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Further studies on the effect of prostaglandin on intraocular pressure in the rabbit
Exp. Eye Res. (1971) 11, 1 7 0 - 1 7 7 Further Studies on the Effect of Prostaglandin on I n t r a o c u l a r P r e s s u r e in the R a b b i t ~/IICHAEL S. STARR
Department of PAysiology and Biochemistry, Institute of Ophthal~nology, University of London, Judd Street, London, WC1H 9QS, Engla~ul (Received 18 December 1970, Z,ondon) The effects o f prostaglandins E on intraocular pressure have been studied h~ rabbits, Prostaglandins Ex mad E, first raised %he intraocular pressure and then lowered it, but. failed to constrict the pupil to any extent. The sensitivity of individual eyes to prostaglandb~ Ex varied more widely with intracameral than intravenous ad/ministration. The latter route gave more reliable pressure responses in both eyes, but slightly bigger doses of prostaglandin were needed. Prior injection of polyphloretin phosphate, into the vitreous completely blocked the rise in intraocular pressure to prostaglandin Ex but not the fall in pressure, while systemic pretreatment ~fith polyptdoretin phosphate was much less effective. 1. I n t r o d u c t i o n I n t h e 1950's, A m b a c h e (1955, 1957, 1959) first d e m o n s t r a t e d t h e p r e s e n c e of a biologically a c t i v e lipid f r a c t i o n i n w a t e r y e x t r a c t s of r a b b i t a n d c a t irides which h,:~, p r o v i s i o n a l l y n a m e d " i r i n " . More r e c e n t l y ( A m b a c h e , B r u m m e r , R o s e a n d Whiting?:, 1966; A m b a c h e and. B r t t m m e r , 1968), t h e a c t i v e c o n s t i t u e n t s of irin h a v e b e e n i d e n t i fied w i t h t h e E a n d F p r o s t a g l a n d i n s , t h e r e b y s t i m u l a t i n g f u r t h e r i n t e r e s t ill t h e ocular actions of these compolmds. I t is n o w well e s t a b l i s h e d t h a t p u r e p r o s t a g l a n d i n s cause a b r e a k d o w n of t h e blooda q u e o u s b a r r i e r a n d a p r o l o n g e d rise in t h e i n t r a o c u l a r pressure ( I O P ) w h e n t h e y are a ~ m l n i s t e r e d i n t o t h e a n t e r i o r c h a m b e r o f t h e r a b b i t ' s (WaitzmaI~ a n d Kin~% 1967 ; B e i t c h a n d E a k i n s , 1969) a n d c a t ' s e y e ( E a k i n s , 1970). T h e d e v e l o p m e n t of t h e s e r e s p o n s e s is i m p a i r e d i n t h e r a b b i t b y p r i o r t r c n t m e n t w i t h p o l y p h l o r e t i n p h o s p h a t e ( P P P ) , a p r o s t a g l a n d i n a n t a g o n i s t ( B e i t e h anqt E a k i n s , 1969). D u r i n g t h e c o u r s e o f s o m e r e c e n t s t u d i e s o f t h e effects o f p r o s t a g l a n d i n s o n b l o o d flow tb.~ough t h e r a b b i C s iris (Start, 1970), i t w a s n o t i c e d t h a t s o m e of t h e featu_~es of t h e I O P r e s p o n s e t o p r o s t a g l a n d i n s differed f r o m t h o s e d e s c r i b e d b y W a i t z m a n a n d K i n g (1967). In. t h i s paper, t h e r e f o r e , s o m e o f t h e a c t i o n s o f p r o s t a g l a n d i n s of t h e E series o n t h e r a b b i t ' s e y e h a v e b e e n r e i n v e s t i g a t e d , a n d t h e a b i l i t y o f p o l y p h l o r e t i n p h o s p h a t e t o m o d i f y t h e m h a s also b e e n s t u d i e d . 2. M e t h o d s
Experimental Male albino (New Zealand) rabbits weighing 2-3 kg were anaesthetized with u r e t h a n e . (1-4--2-0 g/kg i.v.). Drugs were administered systemically via t h e femoral vein or the lingual artery, and int-racamerally via a 27 gauge h y p o d e r m i c needle placed in the anteri~)c c h a m b e r b y means of a specially constructed needle gun. Blood pressure was recorded from t h e femoz'al a r t ~ r y u s i n g a Model SE 4-81 laressure transducer a n d a Model SE 3006 u.v. c h a r t recorder (S.E. Laboratories Ltd.). I 0 P was recorded t h r o u g h 25 gauge needles fired into the anterior c h a m b e r of each eye w i t h t h e needle gun, carefully avoiding t h e iris a n 4 t h e lens. The needles were conn6cted to p r s s s u ~ transducers b y fine 1)olythene tubing. T h e pressure transduceIs were 170
PROSTAGLANDIN
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held at about 40°C in thermostatically controlled metal blocks in order to minimize drift due to changes in ambient temperature.
Dr'J.g8 ProstaglandJJm (PO's) :Ex and :Ea (The Upjohn Company) and polyphloretin phosphate (PPP, Leo Laboratories) were made up freshly in physiological saline. Syster~ric injections were not greater than 1 ml eel. and were always washed in with 0.1 ml saline. A Pahner slow injection apparatus (C. F. Palmer Ltd.) was used to admlnisber the close-arterial ilffusions at injection rates ~>f 0"25-2"0 ml/miu. Xntraeameral injections were prewarmed to 87°C to facilitate mixing with the aqueous, and given in dose .vo:~umes not exceeding 5/xl with the aid of an "Agla" micrometer syringe (3VellGome Rea~e£ts Ltd.). The PG's were normally stored at --4°C as 1 mg/ml solutions of the s~dium salt in saline. 3.
Results
I n a group of 35 rabbits the m e a n resting I O P recorded soon after calmtflating t h e eyes was 18-564-1-28 mmI-Ig (range 15.4-24-3 m m H g ) , a n d in 3 control ardmals t h e I O P r e m a i n e d fairly c o n s t a n t over a period of 3 hr. Occasionally t h e I O P fell spont,aneously during the course of an e x p e r i m e n t b y 2-4 m m H g , b u t a drop in pressure of t,bis n a t u r e was usually associated with t h e t e m p o r a r y 4za~'cular h y p o t e n s i o n a c c o m p a n y i n g an episode of r e s p i r a t o r y distress. 40-
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F ~ o . 1. :Effect o f i n t r a c a m e r a l P O E 1 o n r a b b i $ l O P . V a l u e s a r e s h o w n f o r t h e m e a n r e s t i n g l O P (1~) a n d f o r t h e h i g h e s t ( H ) a n d l o w e s t ( L ) I O P ' s r e c o r d e d a f t e r i n j e c t i n g P G E z. E a c h p o i n t ( ~ : s . ~ . ~ . ) i s t h e m e a n o f a$ l e a s t 4 d e t e r m i n a t i o n s .
