possible mechanism for thiq decrease is relaxation of the ciliary muscle resulting in partial closing of the trabecular meshwork. Stimulation of the sympathetic nerves- to the eye has been demonstrated to reducr facility of outflow probably by this effect (Casey, 1966). Norepinephrinr released under such conditions and the clonidine used in the present experiments may affect t,he samC adrenergic receptors. Previous studies have shown that there are adrenergic alpha-receptors in the hlooti vessels of the ciliary body and the choroid (Bill, 196%). In the retina. on the other hand, there seem to be no alpha-receptors available to respond to the intra-arterial administration of norepinephrine (Aim, 1973). The effects of clonidine on the regional hlood flow in the eye are compatible with this pattern of alpha-recept*or distribution. In clinical practice clonidine has been used to reduce the eye pressure mainly in the treat,ment of open-angle glaucoma but also in acute angle-closure glaucoma,. The present experiments suggest that in angle-closure glaucoma the pressure lowering effect is due to intraocular vasoconstriction and a fall in the blood pressure. Thestb changes cause a reduction in the intraocular blood volume and a fall in the intrascleral venous pressure, resulting in a marked fall in the eye pressure. It heems very possible that in cases with high y?e pressure the vasoconstriction may cause such a reducetl blood flow through the clhary processes that the rate of aqueous secret-ion is reduced which would contribute to the fall in the eye pressure. The use of clonidine in cases of acute angle-closure glaucoma to cause a singular reduction in the intraocular pressure for example as before surgery may be of great advantage (Jiinemann, 1974) and is unlikely to be hazardous. The use of the drug in long-term treatment of open-angle glaucoma, on the other hand, must be questioned. The usefulness of the drug will in fact depend on the mechanism causing the degeneration of the optic nerve head. This mechanism is not clear. At least two alternatives have to be considered (Anderson and Hendrickson, 1974). If degeneration is due to an insufficient blood supply to the optic nerve head as suggested by Gafner and Goldmann (1955), Hayreh (1969). Heilmaml (1972) and Aim and Bill (1973) bhe drug will be valuable only if it increases the hlood flow to the optic nerve head-----which seem!: rather unlikely. If degeneration of the nerve head is due to other factors such as a direct mechanical effect on the nerve fihres at or near the lamina cribrosa, the drug may be of great value in relieving the pressure.
EFFECTS
OF
CLONIDINE
487
If the vascular theory is correct, drugs causing vasoconstriction in the ciliary arteries and their branches and a fall in blood pressure may even be hazardous. The reason is that the major part of the optic nerve head is supplied from arterioles entering the nerve head from the choroid (Hayreh, 1969; Anderson, 1970). A fall in blood pressure in these arterioles and vasoconstriction in these vessels thus may tend to reduce the blood flow through the optic nerve head even if the intraocular pressure is reduced. As long as the vascular theory for the damage of the optic nerve head in glaucoma has not been refuted and the possibility remains that clonidine may reduce the blood tlow in the optic nerve head, it would seem unwise to use the drug without frequent checking of the visual field. This is true for the use of clonidine in the treatment of open-angle glaucoma as well as for its use in arterial hypertension. That the mode of action of drugs used in glaucoma may be important, and not only the fall in eye pressure that they cause, is indicated by the results obtained by Heilmann. A comparison (Heilmann, 19’72,1974) between the effects of acetazolamide and clonidine in glaucoma patients with field defects indicated that for the same fall in eye pressure produced by the two drugs, there was some reversal of the field defects with acetazolamide but not with clonidine. Acetazolamide unlike clonidine exerts its effect directly on the secretory mechanisms in the ciliary processes and has no demonst,rable effect on the uveal blood flow (Bill, 1974). A\CKNOWLEDGMENTS This work was supported by PHS grant EY-00475 from the National Eye Institute and by a grant B75-14X-147 from the Swedish Medical Research Council. We thank Mr U. Ullstriim, Miss M. Thor&n, Mrs A. ostberg and Mr C. Geijer for valuable technical assistance. REFERENCES Xlm,
A. (1972). Effects of norepinephrine, angiotensin, dihydroergotamine, papaverine, isoproterenol, histamine, nicotinic acid and xantinol nicotinate on retinal oxygen tension in
