Animals in parathyroid tetany show decrease or absence of desire for food in direct proportion to the severity of the tetany and cachexia symptoms. Is this refusal of food due to absence of the gastric hunger contractions ?
Our observations were made on three dogs. The dogs were observed every third day for two weeks, so as to secure the average normal gastric tonus and hunger contractions before extirpation of the parathyroid. All three dogs ran a typical course of tetany of varying severity from day to day. Dog I died in tetany on the sixth day after the operation; Dogs II and III died in depression on the eighth and tenth days respectively. The results were practically the same in the three dogs. In this tetany there is depression "of the tonus and contraction of the empty stomach parallel with the severity of the tetany, so that during extreme tetany the stomach is practically atonic, and tonus contractions and hunger contractions are completely absent. The milder stages of the tetany (hyperexcitability of the motor nerves, slight tremors, twitchings, and some salivation) may coexist with considerable gastric tonus and hunger contractions, but the hunger contractions are always slower and weaker than normal.
It is well known that the course of parathyroid tetany, especially in dogs, is usually more or less periodic, the animal recovering spontaneously for periods varying from a few hours to a day or more between the tetany attacks. Dog II showed two such periods of spontaneous recovery of 34 and 20 hours' duration. During these periods the gastric hunger contractions and the gastric tonus also returned to approximately normal conditions, the dog at the same time taking normal interest in food.
The relation of these depressions of hunger and appetite to the gastric hunger contraction is, nevertheless, not a direct one. During the mild stages of tetany the failure of hunger and appetite is usually much greater than one would expect on the basis of the degree of depression of the hunger contractions. In strong tetany there are no gastric hunger contractions and the dogs refuse food, but the dog may refuse food even though fairly strong hunger contractions are present in mild tetany, or if he does eat the amount of food consumed is very small. It is therefore clear that the depression of hunger and appetite in tetany cannot be accounted for solely on the basis of depression of gastric hunger contractions, although this is unquestionably one of the factors. But we must also take into account either a change in the central nervous system, or a change in the character of the nervous impulses from the stomach and other proprioceptor systems.
The condition of parathyroid tetany, so far as it influences the stomach motor activities, depresses both the digestion movements of the filled and the hunger contractions of the empty stomach, but the movements of digestion show less depression than do the hunger contractions. Thus moderately strong tetany may leave the gastric digestion movements practically normal but completely inhibit the hunger contractions of the empty stomach. The tetany condition does not lead to increased motor activity, either in the empty or the filled stomach.
The cause of this depression of the gastric motor activities in parathyroid tetany is not determined. In the case of the digestion movements it was shown not to be due to splanchnic inhibition. This test has not been made in the case of the hunger movements. But one factor in the depression or complete inhibition of the hunger contraction in tetany is the increased excitability of the nerve-endings in the gastric mucosa. Vomiting is a tetany symptom in dogs. And dogs in tetany frequently vomit with nothing in the stomach but bile and saliva. In such dogs water at body temperature introduced into the stomach through a fistula in the fundus causes vomiting. The presence of a delicate rubber balloon in the stomach or the slight inflation of the balloon causes vomiting. This never occurs in normal dogs. The stimulation of the nerve-endings in the gastric mucosa in normal animals (man and dog) * causes inhibition of the gastric hunger contractions through weak and long reflexes. In parathyroid tetany these nerve-endings in the mucosa become so hypersensitive that they are intensely stimulated by saliva, water, bile, and gastric juice. But in addition to these inhibitory reflexes from the gastric mucosa we probably also have a direct depression of the automatic tissue in the stomach, for it is not likely that the inhibitory reflexes, even though very strong, could maintain the sustained extreme depression seen in strong tetany.
By way of summary, we conclude that parathyroid tetany in dogs does not lead to increased tonus or contractions of the empty stomach, but to depression of the tonus and the hunger contractions. The degree of the depression of the motor activities of the empty stomach is on the whole parallel with the severity of the tetany symptoms, and more marked than the depression of the gastric movements of the digestion. The hyperexcitability of the nerve-endings in the gastric mucosa is a factor in this depression. The stimulation of these nerve-endings leads, through local and long reflexes, to inhibition of the tonus and the hunger movements. There is probably also a direct depression of the automatic tissue in the stomach through changes in the blood. The diminution or lack of appetite for food in animals in tetany is on the whole greater than would be expected on the basis of the degree of the depression of the gastric hunger contractions. The cause of the lack of hunger and appetite in tetany is therefore complex. It is due in part to the depression of the gastric hunger contractions. Other factors are the change in the brain, and in the character of the other afferent nervous impulses.