I n t r a c a m e r a l injection of P G E 1 or P G E 2 was followed b y a sustained rise in I O P , the m a g n i t u d e a n d d u r a t i o n of which were dose-dependent. S/lch a response was observed w i t h as little as 25 ng P G E 1, a l t h o u g h lal~er q u a n t i t i e s of this P G were used r o u t i n e l y . F o r example, 0.4/~g P G E 1 e l e v a t e d t h e I O P b y 5-10 mml~g, t h e pressure r e t u r n i n g t o n o r m a l i n 50-70 rain. I n a b o u t o n e - q u a r t e r of She a n i m a l s studied t h e eye pressure t h e n c o n t i n u e d t o .fall below t h e pre-injection level, sometimes b y as m u c h as 10 rnrnHg, a n d s t a y e d low for 180 rni~ or longer. Like ~he preceding presser e~hct, t h e m a g n i t u d e o f t h i s " s e c o n d a r y " or " d e p r e s s o r " response dep e n d e d u p o n t h e concent-ration of P O E x (Fig. 1). Some r a b b i t s failed to e x t i i ~ t t h e usual initial I O P rise a f t e r P O E x b u t still showed
I72
5I. S. S T A R R
a long-lasting ocular hypotension, s t a r t i n g 5-15 rain after t h e P G injection. These reductions in pressure usually p r e d o m i n a t e d in one eye, the c o n t r a l a t e r a l I O P a n d blood pressure b o t h remaining a t a s t e a d y level. P u p i l size was assessed subjectiveIy each time P G E 1 was administered into t h e anterior chamber. :Even after gii,ing 4 tzg P G E x there was no sign of a pupil constriction, although the I O P was elevated b y as much as 40 m m H g with this dose. 20
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F I e . 2. C h a n g e s in ][OP in botl. ayes o f a r a b b i t f o l l o w i n g r e p e a t e d intracarnev~l i n j e c t i o n s o f 0-S t~g P G E 1. N o t e t h e r a p i d d e v e l o p m e n t o f t a c h y p h y l a x i s in t h e left e y e b u t n o t in t h e r i g h t eye.
The most striking f e a t u r e of t h e action of P G E 1 on the r a b b i t ' s eye was its m a r k e d inconsistency. I t varied from no presser response a t all in some animals, to either a pronolmced rise in l O P with the fu'st injection o n l y (followed b y a rapidly-developing t a c h y p h y l a x i s ) , or a c o n t i n u i n g sensitivity to repeated i n t r a c a m e r a l injections of P G E 1 (with h t t l e or no evidence of t a c h y p h y l a x i s ) . Such differences were even a p p a r e n t between t h e bilateral responses of the same rabbit. This finding is well illustrated in Fig. 2, in which i t is p l a i n l y seen t h a t the increase in pressure elicited b y i n t r a o c u l a r P G E 1 (0-8 tzg) in the left eye grew progressively smaller with each successive dose, while 5 such injections dehvered into t h e r i g h t eye were n o t subject to tachyphylaxZu. Far~her experiments were therefore u n d e r t a k e n to determine t h e e x t e n t of this interocular v a r i a t i o n . A comparison was m a d e of t h e sizes of the presser responses induced b y a single dose of P O E z (0-2-0-8/~g) given i n t r a c a m e r a l l y to a group of 18 r a b b i t s in equal c o n c e n t r a t i o n to b o t h eyes. The results are shown i n the scatter d i a g r a m in Fig, 3(a). F r o m t h e value for t h e S p e a r m a n coefficient of r a n k correlation (R = -- 0-263) i t is quite clear t h a t ~he effects produced in the left eyes differed widely from those in t h e r i g h t ( P > 0 - 1 ) . On t h e o t h e r h a n d , it was noticed t h a t similar responses were u s u a l l y evoked i n b o t h cymeswhen PG was a d m i n i s t e r e d systemically. Single doses of 1-4/zg P O E 1 were therefore given i n t r a v e n o u s l y to 20 rabbits a n d t h e changes in eye pressures recorded a n d t r e a t e d statis+Acally in a similar faskion [Fig. 3(b)]. This t i m e the value for R came to + 0.8~:7, i n d i c a t i n g a v e r y h i g h l y sigrdfic a n t correlation between t h e ~wo sets of results ( P < 0 - 0 0 1 ) . F o r this reason the i n t r a venous route of i n j e c t i o n was preferred in studies involving a comparison of t h e e v e n t s Occurring in b o t h eYes. (see later, inlHbitor studies). I n t r a v e n o u s P G E x w a s generally 5-10 t i m e s less effective t h a n i n t r a c a m e r a l PG~] 1 in raising IOP, b u t t h e log dose,response plots for the two routes were a p p r o ~ maCely parallel (Fig. 4). A m a x i m u m response was produced b y a b o u t 0-8 ~g P G E 1
PtlOSTAGLANDIN
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lOP
173
given int~:aeamerally c o m p a r e d to about. 4/xg systemically. The large s t a n d a r d errors a t t a c h e d to each point emphasize t h e wide s c a t t e r of individual points a b o u t the II:lC~lrl S.
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:FJo. 3. l ~ e l ~ . . o u s h i p b e t w e e n t h e i n c r e a s o i n I O P in t h e lef~ a n d r i g h ¢ e y e s o f r ~ b b i t s a f t e r i n t r a e ~ m e r a ! (a) o r i n t r a v e n o u s (b) a d m i n i s t r a t i o n o f : P G E 1.
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~ o . 4. E f f e c t o f i n t r a o a m c r a l ( I . C . ) a n d i n t r a v e n o u s ( I . V . ) P G E z o n r a b b i t I O P . E a c h p o i n t ( ~ - s . ~ . M . ) is the mean of at least 6 estimations.
The n a t u r e of t h e I O P response t 0 i n t r a v e n o u s P G E 1 w a s also s o m e w h a t di~Terent. In h a l f t h e animals tested there was a typical s m o o t h rise in eye pressure afber a delay o f 15-20 rain, while the other, h a l f e~h~bited a t w o - s ~ e pressor response; a n initial small rise of 2-10 mmr~g,- ~ h i c h lasted 10-20 m~ri, followed b y a •more pronounced
174
M. S. S T A r , It
change (10-40 m m H g ) similar to thab observed after intraoeular administration ot' the drug. I n b o t h instances the' eye pressures recovered a n d then t~.nded to fall and remain below t h e b pre-injection levels, as described above.
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i.o~ dose PPP (rag) F~G..5. I n h i b i t i o n b y c l o s e - a r t e r i a l i n f u s i o n o f P P P o f t h e i n c r e a s e i n r a b b i t ' s [ O P p r o d u c e d b y 4 ttg i n t r a v e n o u s ]PGE 1. E a c h p o i n t is t h e m e a n o f 3 e s b i m a t i o n ~ .