cats. Acta Opht?u&nol. 50, 707. A. and Bill, A. (1972). The oxygen supply to the retina, II. Effects of high intraocular pressure on uveal and retinal blood flow in cats. A study with labelled microspheres including flow determinations in brain and other tissues. Acta Physiol. Scand. 84, 306. Alm, A. and Bill, A. (1973). Ocular and optic nerve blood flow at normal and increased intraocular pressures in monkeys (Xacuca irus); a study with radioactively labelled microspheres including flow determinations in brain and some other tissues. Exp. Eye Rea. 15, 15. Anderson, D. R. (1970). Vascular supply to the optic nerve of primates. Amer. J. Ophthnlmol. 70,341. Anderson, D. R. and Hendrickson, A. (1974). Effect of intraocular pressure on rapid axoplasmic transport in monkey optic nerve. Invest. Ophthalmol. 13, 771. B&r&y, E. H. (1964). Simultaneous measurements of changing intraocular pressure and outflow facilit,y in the vervet monkey by constant pressure infusion. Invest. Ophthalmol. 3, 135. Bill, iz. (1962a). Aspects of the drainage of aqueous humor in cats. A.N.A. Arch. Ophthnlmot. 67, 148. Bill, A. (196213). .4 method for quantitative determination of the blood flow through the cat uvea. A.M.A. Arch. Ophthulmol. 67, 156. Bill, A. (1962c). Butonomic nervous control of uveal blood flow. Acta Physiol. Stand. 56, 70. Bill, A. (1966). Formation and drainage of aqueous humor in cats. E.c~. Eye Res. 5, 185. Bill, A. and B&rBny, E. H. (1966). Gross facility, facility of conventional routes and pseudofacility of aqueous humor outflow in the cynomolgus monkey. A.M.A. Arch. Ophthalmol. 75,665. Bill, A. (1971). Effects of longstanding stepwise increments in eye pressure on the rate of aqueous humor formation in a primate (Cercopithecw aethiops). Exp. Eye Res. 12, 184. Bill, A. (1974). Effects of acetazolamide and carotid occlusion on t’he ocular blood flow in unanaesthetized rabbits. Invest. Ophthalmol. 13, 954. Alm,
4xs
A. lSIl,I,
,iNL)
K. HEIl,M.~XX’
Bolme, P., Ishizaki, T.. Porsyth, R. P. and Melmon, K. 1,. (1973). Haemodynamic effect,s and changes in blood flow distribution caused by clonidinr in the conscious monkey. A&n I’7qsioI. &and. (suppl.) 3%,97. Casey, W. (1966). Cervical sympat)hetic stimulation in monkeys and the effects on out,flow f:rrility and intraorular volume. A study in the East African vorvet (Cercopifh~ecus a~fhiops). 7nwst. Ophthnlmol.
5, 33.
Gafner, F. and Goldmann, H. (1955). Experimentelle Unt,ersuchungen iiber den Zusammenhang von Augendrucksteigerung und Gesichtsfeldschgdigung. Ophthalmologica 130, 357. Hasslinger, C. (1969). Catapresan [2-(2,6-Dichlorphenylamino)-2-Imidazolin-hydrochlorid~ -ein neues augendrucksenkendes Medikament,. Klin. Monotsbl. Augenheilk. 154, 9.5. Hayreh, S. S. (1969). Blood supply to the optic nerve and its role in optic atrophy. glaucoma and oedema of the optic disc. &it. J. OphthdwwE. 53, 721. Heilmann. K. (1970). Unt,ersuchungen iiber die Wirkung von Cat,apresan auf den intraokularen Druck. 1. Mitteilung. Klin. Xonatsbl. A,ugenheilk. 157, 182. Heilmann, K. (1972). Augendruck, Blutdrunk und Olnukomschaden. Biicherri de.9 Augennr~tes. Heft 61. Ferdinand Enke, Stuttgart. Heilmann. K. (1974). Clonidin in der Glaukombehanghmg. In Clonidin in der Augenheilkunde. Biicherei des Augennrzfes (Eds MertP, H.-J. and Heilmann. K.). Heft 63, 20. Ferdinand Enke, Stuttgart,. Heilmann, K. (1974). Vergleichende perimetrische Cntersuchungen mit Clonidin. In Clonidi~n in rlr, Augenheilkunde. Biicherei des Augenarzfe.v (Eds Mert,A. H.-J. and Heilmann, K.). Heft 63, 52. Ferdinand Enke, Stuttgart. Jiinemann, G. (1974). Uber die Wirkung des Clonidins auf den intraokularen Druck beim akuten Glaukom. In clonidin in der Augenheilkunde. Biicherei des dugenarztes (Eds Merti, H.-d. and Hrilmann. K.). Heft fi3, 63. Ferdinand Enke, Stuttgart.. Kobinger. W. and Walland, r\. (1967). Kreislaufuntersuchungen mit 2-(2.&dichlorphenylamino)2.imidazolin hydrochlorid. Arzneimittel-Forsch. 17, 292. Makabe. R. (1966). Ophthalmologische Untersuchungen mit Dichlorphenylaminoimidazolin unter besonderer Beriicksicbtigung des Einflusses auf den intraokularen Druck. Deut. Med. Il’och.enschr. 91, 1686. Sayegh, F. and Weigelin. E. (1974). Die intraokulare Drucksenkung durch Clonidin. Ihre Beziehung zu zerebralen StrGmungsrind Blutdruckanderungen. In Clonidin in der Augenheilkunde. Biicherei des Augenarzte.s (Eds MertB, H.-J. and Heilmann, K.). Heft 63, 34. Ferdinand Enke. Stuttgart. neuron blockade by cIonidine: St.arke, K., Wagner. J. and Sohiimann, H. J. (1972). Ad renergic comparison with guanethidine and local anesthetics. Arch. Int. Plwmaeodyn. Ther. 195,291.
