Abstract
In the very careful studies of Dale on the pharmacodynamic action of ergotoxin, it is stated, without qualification, that this drug causes a contraction of the pupil. His studies were made on cats.
I have found that in rabbits, ergotoxin produces exactly the opposite effect. That is to say with all doses a dilatation of the pupil lasting several hours results. With doses of 1 mg. per kilo, which did not cause any marked general intoxication, the pupil dilated to 7 1/2 mm. from an original size of 5 mm. With doses of 2 mg. per kilo, the pupil reached a size of 9 to 10 mm. All injections were in the ear vein.
These larger doses cause a certain amount of disturbance of the respiration, and it might be claimed that the dilatation was due to asphyxia. In fact Dale ascribes a dilatation mentioned by Kobert in a protocol, to this cause. In order to exclude this factor I curarized a number of rabbits and under artificial respiration, injected ergotoxin. The heart was not markedly disturbed, but the dilatation of the pupil was even greater than in most of the normal animals.
We may then assume that the dilatation seen in the rabbit's pupil is not secondary to asphyxia but is due to the action of the drug itself.
Dale states that ergotoxin exerts two distinct actions on the sympathetic nervous system. First a stimulation of the muscle fibers of certain organs, notably the uterus. Second a paralysis of the motor myoneural junctions of the true sympathetic. It is to the latter action that the constriction of the cat's pupil is said to be due. It is possible either that ergotoxin acts in a different manner in the two species, or that the first action, direct stimulation of the muscle fibers is exerted on the dilator muscle of the rabbit's pupil and overcomes the effect of paralysis of the myoneural junction.
Get full access to this article
View all access options for this article.