Both routes of administration suffered disadvantages. I n t r a c a m e r a l injections always elevated the I O P temporarily b y 0.5-4- m m ~ g , b u t this a r t e f a c t was minimized b y keeping the injected volume small. W h e n introduced in this w a y :PGE~ produced entirely localized effects and did not modify blood pressure. B y contrast, intravenous P G E 1 lowered the blood pressure irt a dose-dependent fashion; for example, 4/Lg induced a fall of 30-40 m m H g f r o m a m e a n resting pressure of 93-4=7-2 mini{g: which recovered completely in 5--10 min. I t should be noted, however, t h a t in spit~. of this hypotension there was almost invariably a transient rise in eye pressure during the course of the injection, which a m o u n t e d to 1-5 m m H g a n d lasted for 0.5-4 rain. depending on the dose administered. H a v i n g established the criteria necessary for evoking a reliable 7 0 P response to
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FIe. 6. Inhibition by intravitreal P P P of the increase in rabbit's I O P produced by 4 pg intravenous PGEI. Each point is the m e a n of 3 estimations.
PROSTAGLANDIN
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PGE1, a separate s t u d y was t h e n made to examine t h e properties of the P G inhibitor, P P P . Figure 5 illustrates the inhibitory- avtion of different rates of infusion of P P P (0.25-2 mg/mi~t for 20 ruin) given via the lingual artery, on one side, against the bilateral ocular h y p e r t e n s i o n produced b y i n t r a v e n o u s P G E 1 (4/~g). The degree of inhibition of t h e response in the test eye was arrived a t b y c o m p a r h l g it with t h e control response present in the opposite eye. Low rates of infusion (0.05-0.25 rag/rain) were w i t h o u t m u c h effect, b u t as the flow rate was raised a b o v e 0-25 rag/rain the pressor response to P G E 1 was considerably diminished. Thus a 5~A°/ ~o i a h i b i t i o n (EDr,o) was achieved in the test eye b y infusing 1.43 mg P P P / m i n o~ei:r a time period of 20 rain. A t thLs c o n c e n t r a t i o n P P P did n o t lower blood pressure, i whereas t h e same quantity" of P P P administered over 5 or 10 min, did cause a small hypotension, b u t this quickly recovered on stopping the infusion. Similar resulk~ were obtained when P P P was injected i n t r a ~ i t r e a l l y into one eye 20-24 h r previously, except t h a t here the EDso for t h e a n t a g o n i s m of P G E r b y P P P was a b o u t 50/xg, a b o u t 1/600 of the t o t a l dose administered systemically (see Fig. 6). A n y effect of the i n t r a v i t r e a l injection of P P P on I O P h a d worn off b y t h e time the :PGE 1 was given. A finding of particular interest was t h a t in all 5 r a b b i t s p r e t r e a t e d with the highest dose of P P P (100 tzg), the pressor response to P(~JE 1 was completely abolished, and y e t almost i m m e d i a t e l y after injecting the P G E 1 t h e I O P ' s of the test eyes began to fall (on average by a t o t a l of - - 6 . 9 ± 1 - 4 mini-It), and did n o t recover before the end of the experiment, 60 rain later. 4. Discussion
I t has not been possible to confirm the observations of W a i t z m a n and King (1967) t h a t P G E ' s cause a m a r k e d constriction of t h e r a b b i t ' s pupil. ~taese a u t h o r s n o t e d some reduction in pupil size w i t h n a n o g r a m quantities of PG, while in the present s t u d y microgram a m o u n t s proved to be more or less ineffective in this respect. The reason for ~his discrepancy is n o t known, b u t i t m a y be related to differences in the s t r a i n of r a b b i t and/or to t h e variou~s batches of P G used. On the other hand, the n a t u r e and m a g n i t u d e of the p a r a m e t e r s of the ocular h y p e r t e n s i o n following i n t r a c a m e r a l 1PGE x (i.e. rate of olmet of Eae response, its size and duration) were v e r y similar to those reported b y Beitx~h a n d l~akins (1969), who used slightly smaller concentrations of tbJs drug. B o t h sets. of results indicate t h a t the response lasts no more t h a n 1-2 h r as compared w i t h t h e 3 h r or more observed b y ~Vaitzman a n d K i n g (1967) for tlfis species. However, there are three ways in which the present results differ from those of previous investigators: (i) The first concerns the ~'equeucy with which t a c h y p h y l a x i s occurs. I n c e n ~ a s t to Beiteh and E a k i n s ' (1969) findings, this p h e n o m e n o n did n o t show up in every e x p e r i m e n t with PGE1, a n d when it did appear it was t h e n som~tLmes restricted to one eye. This emphasizes t h a t even the two eyes of one r a b b i t c a n n o t always be relied ou to r e a c t to PG's in a n identical fashion, t h u s m a k i n g i t difficult to c o m p a r e t h e effects of bilateral t r e a t m e n t . :Fortunately bilateral refractorin.ess to t h e pressor a c t i o n of PG's on the I O P did n o t occur t o o often, a n d i t is considered less likely to be due to interocular v a r i a t i o n t h a n t o de~fects in e x p e r h n e n t a l procedure, such as (a) i n a d e q u a t e mixing of t h e drug, ( b ) p o o r positioning of the; i n j e c t i o n needles, ( c ) prior liberation of endogenous PG's from t h e iris b y direc~b c o n t a c t with the i m p l a n t e d needles (Ambache; i ~ a v a n a g h a n d Whiting, 1965). (ii) The P G E ' s , a n d in all-~r0bability other PG's as well, exerl; a pronounced a n d
176
~I. S. S T A R R