Ocular Effects of Clonidine in Cats and Monkeys (Macaca irus)
(Received 12 Mn,y 197??. Hostott ) In monkeys administration of about 0.01 pg clonidine into t’he ant,erior chamber caused no significant change in the facility of outflow. At 10 pg/kg body weight given intravenously from a level of O-51 10.05 ~1. the facility decreased by 0.09*.0.01@ . min- 1 mmHg-’ min-l * mmHg-l. The same dose had no significant effect’ on the rate of aqueous humor formation determined with an indicator dilution technique. Before the administration of t,he drug the formation of aqueous humor was 1.4350.13 $/min, and 30 min later it was 1.32&0.15~l/min. The fall in eye pressure was 1.0-i-O-3 mmHg. In cats Z-lOpg/kg body weight clonidine given intravenously reduced the intrascleral venous pressure and the eye pressure in a way which indicated marked va.soconstriction in the eye. Unilateral carotid injection of the drug at a dose reducing the homolateral eye pressure by P5 mmHg caused a in the blood flow through the 19% reduction in the choroidal blood flow, a 34O, reduction iris and a 4276 reduction in the blood flow in the ciliary body when t*he injected side a-as compared to t.he other.
1. Introduction Clonidine is a drug widely usedin the treatment of arterial hypertension. The pressure lowering effect of the drug is due to activation of adrenergic receptors in the central nervous system which causesreduced activity in the sympathetic nervous system (Kobinger and Walland, 1967). The drug also has effects on peripheral adrenerpie mechanisms: it inhibits adrenergic transmission by a specific interference with the norepinephrine release (Starke, Wagner and Schiimann, 1972) and it also stimulates the post-ganglionic alphareceptors (Kobinger and Walland, 1967). After intravenous administration of the drug the latter effect causesa transient rise in the blood pressnrc which then falls as a result of the other effects. Makabe (1966) reported that intravenous injection of clonidine causesa slight fall in the eye pressurein individuals with normal eyes but a more marked fall in the eye pressurein patients with glaucoma. Tonography indicated that the reduction in eye pressurewas due to a reduced formation of the aqueoushumor. A study on the effects of clonidine given as eye drops (Hasslinger, 1969) indicated that the drug also had an effect on the eye pressurewhen given this way. Tonography indicated that the reduction in eye pressure was due to an increase in the facility of outtlow. Further studies on the effects of clonidine given as eye drops (Heilmann: 1970) indicated that there was a very rapid absorption of the drug from the conjunctiva and the nasal mucosa. Clinically useful reductions in eye pressure were obtained only at doses which also had effects on the blood pressure. At a recent symposium Sayegh and Weigelin (1974) concluded that the effects of clonidine could not be explained only by a reduction in the blood pressurein the ophthalmic artery. What other mechanismsmight be involved was not clear. Clonidine hasbeen reported to causea redistribution of the cardiac output and even low dosestend to causevasoconstriction in some vascular beds such as the skeletal muscle and the skin and gastro-intestinal organs. However, the blood flow to other
4st’
organs
.\.
such as the kidneys
lSlI,I.
.\Sl)
and the liver
K.
HEILMASn-
via the hepatic
artery
is increased
(Rolnlc.
