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Original Paper: Corbett, M. C. and A. B. Richards. 1994. Intraocular adrenaline maintains mydriasis during cataract surgery. British Journal of Ophthalmology 78: 95–98.

The opening in the iris forms the pupil of the eye. The iris is composed of two arrangements of smooth muscle fibers. Constrictor muscles surrounding the pupil are arranged in concentric circles, like tree rings. Peripheral to the constrictor fibers are the dilator fibers, which are arranged like the spokes on a bicycle wheel. When the constrictor muscles contract, the pupil becomes very small, and when the dilators contract, the pupil becomes large.

Aging and exposure to UV light can cause the lens of the eye to become cloudy and nontransparent—a condition known as a cataract. To restore sight, a surgeon must remove the cataract through the pupil. To facilitate this surgery it is important to dilate the pupil as much as possible and to keep it dilated as long as possible. Pupillary dilation is facilitated by the sympathetic nervous system and the adrenal medulla releasing epinephrine (adrenaline) and norepinephrine.

Could epinephrine be used to maintain pupil dilation during cataract surgeries? To find out, researchers studied the effects of adding epinephrine to the irrigation fluid injected into the eye during cataract-removal surgery. Twenty-seven patients received irrigation fluid containing epinephrine at a concentration of 1:1,000,000. Forty-three patients received irrigation fluid without epinephrine. The table below shows the percentage of patients with pupil diameter less than 5 mm at various stages during cataract surgery.

Questions

Question 1

The epinephrine in the irrigation fluid is affecting the dilator muscle fibers. When comparing the two types of surgeries, those that used the epinephrine-containing solution dilated the pupil past the 5-mm diameter and kept it dilated for a longer period of time than those surgeries that did not use the epinephrine-containing irrigation fluid. Since dilator muscle contractions are what cause the pupil to enlarge, these must be the muscle fibers affected by epinephrine.

Question 2

Although both the constrictor muscle fibers and dilator muscle fibers are smooth muscle tissue, only the dilator muscle fibers contain a corresponding receptor protein on their cell surface that will bind to epinephrine. Thus the dilator fibers are target cells for epinephrine while the constrictor fibers are not. Once the epinephrine binds to the receptor protein on the dilator fibers, it begins a signaling cascade within the muscle fibers that promotes strong and prolonged contractions within the fibers. This produces the larger than 5-mm diameter pupil seen in the surgeries that used epinephrine-containing irrigation fluid.

Question 3

Epinephrine affects the cardiovascular system by increasing the heart rate and blood pressure. It causes arterioles in the digestive system and skin to contract but causes the arterioles in the skeletal muscles to dilate. It causes the liver to break down glycogen into glucose and stimulates adipose tissue to break down fat into fatty acids. These different effects are stimulated by the same hormone binding to receptors on the surface of each type of cell. The reason the responses are so varied is because of what happens inside each cell type after the receptor has been activated. Each cell will have a different signaling cascade that results in different intracellular processes. The difference in processes is determined by the type of cell (cardiac muscle vs. smooth muscle vs. hepatocytes vs. adipocytes, etc.) and by the types of intracellular signaling molecules that are created.

Question 4

To elicit a pupillary dilation from the constrictor fibers, these muscle fibers must relax beyond their normal resting state. This is the effect of atropine. Since atropine inhibits the interaction of acetylcholine with its receptor, this must mean that acetylcholine is the neurotransmitter that stimulates constrictor fibers to contract and reduce the pupil size. It also means that there is normally a baseline level of acetylcholine that keeps the constrictor fibers somewhat contracted. Thus the autonomic nervous system controls pupil diameter by releasing both epinephrine and acetylcholine—epinephrine to cause dilator fibers to contract and enlarge the pupil, and acetylcholine to cause constrictor fibers to contract and decrease the pupil size.