reliable presser effect on I O P wh,en t h e y are admhxiatered systemically (c.f. ~ r a i t z m a n a n d King, 1967). So m u c h so, t h a t this r o u t e was used as m a t t e r of policy when bilateral comparisons were to be made, even t h o u g h i n t r a c a m e r a l injectiop required m u c h Iess PGE~ to evoke similar responses, I t seems likely t h a t the initial rise in I O P which followed i n t r a v e n o u s injection of PGE~ in a b o u t half t h e rabbits was due to t h e spasmogenic p r o p e r t y of t h e d r u g on t h e e x t r a o c u l a r muscles, since the response lasted too long to be t h e result of local changes in blood flow, a n d y e t it wm~ over too quickly to be a t t r i b u t a b l e to a direct a l t e r a t i o n in aqueous flow. N e i t h e r this side-effect nor t h e more obvious d i s a d v a n t a g e of widespread vascular d i l a t a t i o n (and hence lowering of blood pressure) a c c o m p a n y i n g the injection, appeared to interfere w i t h t h e subsequent d e v e l o p m e n t of t h e typical prolonged increase in ocular tension. (iii) The ability of P G E 1 to lower I O P in the rabbit, which has n o t been previously reported, occurred sufficiently often in t h e present e x p e r i m e n t s to be recognized as real effect, since it was considerably longer-lasting t h a n t h e more typical rise in pressure which invariably preceded it. I t was also particularly interesting to find t h a t i n t r a v i t r e a l p r e t r e a t m e n t with P P P completely blocked t h e initial hyperte~lsive action of P G E 1 w i t h o u t affecting this fall in pressure. U n d e r these conditions t h e hypotension was more r a p i d in onset, suggesting t h a t in u n t r e a t e d animals ~hese two opposing actions on ocular pressure m a y n o r m a l l y r u n concurrently, and t h a t the depressor phase only becomes e v i d e n t a f t e r t h e stronger presser response has worn off. I n this context, Cole (1961) has already retold t h a t P P P inhibits t h e experimental increase in I O P in the r a b b i t p r o d u c e d either b y paracentesis or by topical application of mustine hydrochloride solution. :Either of these procedures m a y liberate endogenous P G , so t h a t t h e effect of P P P can be explained in t e r m s of P G a n t a g o n i s m alone. Closer e x a m i n a t i o n of Cole's d a t a on P P P - t r e a t e d animals shows that, instead of there being a rise in I O P after p u t t i n g the mustine on to the cornea, there is in f a c t a prolonged drop in eye pressure which closely resembles the aforementioned response to PGE~. The p a p e r of Fries (1960) on the antipblogistie action of P P P in the r a t ' s hind p a w has p r o m p t e d later workers to conduct similar studies of its effect on ocular inflammation. Consequently, n o t only does P P P p r e v e n t t h e mus~ine-induced a p p e a r a n c e of protein in "the r a b b i t ' s aqueous, b u t it also m a i n t a i n s the i n t e g r i t y of the bloodaqueous barrier when P G E x is introduced into the anterior c h a m b e r (Beitch and E a k i n s , 1969). Owing to its high efficacy and low tmdcity, P P P would seem to be a n ideal prospective c a n d i d a t e for a n a g e n t to be used in the t r e a t m e n t of ocular inflaramation in h u m a n subjects. E a t d n s (pers. comm.) has recently t a k e n steps to isolate a p u r e r and m o r e active a n t i - P G fraction of P P P f r o m the more heterogeneous polymeric p a r e n t m~x~tl~re, which is devoid of it~ k n o w n enzymednh~bitor properties (Diczfalusy et al., 1953). F u r t h e r studies w i t h this m a t e r i a l should reveal w h e t h e r it is suitable for use in h u m a n s . T h e f a c t t h a t ocular effects are manifested b y i n t r a v e n o u s P G ' s could h a v e imp e d a n t implications in t h e field of obstetrics, where P G F ~ a n d PGE~ h a v e been infused i n t r a v e n o u s l y in c o m p a r a b l e a m o u n t s to induce l a b o u r in w o m e n a t or n e a r t e r m (Karim, 1969a,b; K a r i m , Trussel, .]~Jllier a n d P a t e l , 1969; K a r i m , I-/iUier, Trussel, P a t e l a n d T a m u s a n g e , 1970), a n d for t h e purpose of inducing t h e r a p e u t i c a b o r t i o n ( K a r i m a n d Filshie, 1970a,b). I t should be obvious t h a t P G ' s p e n e t r a t i n g t h e ocular tissues from t h e circulation of w o m e n who suffer f r o m a n y form of g l a u c o m a could easily a g g r a v a t e thei:c ocular condition. Such t r e a t m e n t in this t y p e of p a t i e n t
P R O S T A G L A N D I N ON R A B B I T I 0 P
177
would therefore ahuost cel~ainly be contra-indicated, unless of course it were possible to pro~ect the eyes from the lmtoward effects of PG's by pretreating them topically with a compound which specifically fllhibits these effects, without interfering with the response of the uterus. Whether PPP is capable of fulfilling such a role remains to be seen, but it is one of the very few compounds which has been shown consistently to possess an anti-PG property ia vitro (EakirLs and Karim, 1970; Eakins, Karim and h'liUer, 1970), and when applied to the rabbit's eye in the form of drops (Eakins, pers. conun.). ACKNOWLEDGMEI~TS
I should like to thank Dr J. E. Pike (The Upjoha Company, Kalamazoo, U.S.A.) for the geuerous gifts of prostaglaudins, and Dr B. HSgberg (Leo Laboratories, Halsingborg, Sweden] ibr the sample of polyphloretin phosphate. The u.v. reoorder and pressure transducers were made available through the generosity of the Wellcome Trust. REFERENCES Ambachc, N. (1955). J. Physiol. (Lomlon) 129, 65P. Ambache, N. (1957). J. Physiol. (LoTu/on) 135, 114. Ambache, N. (1959). J. Physiol. (LoT~don) 146, 255. Ambache, N. and Brummer, I-I. C. (!968). Brit. J. Pharmaz~l. 33, 162. Ambache, N., Brummer, H. C., Rose, J. G. and Whiting, J. M. C. (I966). ;]. Physiol. (LoTgtoa) 185, 77P. Ambachc, N., Kavanagh, L. and Whiting, J. M. C. (1965). J. Physiol'. (LondorO 176, 378. Beitch, B. R. and EakhLs, K. E. (1969). Brit. J. Pharmacol. 37, 158. Cole, D. F. (1961). Brit. J. Ophtlmlmol. 45, 482. Diczfalusy, E., Ferno, O., Fex, H., HSgberg, B., Linderot, T. and Rosenborg, T. (1953). Acta Chem. Stand. 7, 913. Eakins, K. :~]. (1970). Exp. Eye Ra~. 10, 87. Eakin.~, K. E. and Karbn, S. M:. :M:.(1970). Life ,?ci. 9, 1. Eakins, K. E., Karim, S. 5'I..5I. and Miller, J. D. (1970). Brit. J. Pharmacol. 39, 556. Fries, B. (I960). AcZa Chit, Scand. 119, I. Karim, S. M. (1969a). In Prostaglandins, Peptides and Amines, p. 65, (Ed. by Manteg~zza, P. and Itorton, E. W.). Academic Press, Londom Karim, S. M. hi. (I 969b). In Royz! Society of Medicine, Symposium on Kinins and Prostaglandins, November, 1969. Karim, S. M. M. and Filshie, G. 51:. (1970a). Lancet i, 157. Karim, S. 5I. 5L and Filshio, G. M:. (1970b). Brit. Med. J. iii, 198. Karim, S. M:. M:., Trussel, R. R., Hi/licr, K. and Patet, R. C. (1969). J. Obstet. GFnaecol. Bri~. Coramonw. 76, 769. Karim, S. ~I. l~I., Hillier, K., Tru~el, R. R., Patel, R. C. and Tamus.~nge, S. (1970). J. Ob~et. Gyrg~ecol. Bri$. Commonw. 77, 200. Start, M. S. (1971). Exp. Eye Res. 11, 161. Waitzman, 5I. B. and King, C. D. (1967). Amer. J. Ph.ysioI. 212, 329.