Tshizaki, Forsyth and Melmon, 1973). A possible explanation for the eye JWSWIY~ lowering effect of the drug thus could be vasoconstriction in the eye (.Jiinemann. JYX). The purpose of the experiment,s reported here wrasto study the effect! of cloni(lint~ in experimental animals. Effects on the rate of aqueous formation and facility of outflow were studied in monkeys. Effects on the intrascleral venous pressureand t&t> regional
blood
flow in the rye were studied
in cats.
2. Materials and Methods E.zperiments
in monkeys
Cynomolgus monkeys of both sexes and weighing between 1.8 and 3.3 kg were used. The monkeys were anesthetized with sodium methohexital (Brietal Sodium, Lilly) 5(~100 mg given intramuscularly. Anesthesia was maintained with small doses of sodium pentobarbital given every 20-30 min. The animals were placed prone and kept warm with a heating pad. The mean arterial blood pressure was measured in a femoral artery. Effects of intracameral clonic&e. Each eye was connected to a pair of push-pull coupled syringes through needles shot into the eye with a needle gun. A third cannula connected each eye to a reservoir the weight of which was determined continuously (BBrriny, 1964). A T-connection on one of the tubings from the eye to the push-pull syringes connected the anterior chamber to a pressure t,ransducer. On one side the syringes contained 0.1 tug clonidine/ml mock aqueous humor, and on the other side there was only mock aqueous humor (BBrliny, 1964). When the intraocular pressure had stabilized after the cannulntiou, gross outflow facilities (Bill and Bgr;inp, 1966) were det’ermined by measuring the rate of inflow from the reservoirs at two different heights and dividing the change in the inflow rate by the change in the intraocular pressure. Each pressure level was maintained for at least 7 min. Spontaneous pressures were then measured again for about 15 min and then t,he contents of the anterior chambers were mixed with those of the push--pull coupled syringes. Fifteen to 20 min later the facility of outflow was determined in both eyes. In three of these experiments facility measurements were continued for l-2 hr and then IO pg clonidine/kg body weight WAS given intravenously and a last facility determination was made 10-15 min later. Effects of clmidinr on aqueous Aurn.or fortnation. The rate of aqueous humor formation was determined in one eye with an indicator dilution technique described previously (Bill, 1971). The other eye was also cannulated and in some experiments its spontaueous pressure was determined with a pressure transducer; in other experiments the outflow fa.cility was determined. The mean arterial blood pressure was measured in a, cannulated femoral artery. When the rate of aqueous humor formation had been steady for ;It lea,xt 30 min, 10 &kg body weight clonidine was given intravenously over l--IO min. Experiments
in cats
Effects of clonidine OS the eye presswe and the ,intrascleral venous pressure, Cats of both sexes and weighing between 2.2 and 3.3 kg were used. Anesthesia was induced with chloroform and maintained with sodium pentobarbital or chloralose supplemented with urethane. The animal was placed prone and kept warm with a heating pad. The temporal part of the sclera was exposed and a small slit was made through the sclera into one of the branches of the intrascleral venous plexus. To measure the intrascleral venous pressure the tapered tip of a polyethylene tubing was introduced into the vein in such a way that the open end pointed towards the large superior anterior ciliary vein that drains the superior part of the venous plexus (Bill, 1962a). The pressure in the plexus was measured with a pressure transducer. A cannula connected to another pressure transducer was shot into the anterior chamber for measurements of the intraocular pressure. The arterial blood
EFFECTS
OF
CLONIDIXE
483
pressure was measured through a cannulated femoral artery. When the pressures had been stable for at least 5 min, 2-10 pg/kg body weight clonidine was injected intravenously. Effects of clonidineon thebloodjow in theeye.Cats anesthetized as mentioned above were tracheotomized and artificially ventilated. Ventilation was adjusted to give normal values for the oxygen tension, the pH and the P ~0~. A thyroidal artery was isolated and can-
nulated in such a way that injections could be madeinto the commoncarotid artery. The heart was exposed through a thoracotomy and a cannula connected to a polyethylene tubing was introduced into the left ventricle. The eye pressure in both eyes were measured with pressure transducers connected to the eyes by needles and polyethylene tubings. A O+)Olq/, solution of clonidine was given intra-arterially at a rate of about 0.1 ml/l0 set-. When a fall in eye pressure was observed on the side of the injection, the injection was stopped and l-2 ml of a l”/b suspension of 15 pm *5Sr labelled microspheres (3 M Company, St Paul, Minnesota) were injected into the left. heart. A reference flow (Alm and Bill, 1972) of about, l-2 ml/min was obtained from one femora,l artery. The blood pressure was monit,ored via the other. One min after the start of the inject’ion of the labelled spheres the animal was put to death with a sat,urated solution of KC’1 given into the heart. The eyes \\-ere dissected and regional blood flow rates were determined as described in a previous report (Altn and Bill, 19’72).
