www.cuwai.com The canine eye: lens depression and aqueous outflow E. Michael Van Buskirk The effects on facility of outflow of lens depression (controlled mechanical retrodisplaceinent of the crystalline lens to simulate cyclotonia in excised eye preparations), prolonged perfusion, and intracameral hyaluronidase were evaluated in 16 pairs offreshly excised canine eyes. Lens depression increased facility of aqueous outflow by an amount greater than could he attributed to mechanical or temporal disruption of the hyaluronidase-sensitive outflow barrier and ap- pears to act, at least in part, on the hyaluronidase-resistant portion of the outflow barrier. These data suggest that in addition to accelerating the washout effect, mechanical tension on the iridocorneal angle in the dog increases facility of aqueous outflow in a manner similar to the direct trabecular component of cyclotonic-induced facility increases observed in primate eyes. Key words: canine eye, facility of outflow, cyclotonia, lens depression, "washout", hyaluronidase-sensitive barrier I revious segments of these studies have effects may be paradoxical and, if exploited, described characteristic features of the ca- may provide insight into common mecha- nine aqueous outflow system, many of which nisms for aqueous outflow shared by the have contrasted the canine with the primate spectrum of mammalian species. eye. 1 " 3 Both mammalian groups, however, Parasympathomimetic agents increase fa- appear to share some features, including an cility of outflow in primates by means of ocular hypotensive effect from pilocarpine ciliary muscle contraction against trabecular administration to glaucomatous eyes. 4 " 6 At meshwork, in part, through scleral spur.7 first glance, a common pharmacologic effect The experimental technique of lens depres- between two mammalian species appears in- sion simulates this effect by mechanically tuitively predictable. On the other hand, pulling on the ciliary body through the lens considering the numerous differences be- zonules. Although force vectors of lens de- tween the two outflow systems, these similar pression are more centripetally oriented than those of true ciliary muscle contraction, their combined effect is sufficiently comparable to increase, reversibly and reproducibly, facil- From the Division of Ophthalmology, Department of ity of aqueous outflow in excised primate Surgery, The Milton S. Hershey Medical Center, The eyes.8"10 The present study applies this Pennsylvania State University College of Medicine, Hershey. technique to enucleated canine eyes, recog- This study was supported by NIH Research grant nizing that changes in facility which may be EYO1563 and Research Career Development Award quantitatively comparable to those observed EYO0051 from the National Eye Institute. in the primate need to be assessed in the Submitted for publication April 16, 1979. light of known anatomic and physiologic Reprint requests: E. M. Van Buskirk, M.D., Depart- ment of Ophthalmology, University of Oregon Health differences among the separate mammalian Sciences Center, Portland, Ore. 97201. species. 0146-0404/80/070789+04$00.40/0 © 1980 Assoc. for Res. in Vis. and Ophthal., Inc. 789 www.cuwai.com Invest. Ophthalmol. Vis. Sci. 790 Van Buskirk July 1980 terior chambers of six pairs of eyes were irrigated with 10 ml of Hanks' solution containing 300 U/ml bovine testicular hyaluronidase (Wydase; Wyeth Laboratories, Philadelphia, Pa.). This solution was allowed to remain in the anterior chamber, with a. 2.0 the corneal fitting removed, at ambient pressure 5 for 30 min and was then removed by irrigation o £ 1.6 with 10 ml of Hanks' solution. Because of the non- specificity of bovine testicular hyaluronidase for canine anterior chamber angle mucopolysac- > = 1.2 charides, the relatively high dosage of hyaluroni- dase was chosen on the basis of previously re- 0.8 ported data to ensure as complete a dissolution of the hyaluronidase-sensitive barrier to aqueous NO HYALURONIDASE • — • — • WITHOUT LENS DEPRESSION outflow as possible.1" 2 0.4 NO HYALURONIDASE O - O WITH LENS DEPRESSION IC. HYALURONIDASE A A WITHOUT LENS DEPRESSION IC. HYALURONIDASE A A WIIH LENS DEPRESSION Results TIME (minutes) 60 120 180 For the eyes not treated with hyaluroni- LENS DEPRESSION (units) 2 4 6 dase, mean baseline facility of outflow was Fig. 1. Effects of prolonged perfusion and lens 0.39 ± 0.03 /Ltl/min/mm Hg* for both the 10 depression on facility of outflow in 16 pairs of eyes to be subjected to lens depression and canine eyes, of which six pairs had been treated the 10 fellow control eyes. After 6 U of lens with intracameral hyaluronidase. depression, mean facility of outflow increased to 1.43 ± 0.08 /Ltl/min/mm Hg for an average Methods total increase of 275% ± 102 or an average Facility of aqueous outflow was estimated in 16 facility increase of 96% ± 40 per hour. In the pairs of