Department of PAysiology and Biochemistry, Institute of Ophthal~nology, University of London, Judd Street, London, WC1H 9QS, Engla~ul (Received 18 December 1970, Z,ondon) The effects o f prostaglandins E on intraocular pressure have been studied h~ rabbits, Prostaglandins Ex mad E, first raised %he intraocular pressure and then lowered it, but. failed to constrict the pupil to any extent. The sensitivity of individual eyes to prostaglandb~ Ex varied more widely with intracameral than intravenous ad/ministration. The latter route gave more reliable pressure responses in both eyes, but slightly bigger doses of prostaglandin were needed. Prior injection of polyphloretin phosphate, into the vitreous completely blocked the rise in intraocular pressure to prostaglandin Ex but not the fall in pressure, while systemic pretreatment ~fith polyptdoretin phosphate was much less effective. 1. I n t r o d u c t i o n I n t h e 1950's, A m b a c h e (1955, 1957, 1959) first d e m o n s t r a t e d t h e p r e s e n c e of a biologically a c t i v e lipid f r a c t i o n i n w a t e r y e x t r a c t s of r a b b i t a n d c a t irides which h,:~, p r o v i s i o n a l l y n a m e d " i r i n " . More r e c e n t l y ( A m b a c h e , B r u m m e r , R o s e a n d Whiting?:, 1966; A m b a c h e and. B r t t m m e r , 1968), t h e a c t i v e c o n s t i t u e n t s of irin h a v e b e e n i d e n t i fied w i t h t h e E a n d F p r o s t a g l a n d i n s , t h e r e b y s t i m u l a t i n g f u r t h e r i n t e r e s t ill t h e ocular actions of these compolmds. I t is n o w well e s t a b l i s h e d t h a t p u r e p r o s t a g l a n d i n s cause a b r e a k d o w n of t h e blooda q u e o u s b a r r i e r a n d a p r o l o n g e d rise in t h e i n t r a o c u l a r pressure ( I O P ) w h e n t h e y are a ~ m l n i s t e r e d i n t o t h e a n t e r i o r c h a m b e r o f t h e r a b b i t ' s (WaitzmaI~ a n d Kin~% 1967 ; B e i t c h a n d E a k i n s , 1969) a n d c a t ' s e y e ( E a k i n s , 1970). T h e d e v e l o p m e n t of t h e s e r e s p o n s e s is i m p a i r e d i n t h e r a b b i t b y p r i o r t r c n t m e n t w i t h p o l y p h l o r e t i n p h o s p h a t e ( P P P ) , a p r o s t a g l a n d i n a n t a g o n i s t ( B e i t e h anqt E a k i n s , 1969). D u r i n g t h e c o u r s e o f s o m e r e c e n t s t u d i e s o f t h e effects o f p r o s t a g l a n d i n s o n b l o o d flow tb.~ough t h e r a b b i C s iris (Start, 1970), i t w a s n o t i c e d t h a t s o m e of t h e featu_~es of t h e I O P r e s p o n s e t o p r o s t a g l a n d i n s differed f r o m t h o s e d e s c r i b e d b y W a i t z m a n a n d K i n g (1967). In. t h i s paper, t h e r e f o r e , s o m e o f t h e a c t i o n s o f p r o s t a g l a n d i n s of t h e E series o n t h e r a b b i t ' s e y e h a v e b e e n r e i n v e s t i g a t e d , a n d t h e a b i l i t y o f p o l y p h l o r e t i n p h o s p h a t e t o m o d i f y t h e m h a s also b e e n s t u d i e d . 2. M e t h o d s
Experimental Male albino (New Zealand) rabbits weighing 2-3 kg were anaesthetized with u r e t h a n e . (1-4--2-0 g/kg i.v.). Drugs were administered systemically via t h e femoral vein or the lingual artery, and int-racamerally via a 27 gauge h y p o d e r m i c needle placed in the anteri~)c c h a m b e r b y means of a specially constructed needle gun. Blood pressure was recorded from t h e femoz'al a r t ~ r y u s i n g a Model SE 4-81 laressure transducer a n d a Model SE 3006 u.v. c h a r t recorder (S.E. Laboratories Ltd.). I 0 P was recorded t h r o u g h 25 gauge needles fired into the anterior c h a m b e r of each eye w i t h t h e needle gun, carefully avoiding t h e iris a n 4 t h e lens. The needles were conn6cted to p r s s s u ~ transducers b y fine 1)olythene tubing. T h e pressure transduceIs were 170
PROSTAGLANDIN
ON RABBIT
I0P
171
held at about 40°C in thermostatically controlled metal blocks in order to minimize drift due to changes in ambient temperature.
Dr'J.g8 ProstaglandJJm (PO's) :Ex and :Ea (The Upjohn Company) and polyphloretin phosphate (PPP, Leo Laboratories) were made up freshly in physiological saline. Syster~ric injections were not greater than 1 ml eel. and were always washed in with 0.1 ml saline. A Pahner slow injection apparatus (C. F. Palmer Ltd.) was used to admlnisber the close-arterial ilffusions at injection rates ~>f 0"25-2"0 ml/miu. Xntraeameral injections were prewarmed to 87°C to facilitate mixing with the aqueous, and given in dose .vo:~umes not exceeding 5/xl with the aid of an "Agla" micrometer syringe (3VellGome Rea~e£ts Ltd.). The PG's were normally stored at --4°C as 1 mg/ml solutions of the s~dium salt in saline. 3.
Results
I n a group of 35 rabbits the m e a n resting I O P recorded soon after calmtflating t h e eyes was 18-564-1-28 mmI-Ig (range 15.4-24-3 m m H g ) , a n d in 3 control ardmals t h e I O P r e m a i n e d fairly c o n s t a n t over a period of 3 hr. Occasionally t h e I O P fell spont,aneously during the course of an e x p e r i m e n t b y 2-4 m m H g , b u t a drop in pressure of t,bis n a t u r e was usually associated with t h e t e m p o r a r y 4za~'cular h y p o t e n s i o n a c c o m p a n y i n g an episode of r e s p i r a t o r y distress. 40-
3O -rE
E
0a_ 2 0 - 0 ' 2 /~.g PGE t 0"4/.,.g I0
0 t
J. 0 - 8 ~ g
!
t
I
R
H
L
F ~ o . 1. :Effect o f i n t r a c a m e r a l P O E 1 o n r a b b i $ l O P . V a l u e s a r e s h o w n f o r t h e m e a n r e s t i n g l O P (1~) a n d f o r t h e h i g h e s t ( H ) a n d l o w e s t ( L ) I O P ' s r e c o r d e d a f t e r i n j e c t i n g P G E z. E a c h p o i n t ( ~ : s . ~ . ~ . ) i s t h e m e a n o f a$ l e a s t 4 d e t e r m i n a t i o n s .
