Parathyroids and calcium metabolism

Larvæ of the salamanders Amblystoma maculatum, Amblystoma opacum and Eurycea bislineata when fed on thymus exhibit severe tetanic convulsions. In contradiction to other amphibian larvæ which do not show this reaction, they possess no parathyroids; the tetanic convulsions of the thymus-fed salamanders are identical with a true parathyreoprival tetany.

The tetanic action of the thymus is due to the presence in the organ of the tetany toxin which can be antagonized only by the parathyroids, but not by the substances contained in normal food. The addition to the thymus diet of normal diet in amounts sufficiently large to keep a salamander larva in a perfectly normal condition does not prevent the tetanic convulsions. Tetany is caused by a toxic substance contained and excreted by the thymus.

Calcium salts, when dissolved in the water in which the thymus-fed larvæ live, do suppress, to some extent, the tetanic convulsions. Magnesium salts, however, assert the same action and are still more effective than calcium salts. The suppression of the tetanic convulsions by the salts of calcium is not a specific action of the calcium.

Neither calcium nor magnesium is able to prevent the development in the thymus-fed larva: of severe and permanent lesions of the muscular system, lesions which are caused probably by injuries inflicted upon the central nervous system by the tetany toxin. These nervous lesions develop even in the presence of calcium or magnesium and even without any convulsions at all, and finally lead to the death of the animals.

The most dangerous and important action of the tetany toxin is its highly injurious effect upon the central nervous system as expressed by the lesions of the muscular system (permanent spasmodic contractions and paralysis). The most important function of the parathyroids is to prevent the tetany toxin from coming in contact with the central nervous system, and this function cannot be substituted by any substance such as calcium or magnesium, since these substances do not antagonize the tetany toxin.