3. Results E$ects on facility of outjiow
The effects of intracameral administration of about 0.01 pg clonidine on the faciiity of outflow were determined in 10 eyes. The facility of outflow before the administration of clonidine was 0.35f0.05 $*min-l*mmHg-l, after clonidine it was 0.38&0.03 ~1. minel*mmHg-l . In the control eyes the facility values were 0*37&0*3 $*rnin-l* mmHg-l and 0.39-&0*03$*min-l*mmHg-l, respectively. The spontaneous change in the facility of outflow in the control eyes t,hus was almost the same as the change in the treated eyes.
0
o-2 Facility
0.4
0.6
O-8
I,pl.min-‘.mmHg-’
PIG. 1. Facility I was determined before the intravenous injection of 10 pg/kg body weight facilit,y II was determined lo-15 min after t,he injection. The line shown is the identity line.
The effect of intravenous
administration
of clonidine
in eyes not previously
clonidinr.
injected
with the drug was also determined in 10 animals. Figure 1 shows a plot of facilities before and after the administration of the drug. Ten pg/kg body weight reduced the mean facility of outflow from 0*51+0*05 ~l*min-l*mmHg-l mmHg-l. The fall in facility was 0*09+0*01 $min-l*mmHg-l. significant at the P < 0.001 level.
to 0.43 *O-O5 dmin+ The change was
The effects of int.rHvr>nousinjection of 10 pg/kg l~orty weight cionitlinc~ was iii vestigatetl in 17 aninlals. There was a biphasic efiect, on thr I~lootl prrlssurcl -first ;i
transient rise over a few lllinut,es and then a.long-lasting fall in the pressure. The +J!.ti pressure fell l*O+W3 mmHg after the injection of the clonidine (11= 11). Thr fall watt statistically significant. P < O-01. There was no con&em? change in the rate ot aqueous formation. The mean flon just I)eforc t,htx inject,ion of the clonidint~ \~a\ 1.4310.13 #min. The mean arterial blood pressure at this time was X8+:7 rnrnHg. Thirty min later when t,he blood pressure was about, 20 mmHg lower. tht IIICHII rates of aqueous hnnlor forniation was 1-Z +0.15 ~l/iiiin.
?‘r IOP
15 ,o T P w5 8 - Ok---
5m1n
Effects of dondine
ON pye pressure
md ~ntmsclernl
veytous pressure
In cats intravenous injection of 2-10 tug clonidine per kg body weight causeda longlasting fall both in the eye pressure and the intrascleral venous pressure. Figure 2 shows the effect of 2 pg/kg body weight’ in one animal. The fall in eye pressure and venous pressure had already started during the phase of increased arterial blood pressure and lasted for the rest of the experiments~--that is for more than 60 min. The fall in IOP was faster than the fall in the intrascleral venous pressure. At, 15-30 min after the injection of the drug both the eye pressuresand the intrascleral venous pressureswere relatively constant. In three of five casesthe two pressure> were reduced by about the sameamount,. In two eyes the fall in the eye pressurewalmore marked than t,he fall in the intrascleral venous pressure(Table I).
Values of intraocular pressure, IOP, intrascleral 2‘emzi.s pressure, ISVP: and mewtt arterial blood pressure, MAP, before and k-30 W&Lafter the injection of 2-10 ~glk~q body weight clonidine in Jive cats ISVI’ mmHg
IOP
Before
21 40 26 32 19
mmHR
After
17 23 23 18 16
Before
12 18 14 23 12
MAP mmHg Aft,rr
Before
9 8 11 9 x
107 129 154 I69 184
Bftel
74 81 147 96 147
EFFECTS
Effects qf clom&ae
on the regional
OF
C’LONIDIRE
4x5
blood fEow in the eye
In cats t.he intra-arterial injection of l-3 pg clonidine over 15-30 set causeda fall in the homolateral eye pressure of 4-5 mmHg but little change in the pressure of the other eye and in the blood pressure.Figure 3 showsthat in the 10 animals investigated the blood flow in the eye on the t,reated side was reduced compared to that on the control side. The fall in blood flow was marked in the iris and the ciliary body. 33*8&6*5°h and 42.4f4.2% in the two tissues respectively. It was statistically significant in both tissues P < 0.001. In the choroid the fall in blood flow was 18.6+f*l”/,. P .’ 0.005. There was no significant effect on the ret,inal blood flow.