I n t r a c a m e r a l injection of P G E 1 or P G E 2 was followed b y a sustained rise in I O P , the m a g n i t u d e a n d d u r a t i o n of which were dose-dependent. S/lch a response was observed w i t h as little as 25 ng P G E 1, a l t h o u g h lal~er q u a n t i t i e s of this P G were used r o u t i n e l y . F o r example, 0.4/~g P G E 1 e l e v a t e d t h e I O P b y 5-10 mml~g, t h e pressure r e t u r n i n g t o n o r m a l i n 50-70 rain. I n a b o u t o n e - q u a r t e r of She a n i m a l s studied t h e eye pressure t h e n c o n t i n u e d t o .fall below t h e pre-injection level, sometimes b y as m u c h as 10 rnrnHg, a n d s t a y e d low for 180 rni~ or longer. Like ~he preceding presser e~hct, t h e m a g n i t u d e o f t h i s " s e c o n d a r y " or " d e p r e s s o r " response dep e n d e d u p o n t h e concent-ration of P O E x (Fig. 1). Some r a b b i t s failed to e x t i i ~ t t h e usual initial I O P rise a f t e r P O E x b u t still showed
I72
5I. S. S T A R R
a long-lasting ocular hypotension, s t a r t i n g 5-15 rain after t h e P G injection. These reductions in pressure usually p r e d o m i n a t e d in one eye, the c o n t r a l a t e r a l I O P a n d blood pressure b o t h remaining a t a s t e a d y level. P u p i l size was assessed subjectiveIy each time P G E 1 was administered into t h e anterior chamber. :Even after gii,ing 4 tzg P G E x there was no sign of a pupil constriction, although the I O P was elevated b y as much as 40 m m H g with this dose. 20
E
~2 ~
~
R!ght eye
8
0
1
!~
I
2
I ~
4
.5
Number of doses PGE~
F I e . 2. C h a n g e s in ][OP in botl. ayes o f a r a b b i t f o l l o w i n g r e p e a t e d intracarnev~l i n j e c t i o n s o f 0-S t~g P G E 1. N o t e t h e r a p i d d e v e l o p m e n t o f t a c h y p h y l a x i s in t h e left e y e b u t n o t in t h e r i g h t eye.
The most striking f e a t u r e of t h e action of P G E 1 on the r a b b i t ' s eye was its m a r k e d inconsistency. I t varied from no presser response a t all in some animals, to either a pronolmced rise in l O P with the fu'st injection o n l y (followed b y a rapidly-developing t a c h y p h y l a x i s ) , or a c o n t i n u i n g sensitivity to repeated i n t r a c a m e r a l injections of P G E 1 (with h t t l e or no evidence of t a c h y p h y l a x i s ) . Such differences were even a p p a r e n t between t h e bilateral responses of the same rabbit. This finding is well illustrated in Fig. 2, in which i t is p l a i n l y seen t h a t the increase in pressure elicited b y i n t r a o c u l a r P G E 1 (0-8 tzg) in the left eye grew progressively smaller with each successive dose, while 5 such injections dehvered into t h e r i g h t eye were n o t subject to tachyphylaxZu. Far~her experiments were therefore u n d e r t a k e n to determine t h e e x t e n t of this interocular v a r i a t i o n . A comparison was m a d e of t h e sizes of the presser responses induced b y a single dose of P O E z (0-2-0-8/~g) given i n t r a c a m e r a l l y to a group of 18 r a b b i t s in equal c o n c e n t r a t i o n to b o t h eyes. The results are shown i n the scatter d i a g r a m in Fig, 3(a). F r o m t h e value for t h e S p e a r m a n coefficient of r a n k correlation (R = -- 0-263) i t is quite clear t h a t ~he effects produced in the left eyes differed widely from those in t h e r i g h t ( P > 0 - 1 ) . On t h e o t h e r h a n d , it was noticed t h a t similar responses were u s u a l l y evoked i n b o t h cymeswhen PG was a d m i n i s t e r e d systemically. Single doses of 1-4/zg P O E 1 were therefore given i n t r a v e n o u s l y to 20 rabbits a n d t h e changes in eye pressures recorded a n d t r e a t e d statis+Acally in a similar faskion [Fig. 3(b)]. This t i m e the value for R came to + 0.8~:7, i n d i c a t i n g a v e r y h i g h l y sigrdfic a n t correlation between t h e ~wo sets of results ( P < 0 - 0 0 1 ) . F o r this reason the i n t r a venous route of i n j e c t i o n was preferred in studies involving a comparison of t h e e v e n t s Occurring in b o t h eYes. (see later, inlHbitor studies). I n t r a v e n o u s P G E x w a s generally 5-10 t i m e s less effective t h a n i n t r a c a m e r a l PG~] 1 in raising IOP, b u t t h e log dose,response plots for the two routes were a p p r o ~ maCely parallel (Fig. 4). A m a x i m u m response was produced b y a b o u t 0-8 ~g P G E 1
PtlOSTAGLANDIN
01~ RABBIT
lOP
173
given int~:aeamerally c o m p a r e d to about. 4/xg systemically. The large s t a n d a r d errors a t t a c h e d to each point emphasize t h e wide s c a t t e r of individual points a b o u t the II:lC~lrl S.
32
(a)
(b}
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Q
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o •
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--
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o
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f
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20
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in
right
I
I
!
I
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,5
tO
if5
lOP
(mm
1.
20
I
25
Hg)
:FJo. 3. l ~ e l ~ . . o u s h i p b e t w e e n t h e i n c r e a s o i n I O P in t h e lef~ a n d r i g h ¢ e y e s o f r ~ b b i t s a f t e r i n t r a e ~ m e r a ! (a) o r i n t r a v e n o u s (b) a d m i n i s t r a t i o n o f : P G E 1.
24 .~
f8 -
r-
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!
0-2
! ......
£~4
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_ I
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Log dose PGE I ( ~ g )
~ o . 4. E f f e c t o f i n t r a o a m c r a l ( I . C . ) a n d i n t r a v e n o u s ( I . V . ) P G E z o n r a b b i t I O P . E a c h p o i n t ( ~ - s . ~ . M . ) is the mean of at least 6 estimations.
The n a t u r e of t h e I O P response t 0 i n t r a v e n o u s P G E 1 w a s also s o m e w h a t di~Terent. In h a l f t h e animals tested there was a typical s m o o t h rise in eye pressure afber a delay o f 15-20 rain, while the other, h a l f e~h~bited a t w o - s ~ e pressor response; a n initial small rise of 2-10 mmr~g,- ~ h i c h lasted 10-20 m~ri, followed b y a •more pronounced
174
M. S. S T A r , It
change (10-40 m m H g ) similar to thab observed after intraoeular administration ot' the drug. I n b o t h instances the' eye pressures recovered a n d then t~.nded to fall and remain below t h e b pre-injection levels, as described above.
I00 i
/
.....
80
~
6o
40
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!
.t_
15
:5O
.__t~
50
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i.o~ dose PPP (rag) F~G..5. I n h i b i t i o n b y c l o s e - a r t e r i a l i n f u s i o n o f P P P o f t h e i n c r e a s e i n r a b b i t ' s [ O P p r o d u c e d b y 4 ttg i n t r a v e n o u s ]PGE 1. E a c h p o i n t is t h e m e a n o f 3 e s b i m a t i o n ~ .