anatomically normal eyes, excised within 3 fellow, control eyes perfused over the same hr postmortem from mongrel dogs, by quantita- length of time, mean facility increased by an tive aqueous perfusion at room temperature with average total of 163% ± 25 or 54% ± 8 per Hanks' balanced salt solution at an intraocular hour to a mean of 1.00 ± 0.23 /Ltl/min/mm pressure of 20 mm Hg. The eyes were stored and Hg. The mean rate of facility increase per prepared for perfusion as previously described.' hour was significantly higher in the eyes sub- Following removal of a 5 mm button from the cen- jected to lens depression than in the control tral cornea, a radial iridotomy provided communi- eyes (0.01 < p < 0.02) (Fig. 1). cation between the anterior and posterior cham- bers. The lens was carefully inspected to ensure an Of the six pairs of eyes which were pre- intact anterior capsule. The anterior chamber was treated with hyaluronidase, initially mea- perfused by means of an acrylic 5 mm corneal sured facility of outflow averaged 1.06 ± 0.06 fitting described by Grant." The lens depression /Ltl/min/mm Hg for those to be subjected to apparatus was fitted to one member of each pair. lens depression and 1.33 ± 0.63 for the con- As before, the facility of outflow estimated from trol eyes pretreated with hyaluronidase but the flow measured after 1 hr was arbitrarily desig- not subjected to lens depression eyes. Facil- nated "baseline facility of outflow" to permit ade- ity increased with lens depression to 1.61 ± quate time for viscoelastic equilibration.1 The 0.16 /Ltl/min/mm Hg for an average total in- plunger of the lens depression apparatus was ad- crease of 58% ± 35 or 19.33% ± 11 per hour vanced from the faceplate of the corneal fitting a distance of 2 U (1.1 mm) every hour to a maximum (Fig. 1). There was no significant difference of 6 U over 3 hr of additional perfusion; the fellow, between measured facility of outflow with control eye was continuously perfused for 4 hr 0 U and 2 U of lens depression (0.1 < without lens depression. The plunger was approx- p < 0.2), but the facilities measured at 0 U imately in apposition with the anterior surface of and 6 U of lens depression were significantly the lens with advancement of 2 U. Before insertion of the corneal fitting, the an- * All means are expressed ± S.E.M. www.cuwai.com Volume 19 Number 1 Canine eye: lens depression, aqueous outflow 791 different (0.01 < p < 0.02). Over the same the values for posthyaluronidase facility were length of time, facility of the control eyes in- variable. It is likely that transtrabecular driv- creased from 1.33 ± 0.63 /ul/min/mm Hg to ing pressure facilitates the action of in- 1.46 ± 0.53, for an average total increase of tracameral hyaluronidase by delivering the 16.7% ± 25.0 or 5.5% ±8.1 per hour. enzyme to its site of action deep in the irido- These initial and final facilities were not sig- corneal angle and by washing downstream nificantly different (0.6 < p < 0.7). residual mucopolysaccharide by-products. In spite of pretreatment with a high dose of Since the experimental procedure of lens de- intracameral hyaluronidase, some time-de- pression necessitates increased anterior seg- pendent facility increase persisted without ment manipulation, undoubtedly the proce- lens depression in two eyes by as much as dure itself contributes a kind of washout 18%/hr whereas it was under 3%/hr in the effect by partially disrupting the hyaluroni- remaining 4 eyes. Among the hyaluronidase- dase-sensitive portion of the outflow barrier. treated eyes which were subjected to lens This effect cannot be clearly separated, by depression, the facility increase exceeded the techniques employed in these studies, 20%/hr in four of six eyes, but the differences from any direct action of lens depression on between the mean hourly facility increase the nonmucopolysaccharide elements of the with and without lens depression were filtration meshwork. However, lens depres- not quite statistically significant (0.05 < sion did increase facility in most of the eye p<0.1). studies by more than could clearly be attri- buted to washout alone. Moreover, lens de- Discussion pression continued to increase facility in the Although mean baseline facility of outflow hyaluronidase-treated eyes in which the in these canine eyes was comparable to that hyaluronidase-sensitive component of the observed previously, it was somewhat high- outflow barrier had been previously elimi- er. ' Because of the mechanical fragility of the nated or markedly diminished. Although lens hyaluronidase-sensitive component of the depression-induced dissolution of iridocor- canine aqueous outflow barrier, the higher neal angle mucopolysaccharides probably ac- facilities which were observed in these eyes counted for a portion of the facility increases may have been attributable to the greater observed, the remaining portion may well