Ic’rc:. 3. The effect of clonidine on re,+xml blood H
via R side.
4. Discussion In cats a great part of the blood leaving the urea is drained into an intrascleral venous plexus (Bill, 1962a). The pressurein this plexus tends to fall if the blood flow through the uvea is reduced (Bill, 1962a,b). The aqueoushumor is also drained into the venous plexus (Bill, 1962a, 1966). Changesin t.he venous pressuretherefore cause parallel changesin the eye pressureif the aqueousflow into the plexus and t,he outflow facility remain constant. The experiments in cats with determination of the regional blood flow in both eyes after unilateral injection of clonidine demonstrated that the drug causesrapid and pronounced vaeoconstrict,ion in the iris and the ciliary body but a lesspronounced vasoconstriction in the choroid. Vasoconstriction can be assumedto affect the eye pressurein two different ways. One is by reducing the blood volume in the eye. The other is by reducing the intrascleral venous pressme. The former effect is transient since blood lost from t,he vascular compartment is soon compeasated by an increase in the volume of the aqueoushumor within the eye. The latter effect of clonidine will remain as long as the vasoconstrict,ion remains. The reasonis that aslong as the blood flow is reduced there is also a reduced intrasclelal venous pressure. In the cat experiments the rapid fall in eye pressurewasno doubt due t,o a reduction in the intraocular blood volume. The longstanding reduction in pressureoccurred very much in parallel with the reduction in the intrascleral venous pressure and can be assumedto be caused entirely by the latter in three of five animals. In two animals the fall in eye pressure was greater than the fall in venous pressure.The reason for this is not clear. One possibility is that the venous pressure measuredwas not representative for the average venous pressure in the plexus affecting the outflow of the aqueous humor. This is quite possiblesince there are considerable variations in pressurein the plexus from one part to anot,her (Bill, 1962a). Another possibility is that in two animals the F
4%
.i.
KILI,
:1X1)
K.
HEILMAKK
drug had an effect on the facility of outflow or the formation of aqueous humor. ‘I’hr combined effect of a vasoconstriction in the ciliary body and a fall iu the perfusion pressure caused by a marked fall in t’he blood pressure---not compensated 11ya sinlilal fall in the eye pressure---may he a reduced rate of aqueous format.ion. In monkeys the effects of clonidine on the eye pressure were much stnaller than iu cats-the fall being only about 1 mmHg. Determinations of the effects of the drug on the outflow facility and the rate of aqueous humor formation gave no explanabions for the fall in the eye pressure. It seems likely therefore that in monkeys the fall in eye pressure was also secondary to intraocular vasoconstriction and the retluced perfusion pressure, resulting in a fall in the pressure in the anterior ciliary veins. In monkeys intracameral clonidine had no significant effect on the facilit,y of outflow. Intravenous injection of the drug on the other hand seemed to cause a fall in the facility of outflow. Spont,aneous changes in the facility of outflow (luring experiments of the type conducted usually are in the opposite direction. i.cl. thr facility tends to increase. For that reason it seems unlikely that the chanail after clonidine was due to a spontaneous change. It thus seems that, if anvthing. the drug tends to decrease the facility of outflow. A possible mechanism for thiq decrease is relaxation of the ciliary muscle resulting in partial closing of the trabecular meshwork. Stimulation of the sympathetic nerves- to the eye has been demonstrated to reducr facility of outflow probably by this effect (Casey, 1966). Norepinephrinr released under such conditions and the clonidine used in the present experiments may affect t,he samC adrenergic receptors. Previous studies have shown that there are adrenergic alpha-receptors in the hlooti vessels of the ciliary body and the choroid (Bill, 196%). In the retina. on the other hand, there seem to be no alpha-receptors available to respond to the intra-arterial administration of norepinephrine (Aim, 1973). The effects of clonidine on the regional hlood flow in the eye are compatible with this pattern of alpha-recept*or distribution. In clinical practice clonidine has been used to reduce the eye pressure mainly in the treat,ment of open-angle glaucoma but also in acute angle-closure glaucoma,. The present experiments suggest that in angle-closure glaucoma the pressure lowering effect is due to intraocular vasoconstriction and a fall in the blood pressure. Thestb changes cause a reduction in the intraocular blood volume and a fall in the intrascleral venous pressure, resulting in a marked fall in the eye pressure. It heems very possible that in cases with high y?e pressure the vasoconstriction may cause such a reducetl blood flow through the clhary processes that the rate of aqueous secret-ion is reduced which would contribute to the fall in the eye pressure. The use of clonidine in cases of acute angle-closure glaucoma to cause a singular reduction in the intraocular pressure for example as before surgery may be of great advantage (Jiinemann, 1974) and is unlikely to be hazardous. The use of the drug in long-term treatment of open-angle glaucoma, on the other hand, must be questioned. The usefulness of the drug will in fact depend on the mechanism causing the degeneration of the optic nerve head. This mechanism is not clear. At least two alternatives have to be considered (Anderson and Hendrickson, 1974). If degeneration is due to an insufficient blood supply to the optic nerve head as suggested by Gafner and Goldmann (1955), Hayreh (1969). Heilmaml (1972) and Aim and Bill (1973) bhe drug will be valuable only if it increases the hlood flow to the optic nerve head-----which seem!: rather unlikely. If degeneration of the nerve head is due to other factors such as a direct mechanical effect on the nerve fihres at or near the lamina cribrosa, the drug may be of great value in relieving the pressure.
