Response of the Iris to Pitressin and Pitocin

Abstract

Posterior pituitary extract (pituitrin) has been reported 1 to cause a dilatation of the pupil when dropped into the conjunctival sac of the normal eye of the rabbit, averaging 0.65 mm. (minimum: 0.25; maximum: 1.5) in 31 of 32 experiments. In one of 32 experiments the pupil was found to contract and stay contracted following the use of pituitrin.

In 8 experiments we have found the posterior pituitary extract prepared by 2 manufacturers (4 experiments with each) to cause a constriction of the pupil of the normal eye of the rabbit amounting to 1.0 mm. in each of the 8 experiments.

The active constituents of the posterior pituitary have been separated into 2 fractions, one possessing vasopressor, the other oxytoxic actions. 2 These fractions are commonly available under the names “pitressin” and “pitocin”, respectively. Pitressin in the normal rabbit eye caused a constriction in each of 7 experiments, averaging 2.2 mm. (minimum: 1.5; maximum: 4.0). Pitressin placed in the conjunctival sac following the use of 1% atropin until the light reflex was abolished, caused a constriction of the pupil in each of 9 experiments, averaging 4.9 mm. (minimum: 4.0; maximum: 6.3).

Pitressin placed in the conjunctival sac following pronounced dilatation of the pupil from 4% cocaine caused constriction of 6.5 mm. in each of 2 experiments. Pitressin following ephedrine 3% solution, resulting in dilatation of the pupil averaging 2.0 mm. increased diameter, failed to influence the size of the pupil in each of 4 experiments. Pitressin following epinephrine 1 :1,000 solution, resulting in 0.5 mm. dilatation of the pupil, caused constriction of 1.0 and 1.5 mm. in 2 experiments.

Pitocin in the normal rabbit eye caused no change in the size of the pupil in each of 7 experiments. Pitocin in the eye with pupil inactive to light following atropine failed to alter the size of the pupil in each of 3 experiments.

Pitocin (10 unit strength) when mixed with pitressin (10 unit strength) did not appear to influence the degree or duration of pitressin constriction of the normal rabbit pupil (average: 2.6 mm.; minimum: 1 mm.; maximum: 3.1 mm.) in 4 experiments. Pitocin in 3 experiments did not appear to alter the action of pitressin in constricting the pupil dilated with atropine.