Both routes of administration suffered disadvantages. I n t r a c a m e r a l injections always elevated the I O P temporarily b y 0.5-4- m m ~ g , b u t this a r t e f a c t was minimized b y keeping the injected volume small. W h e n introduced in this w a y :PGE~ produced entirely localized effects and did not modify blood pressure. B y contrast, intravenous P G E 1 lowered the blood pressure irt a dose-dependent fashion; for example, 4/Lg induced a fall of 30-40 m m H g f r o m a m e a n resting pressure of 93-4=7-2 mini{g: which recovered completely in 5--10 min. I t should be noted, however, t h a t in spit~. of this hypotension there was almost invariably a transient rise in eye pressure during the course of the injection, which a m o u n t e d to 1-5 m m H g a n d lasted for 0.5-4 rain. depending on the dose administered. H a v i n g established the criteria necessary for evoking a reliable 7 0 P response to
/
6°I 50o
/
~- 4 0 c_ z-
30
20-
-
J
I I ! __ L 5 I0 20 4.0 Log dose PPP (~g/2Omin)
FIe. 6. Inhibition by intravitreal P P P of the increase in rabbit's I O P produced by 4 pg intravenous PGEI. Each point is the m e a n of 3 estimations.
PROSTAGLANDIN
ON RABBIT
IOP
175
PGE1, a separate s t u d y was t h e n made to examine t h e properties of the P G inhibitor, P P P . Figure 5 illustrates the inhibitory- avtion of different rates of infusion of P P P (0.25-2 mg/mi~t for 20 ruin) given via the lingual artery, on one side, against the bilateral ocular h y p e r t e n s i o n produced b y i n t r a v e n o u s P G E 1 (4/~g). The degree of inhibition of t h e response in the test eye was arrived a t b y c o m p a r h l g it with t h e control response present in the opposite eye. Low rates of infusion (0.05-0.25 rag/rain) were w i t h o u t m u c h effect, b u t as the flow rate was raised a b o v e 0-25 rag/rain the pressor response to P G E 1 was considerably diminished. Thus a 5~A°/ ~o i a h i b i t i o n (EDr,o) was achieved in the test eye b y infusing 1.43 mg P P P / m i n o~ei:r a time period of 20 rain. A t thLs c o n c e n t r a t i o n P P P did n o t lower blood pressure, i whereas t h e same quantity" of P P P administered over 5 or 10 min, did cause a small hypotension, b u t this quickly recovered on stopping the infusion. Similar resulk~ were obtained when P P P was injected i n t r a ~ i t r e a l l y into one eye 20-24 h r previously, except t h a t here the EDso for t h e a n t a g o n i s m of P G E r b y P P P was a b o u t 50/xg, a b o u t 1/600 of the t o t a l dose administered systemically (see Fig. 6). A n y effect of the i n t r a v i t r e a l injection of P P P on I O P h a d worn off b y t h e time the :PGE 1 was given. A finding of particular interest was t h a t in all 5 r a b b i t s p r e t r e a t e d with the highest dose of P P P (100 tzg), the pressor response to P(~JE 1 was completely abolished, and y e t almost i m m e d i a t e l y after injecting the P G E 1 t h e I O P ' s of the test eyes began to fall (on average by a t o t a l of - - 6 . 9 ± 1 - 4 mini-It), and did n o t recover before the end of the experiment, 60 rain later. 4. Discussion
I t has not been possible to confirm the observations of W a i t z m a n and King (1967) t h a t P G E ' s cause a m a r k e d constriction of t h e r a b b i t ' s pupil. ~taese a u t h o r s n o t e d some reduction in pupil size w i t h n a n o g r a m quantities of PG, while in the present s t u d y microgram a m o u n t s proved to be more or less ineffective in this respect. The reason for ~his discrepancy is n o t known, b u t i t m a y be related to differences in the s t r a i n of r a b b i t and/or to t h e variou~s batches of P G used. On the other hand, the n a t u r e and m a g n i t u d e of the p a r a m e t e r s of the ocular h y p e r t e n s i o n following i n t r a c a m e r a l 1PGE x (i.e. rate of olmet of Eae response, its size and duration) were v e r y similar to those reported b y Beitx~h a n d l~akins (1969), who used slightly smaller concentrations of tbJs drug. B o t h sets. of results indicate t h a t the response lasts no more t h a n 1-2 h r as compared w i t h t h e 3 h r or more observed b y ~Vaitzman a n d K i n g (1967) for tlfis species. However, there are three ways in which the present results differ from those of previous investigators: (i) The first concerns the ~'equeucy with which t a c h y p h y l a x i s occurs. I n c e n ~ a s t to Beiteh and E a k i n s ' (1969) findings, this p h e n o m e n o n did n o t show up in every e x p e r i m e n t with PGE1, a n d when it did appear it was t h e n som~tLmes restricted to one eye. This emphasizes t h a t even the two eyes of one r a b b i t c a n n o t always be relied ou to r e a c t to PG's in a n identical fashion, t h u s m a k i n g i t difficult to c o m p a r e t h e effects of bilateral t r e a t m e n t . :Fortunately bilateral refractorin.ess to t h e pressor a c t i o n of PG's on the I O P did n o t occur t o o often, a n d i t is considered less likely to be due to interocular v a r i a t i o n t h a n t o de~fects in e x p e r h n e n t a l procedure, such as (a) i n a d e q u a t e mixing of t h e drug, ( b ) p o o r positioning of the; i n j e c t i o n needles, ( c ) prior liberation of endogenous PG's from t h e iris b y direc~b c o n t a c t with the i m p l a n t e d needles (Ambache; i ~ a v a n a g h a n d Whiting, 1965). (ii) The P G E ' s , a n d in all-~r0bability other PG's as well, exerl; a pronounced a n d
176
~I. S. S T A R R