EFFECTS
OF
CLONIDINE
487
If the vascular theory is correct, drugs causing vasoconstriction in the ciliary arteries and their branches and a fall in blood pressure may even be hazardous. The reason is that the major part of the optic nerve head is supplied from arterioles entering the nerve head from the choroid (Hayreh, 1969; Anderson, 1970). A fall in blood pressure in these arterioles and vasoconstriction in these vessels thus may tend to reduce the blood flow through the optic nerve head even if the intraocular pressure is reduced. As long as the vascular theory for the damage of the optic nerve head in glaucoma has not been refuted and the possibility remains that clonidine may reduce the blood tlow in the optic nerve head, it would seem unwise to use the drug without frequent checking of the visual field. This is true for the use of clonidine in the treatment of open-angle glaucoma as well as for its use in arterial hypertension. That the mode of action of drugs used in glaucoma may be important, and not only the fall in eye pressure that they cause, is indicated by the results obtained by Heilmann. A comparison (Heilmann, 19’72,1974) between the effects of acetazolamide and clonidine in glaucoma patients with field defects indicated that for the same fall in eye pressure produced by the two drugs, there was some reversal of the field defects with acetazolamide but not with clonidine. Acetazolamide unlike clonidine exerts its effect directly on the secretory mechanisms in the ciliary processes and has no demonst,rable effect on the uveal blood flow (Bill, 1974). A\CKNOWLEDGMENTS This work was supported by PHS grant EY-00475 from the National Eye Institute and by a grant B75-14X-147 from the Swedish Medical Research Council. We thank Mr U. Ullstriim, Miss M. Thor&n, Mrs A. ostberg and Mr C. Geijer for valuable technical assistance. REFERENCES Xlm,
A. (1972). Effects of norepinephrine, angiotensin, dihydroergotamine, papaverine, isoproterenol, histamine, nicotinic acid and xantinol nicotinate on retinal oxygen tension in
cats. Acta Opht?u&nol. 50, 707. A. and Bill, A. (1972). The oxygen supply to the retina, II. Effects of high intraocular pressure on uveal and retinal blood flow in cats. A study with labelled microspheres including flow determinations in brain and other tissues. Acta Physiol. Scand. 84, 306. Alm, A. and Bill, A. (1973). Ocular and optic nerve blood flow at normal and increased intraocular pressures in monkeys (Xacuca irus); a study with radioactively labelled microspheres including flow determinations in brain and some other tissues. Exp. Eye Rea. 15, 15. Anderson, D. R. (1970). Vascular supply to the optic nerve of primates. Amer. J. Ophthnlmol. 70,341. Anderson, D. R. and Hendrickson, A. (1974). Effect of intraocular pressure on rapid axoplasmic transport in monkey optic nerve. Invest. Ophthalmol. 13, 771. B&r&y, E. H. (1964). Simultaneous measurements of changing intraocular pressure and outflow facilit,y in the vervet monkey by constant pressure infusion. Invest. Ophthalmol. 3, 135. Bill, iz. (1962a). Aspects of the drainage of aqueous humor in cats. A.N.A. Arch. Ophthnlmot. 67, 148. Bill, A. (196213). .4 method for quantitative determination of the blood flow through the cat uvea. A.M.A. Arch. Ophthulmol. 67, 156. Bill, A. (1962c). Butonomic nervous control of uveal blood flow. Acta Physiol. Stand. 