Vector-Selective Contractility in Monkey Ciliary Muscle

Glaucoma is a common affliction of the eye of older people which, if left untreated, often progresses to the development of ‘tunnel vision’, and ultimately blindness, over a period of a decade or more. The disease is characterized by an increase in pressure inside the eyeball (caused by a partial impedance of outflow of fluid – aqueous humor – from the inside of the eye to the bloodstream) which is damaging to the optic nerve at the back of the eye. This nerve conveys visual images from the eye to the brain. One approach to treatment of the disease is the instillation of eye drops containing a drug which causes a muscle (known as the ciliary muscle – CM) inside the eye to contract. This improves the outflow of aqueous humor from the eye to the bloodstream, which leads to a reduction in eye pressure. Unfortunately, contraction of the CM also causes the lens inside the eye to become more round (a phenomenon known as accommodation). This limits clear focus to close-up objects only, with anything further away appearing as a blur. One possible approach to eliminate this problem would be to find a drug which will selectively contract that part of the CM involved in outflow of aqueous humor, but not that part involved in accommodation.

Hypothesis

The crucial role of the CM in modulating accommodation and aqueous humor outflow has prompted some investigators to study the contractile properties of CM’s removed from animal eyes and kept alive artificially under laboratory conditions. The response of such ‘in vitro’ CM preparations to a number of different types of drugs has been characterized, but no investigator has ever studied the contractile properties of the in vitro CM in more than one contractile vector simultaneously. The CM is capable of contracting in three different vectors, and possesses microscopic contractile muscle fibers running in three different planes. Contraction of the longitudinal fibers is thought to improve fluid outflow and thus reduce eye pressure whereas contraction of the circular fibers leads to accommodation. No drug in current clinical use for the treatment of glaucoma can selectively contract only the longitudinal fibers, without also causing contraction of the circular fibers. Based on structural and functional characteristics of the CM, we believe that identification of such drugs may be possible.

Specific Aims

We have developed an apparatus capable of measuring contractile force of the in vitro CM of monkeys in both the longitudinal and circular vectors simultaneously, in response to drugs. We now want to test the hypothesis that the contractile force of the CM in the longitudinal vector can be selectively induced by an appropriate drug or combination of drugs. We will test drugs from several major groups which, from other information, are promising candidates. To study in vitro CM preparations in this way, it is necessary to obtain CM from eyes immediately after death, making it impractical to utilize human CM’s, which would not become available to us for many hours after death. We therefore use animal CM. Since this tissue is highly variable in its biological characteristics from one species to another, we use CM obtained from rhesus monkeys at the Wisconsin Regional Primate Research Center, which have eyes much like the human. Once the fresh CM is obtained, we can maintain it in a living condition for several days, enabling us to perform many experiments.

2nd Year

Glaucoma is a common affliction of the eye of older people which, if left untreated, often progresses to the development of ‘tunnel vision’, and ultimately blindness, over a period of a decade or more. The disease is characterized by an increase in pressure inside the eyeball (caused by a partial obstruction of drainage of fluid- aqueous humor- from the inside of the eye to the bloodstream) which is damaging to the optic nerve at the back of the eye. This nerve conveys visual images from the eye to the brain.

Hypothesis

One approach to treatment of the disease is the instillation of eye drops containing a drug which causes a muscle (known as the ciliary muscle – CM) inside the eye to contract. This improves the drainage of aqueous humor from the eye to the bloodstream, which leads to a reduction in eye pressure. Unfortunately, contraction of the CM also causes the lens inside the eye to become more round (a phenomenon known as accommodation). This limits clear focus to close-up objects only, with anything further away appearing as a blur. One possible approach to eliminate this problem would be to find a drug which will selectively contract that part of the CM involved in drainage of aqueous humor, but not that part involved in accommodation. The crucial role of the CM in modulating accommodation and aqueous humor outflow has prompted some investigators to study the contractile properties of CMs removed from animal eyes and kept alive artificially under laboratory conditions. The response of such ‘in vitro’ CM preparations to a number of different types of drugs has been characterized, but no investigator has ever studied the contractile properties of the in vitro CM in more than one direction simultaneously. The CM is capable of contracting in three different directions, corresponding to the microscopic muscle fibers running in three different planes. Contraction of the longitudinal fibers is thought to improve fluid drainage and thus reduce eye pressure, whereas contraction of the circular fibers leads to accommodation. No drug in current clinical use for the treatment of glaucoma can selectively contract only the longitudinal fibers, without also causing contraction of the circular fibers. Based on structural and functional characteristics of the CM, we believe that identification of such drugs may be possible.

Specific Aims

We have developed an apparatus capable of measuring contractile force of the in vitro CM of monkeys in both the longitudinal and circular directions simultaneously, in response to drugs. We now want to test the hypothesis that the contractile force of the CM in the longitudinal vector can be selectively induced by an appropriate drug or combination of drugs. We will test drugs from several major groups which, from other information, are promising candidates. We hope to identify new drug treatments for glaucoma, based on selective contraction of the longitudinal fibers of the CM. Such treatments would be free of some of the annoying side effects which limit the usefulness of current drugs acting via CM contraction. To study in vitro CM preparations in this way, it is necessary to obtain CM from eyes immediately after death, making it impractical to utilize human eye bank CMs (which usually do not become available to us for many hours after death), on a regular basis. We therefore use primarily CM obtained from rhesus monkeys (which have eyes much like the human) at the Wisconsin Regional Primate Research Center. Once the fresh CM is obtained, we can maintain it in a living condition for several days, enabling us to perform many experiments.

Progress Toward Goals

We have duplicated the experimental apparatus. This represents an important step forward, as we are now able to double our productivity. We are now in the process of computerizing the data acquisition and analysis system so that experimental results can be processed more rapidly. In addition, our occasional source of fresh human CM enables us to compare the effects of drugs on monkey and human tissue to a limited extent. One group of drugs (endothelins) has proved to have little effect on CM contraction, and so further study with these drugs has been deferred. Work with another drug group (muscarinics) has verified that the M3 receptor subtype is primarily responsible for contraction induced by miotic antiglaucoma drugs, but unfortunately in both vectors with little selectivity for the compounds thus far studied.