reliable presser effect on I O P wh,en t h e y are admhxiatered systemically (c.f. ~ r a i t z m a n a n d King, 1967). So m u c h so, t h a t this r o u t e was used as m a t t e r of policy when bilateral comparisons were to be made, even t h o u g h i n t r a c a m e r a l injectiop required m u c h Iess PGE~ to evoke similar responses, I t seems likely t h a t the initial rise in I O P which followed i n t r a v e n o u s injection of PGE~ in a b o u t half t h e rabbits was due to t h e spasmogenic p r o p e r t y of t h e d r u g on t h e e x t r a o c u l a r muscles, since the response lasted too long to be t h e result of local changes in blood flow, a n d y e t it wm~ over too quickly to be a t t r i b u t a b l e to a direct a l t e r a t i o n in aqueous flow. N e i t h e r this side-effect nor t h e more obvious d i s a d v a n t a g e of widespread vascular d i l a t a t i o n (and hence lowering of blood pressure) a c c o m p a n y i n g the injection, appeared to interfere w i t h t h e subsequent d e v e l o p m e n t of t h e typical prolonged increase in ocular tension. (iii) The ability of P G E 1 to lower I O P in the rabbit, which has n o t been previously reported, occurred sufficiently often in t h e present e x p e r i m e n t s to be recognized as real effect, since it was considerably longer-lasting t h a n t h e more typical rise in pressure which invariably preceded it. I t was also particularly interesting to find t h a t i n t r a v i t r e a l p r e t r e a t m e n t with P P P completely blocked t h e initial hyperte~lsive action of P G E 1 w i t h o u t affecting this fall in pressure. U n d e r these conditions t h e hypotension was more r a p i d in onset, suggesting t h a t in u n t r e a t e d animals ~hese two opposing actions on ocular pressure m a y n o r m a l l y r u n concurrently, and t h a t the depressor phase only becomes e v i d e n t a f t e r t h e stronger presser response has worn off. I n this context, Cole (1961) has already retold t h a t P P P inhibits t h e experimental increase in I O P in the r a b b i t p r o d u c e d either b y paracentesis or by topical application of mustine hydrochloride solution. :Either of these procedures m a y liberate endogenous P G , so t h a t t h e effect of P P P can be explained in t e r m s of P G a n t a g o n i s m alone. Closer e x a m i n a t i o n of Cole's d a t a on P P P - t r e a t e d animals shows that, instead of there being a rise in I O P after p u t t i n g the mustine on to the cornea, there is in f a c t a prolonged drop in eye pressure which closely resembles the aforementioned response to PGE~. The p a p e r of Fries (1960) on the antipblogistie action of P P P in the r a t ' s hind p a w has p r o m p t e d later workers to conduct similar studies of its effect on ocular inflammation. Consequently, n o t only does P P P p r e v e n t t h e mus~ine-induced a p p e a r a n c e of protein in "the r a b b i t ' s aqueous, b u t it also m a i n t a i n s the i n t e g r i t y of the bloodaqueous barrier when P G E x is introduced into the anterior c h a m b e r (Beitch and E a k i n s , 1969). Owing to its high efficacy and low tmdcity, P P P would seem to be a n ideal prospective c a n d i d a t e for a n a g e n t to be used in the t r e a t m e n t of ocular inflaramation in h u m a n subjects. E a t d n s (pers. comm.) has recently t a k e n steps to isolate a p u r e r and m o r e active a n t i - P G fraction of P P P f r o m the more heterogeneous polymeric p a r e n t m~x~tl~re, which is devoid of it~ k n o w n enzymednh~bitor properties (Diczfalusy et al., 1953). F u r t h e r studies w i t h this m a t e r i a l should reveal w h e t h e r it is suitable for use in h u m a n s . T h e f a c t t h a t ocular effects are manifested b y i n t r a v e n o u s P G ' s could h a v e imp e d a n t implications in t h e field of obstetrics, where P G F ~ a n d PGE~ h a v e been infused i n t r a v e n o u s l y in c o m p a r a b l e a m o u n t s to induce l a b o u r in w o m e n a t or n e a r t e r m (Karim, 1969a,b; K a r i m , Trussel, .]~Jllier a n d P a t e l , 1969; K a r i m , I-/iUier, Trussel, P a t e l a n d T a m u s a n g e , 1970), a n d for t h e purpose of inducing t h e r a p e u t i c a b o r t i o n ( K a r i m a n d Filshie, 1970a,b). I t should be obvious t h a t P G ' s p e n e t r a t i n g t h e ocular tissues from t h e circulation of w o m e n who suffer f r o m a n y form of g l a u c o m a could easily a g g r a v a t e thei:c ocular condition. Such t r e a t m e n t in this t y p e of p a t i e n t
P R O S T A G L A N D I N ON R A B B I T I 0 P
177
would therefore ahuost cel~ainly be contra-indicated, unless of course it were possible to pro~ect the eyes from the lmtoward effects of PG's by pretreating them topically with a compound which specifically fllhibits these effects, without interfering with the response of the uterus. Whether PPP is capable of fulfilling such a role remains to be seen, but it is one of the very few compounds which has been shown consistently to possess an anti-PG property ia vitro (EakirLs and Karim, 1970; Eakins, Karim and h'liUer, 1970), and when applied to the rabbit's eye in the form of drops (Eakins, pers. conun.). ACKNOWLEDGMEI~TS
I should like to thank Dr J. E. Pike (The Upjoha Company, Kalamazoo, U.S.A.) for the geuerous gifts of prostaglaudins, and Dr B. HSgberg (Leo Laboratories, Halsingborg, Sweden] ibr the sample of polyphloretin phosphate. The u.v. reoorder and pressure transducers were made available through the generosity of the Wellcome Trust. REFERENCES Ambachc, N. (1955). J. Physiol. (Lomlon) 129, 65P. Ambache, N. (1957). J. Physiol. (LoTu/on) 135, 114. Ambache, N. (1959). J. Physiol. (LoT~don) 146, 255. Ambache, N. and Brummer, I-I. C. (!968). Brit. J. Pharmaz~l. 33, 162. Ambache, N., Brummer, H. C., Rose, J. G. and Whiting, J. M. C. (I966). ;]. Physiol. (LoTgtoa) 185, 77P. Ambachc, N., Kavanagh, L. and Whiting, J. M. C. (1965). J. Physiol'. (LondorO 176, 378. Beitch, B. R. and EakhLs, K. E. (1969). Brit. J. Pharmacol. 37, 158. Cole, D. F. (1961). Brit. J. Ophtlmlmol. 45, 482. Diczfalusy, E., Ferno, O., Fex, H., HSgberg, B., Linderot, T. and Rosenborg, T. (1953). Acta Chem. Stand. 7, 913. Eakins, K. :~]. (1970). Exp. Eye Ra~. 10, 87. Eakin.~, K. E. and Karbn, S. M:. :M:.(1970). Life ,?ci. 9, 1. Eakins, K. E., Karim, S. 5'I..5I. and Miller, J. D. (1970). Brit. J. Pharmacol. 39, 556. Fries, B. (I960). AcZa Chit, Scand. 119, I. Karim, S. M. (1969a). In Prostaglandins, Peptides and Amines, p. 65, (Ed. by Manteg~zza, P. and Itorton, E. W.). Academic Press, Londom Karim, S. M. hi. (I 969b). In Royz! Society of Medicine, Symposium on Kinins and Prostaglandins, November, 1969. Karim, S. M. M. and Filshie, G. 51:. (1970a). Lancet i, 157. Karim, S. 5I. 5L and Filshio, G. M:. (1970b). Brit. Med. J. iii, 198. Karim, S. M:. M:., Trussel, R. R., Hi/licr, K. and Patet, R. C. (1969). J. Obstet. GFnaecol. Bri~. Coramonw. 76, 769. Karim, S. ~I. l~I., Hillier, K., Tru~el, R. R., Patel, R. C. and Tamus.~nge, S. (1970). J. Ob~et. Gyrg~ecol. Bri$. Commonw. 77, 200. Start, M. S. (1971). Exp. Eye Res. 11, 161. Waitzman, 5I. B. and King, C. D. (1967). Amer. J. Ph.ysioI. 212, 329.