56, 70. Bill, A. (1966). Formation and drainage of aqueous humor in cats. E.c~. Eye Res. 5, 185. Bill, A. and B&rBny, E. H. (1966). Gross facility, facility of conventional routes and pseudofacility of aqueous humor outflow in the cynomolgus monkey. A.M.A. Arch. Ophthalmol. 75,665. Bill, A. (1971). Effects of longstanding stepwise increments in eye pressure on the rate of aqueous humor formation in a primate (Cercopithecw aethiops). Exp. Eye Res. 12, 184. Bill, A. (1974). Effects of acetazolamide and carotid occlusion on t’he ocular blood flow in unanaesthetized rabbits. Invest. Ophthalmol. 13, 954. Alm,
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Bolme, P., Ishizaki, T.. Porsyth, R. P. and Melmon, K. 1,. (1973). Haemodynamic effect,s and changes in blood flow distribution caused by clonidinr in the conscious monkey. A&n I’7qsioI. &and. (suppl.) 3%,97. Casey, W. (1966). Cervical sympat)hetic stimulation in monkeys and the effects on out,flow f:rrility and intraorular volume. A study in the East African vorvet (Cercopifh~ecus a~fhiops). 7nwst. Ophthnlmol.
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Gafner, F. and Goldmann, H. (1955). Experimentelle Unt,ersuchungen iiber den Zusammenhang von Augendrucksteigerung und Gesichtsfeldschgdigung. Ophthalmologica 130, 357. Hasslinger, C. (1969). Catapresan [2-(2,6-Dichlorphenylamino)-2-Imidazolin-hydrochlorid~ -ein neues augendrucksenkendes Medikament,. Klin. Monotsbl. Augenheilk. 154, 9.5. Hayreh, S. S. (1969). Blood supply to the optic nerve and its role in optic atrophy. glaucoma and oedema of the optic disc. &it. J. OphthdwwE. 53, 721. Heilmann. K. (1970). Unt,ersuchungen iiber die Wirkung von Cat,apresan auf den intraokularen Druck. 1. Mitteilung. Klin. Xonatsbl. A,ugenheilk. 157, 182. Heilmann, K. (1972). Augendruck, Blutdrunk und Olnukomschaden. Biicherri de.9 Augennr~tes. Heft 61. Ferdinand Enke, Stuttgart. Heilmann. K. (1974). Clonidin in der Glaukombehanghmg. In Clonidin in der Augenheilkunde. Biicherei des Augennrzfes (Eds MertP, H.-J. and Heilmann. K.). Heft 63, 20. Ferdinand Enke, Stuttgart,. Heilmann, K. (1974). Vergleichende perimetrische Cntersuchungen mit Clonidin. In Clonidi~n in rlr, Augenheilkunde. Biicherei des Augenarzfe.v (Eds Mert,A. H.-J. and Heilmann, K.). Heft 63, 52. Ferdinand Enke, Stuttgart. Jiinemann, G. (1974). Uber die Wirkung des Clonidins auf den intraokularen Druck beim akuten Glaukom. In clonidin in der Augenheilkunde. Biicherei des dugenarztes (Eds Merti, H.-d. and Hrilmann. K.). Heft fi3, 63. Ferdinand Enke, Stuttgart.. Kobinger. W. and Walland, r\. (1967). Kreislaufuntersuchungen mit 2-(2.&dichlorphenylamino)2.imidazolin hydrochlorid. Arzneimittel-Forsch. 17, 292. Makabe. R. (1966). Ophthalmologische Untersuchungen mit Dichlorphenylaminoimidazolin unter besonderer Beriicksicbtigung des Einflusses auf den intraokularen Druck. Deut. Med. Il’och.enschr. 91, 1686. Sayegh, F. and Weigelin. E. (1974). Die intraokulare Drucksenkung durch Clonidin. Ihre Beziehung zu zerebralen StrGmungsrind Blutdruckanderungen. In Clonidin in der Augenheilkunde. Biicherei des Augenarzte.s (Eds MertB, H.-J. and Heilmann, K.). Heft 63, 34. Ferdinand Enke. Stuttgart. neuron blockade by cIonidine: St.arke, K., Wagner. J. and Sohiimann, H. J. (1972). Ad renergic comparison with guanethidine and local anesthetics. Arch. Int. Plwmaeodyn. Ther. 195,291.