manipulation necessitated by the corneal have resulted from direct action on the re- trephination and radial iridotomy. On the sidual hyaluronidase-resistant portion of the other hand, initially measured posthyaluron- canine outflow barrier. It is this latter effect idase facility tended to be lower in this study which may be relevant to outflow modulation than previously observed, an effect also at- in the primate. tributable to the change in technique. Al- Increases in facility of outflow induced by though this difference may simply reflect the cyclotonia in living, or by lens depression in relatively small number of eyes studied, the excised, primate eyes may be partially but lack of transtrabecular aqueous flow at the not entirely attributable to separation of the ambient intraocular pressure extant during opposing walls of Schlemm's canal and resul- enzyme exposure undoubtedly was a major tant partial intracanalicular circumferential factor. A large volume of hyaluronidase, em- flow.8- 12~13 Schlemm's canal canalicular wall ployed in an attempt to compensate for the separation is anatomically precluded in the lack of transtrabecular driving pressure, suc- dog by the absence of a Schlemm's canal ho- cessfully eliminated the subsequent time-de- mology.3 On the other hand, the remaining pendent increase in facility (washout) in only component of cyclotonic-induced facility in- four of the six control eyes. This meant that a creases in the primate probably derives from portion of the hyaluronidase-sensitive com- a direct effect on the trabecular meshwork ponent persisted in some eyes even after in- itself.14 In previous lens depression experi- tracameral hyaluronidase irrigation and that ments in primates, an attempt was made to www.cuwai.com Invest. Ophthalmol. Vis. Sci. 792 Van Buskirk July 1980 differentiate the portions of facility increases glaucoma in the beagle. INVEST OPHTHALMOL VI- SUAL SCI 16:1135, 1977. resulting from induction of circumferential 7. Kaufman PL and Barany EH: Loss of acute pilocar- flow and from a direct effect on trabecular pine effect on outflow facility following surgical dis- meshwork. Interestingly, the magnitude of insertion and retrodisplacement of the ciliary mus- the total lens depression-induced facility in- cle from the scleral spur in the cynomolgus monkey. crease in the dog is less than the total effect of INVEST OPHTHALMOL 15:793, 1976. lens depression in primate eyes but is quanti- 8. Van Buskirk EM and Grant VVM: Lens depression and aqueous outflow in enucleated primate eyes. tatively comparable to the trabecular compo- Am J Ophthalmol 76:632, 1973. nent of facility increases in the primate.8 9. Van Buskirk EM: Changes in the facility of aqueous In spite of many important anatomic and outflow induced by lens depression and intraocular physiologic differences between the aqueous pressure in excised human eyes. Am J Ophthalmol outflow systems of the primate and canine 82:736, 1976. 10. Epstein DL, Hashimoto JM, and Grant WM: Serum eyes, they appear to share some aspects of obstruction of aqueous outflow in enucleated eyes. these responses to mechanical tension on Am J Ophthalmol 86:101, 1978. their respective filtration mesh works. 11. Grant WM: Further studies on facility of flow through the trabecular meshwork. Arch Ophthalmol 60:523, 1958. REFERENCES 12. Rohen JW, Liitjen E, and Barany E: The relation 1. Van Buskirk EM and Brett J: The canine eye: in between the ciliary muscle and the trabecular vitro dissolution of the barriers to aqueous outflow. meshwork and its importance for the effect of mi- INVEST OPHTHALMOL VISUAL SCI 17:258, 1978. otics on aqueous outflow resistance. A study in two 2. Van Buskirk EM and Brett J: The canine eye: in contrasting monkey species, Macaco irus and Cer- vitro studies of the intraocular pressure and facility copithecus aethiops. Albrecht von Graefes Arch Klin of aqueous outflow. INVEST OPHTHALMOL VISUAL SCI Exp Ophthalmol 172:23, 1967. 17:373, 1978. 13. Barany EH, Linner E, Liitjen-Drecoll E, and 3. Van Buskirk EM: The canine eye: the vessels of Rohen JW: Structural and functional effects of aqueous drainage. INVEST OPHTHAL VISUAL SCI trabeculectomy in cynomolgus monkeys. Albrecht 18:223, 1979. von Graefes Arch Klin Exp Ophthalmol 184:1, 1972. 4. Gelatt KN, Peiffer RL Jr, Gwin RM, and Sauk JJ Jr: 14. Barany EH: The influence of extraocular venous Glaucoma in the beagle. Trans Am Acad Ophthal- pressure on outflow facility in Cercopithecus ethiops mol Otolaryngol 81:636, 1976. and Macaca fascicularis. INVEST OPHTHALMOL VI- 5. Gelatt KN: Animal models for glaucoma. INVEST SUAL SCI 17:711, 1978. OPHTHALMOL VISUAL SCI 16:592, 1977. 15. Nesterov AP, Hasanova NH, and Batmanov YE: 6. Gelatt KN, Peiffer RL Jr, Gwin RM, Gum GG, and Schlemm's canal and scleral spur in normal and Williams LW: Clinical manifestations of inherited glaucomatous eyes. Acta Ophthalmol 52:634, 1974.