The experiments described in the three papers of this series were undertaken to determine if possible the significance of the parathyroid hypertrophy that has repeatedly been found in rabbits after exposure to ultra-violet light. The fact that the hypertrophy was not accompanied by a corresponding increase in blood calcium led us to infer that the upper level of blood calcium concentration is governed by factors independent of parathyroid control, and that the gland hypertrophy under the influence of ultra-violet light might represent a potential functional capacity, or increased factor of safety, to protect the calcium level under conditions of stress or emergency. Partial parathyroidectomy in irradiated rabbits, compared with the same operation in normal controls, resulted in a similar immediate drop in blood calcium; but a more rapid restoration of the calcium level in the irradiated rabbits indicated a more prompt and active response to the emergency on the part of the remaining glands. In the second group of experiments no significant differences were found in the reactions of irradiated, normal, or partially parathyroidectomized rabbits to injections of calcium chloride or of disodium hydrogen phosphate. All three groups of rabbits rapidly eliminated the excess calcium from the blood. Their reactions to large doses of sodium phosphate, given subcutaneously, appeared to be related to phosphate elimination or retention in the individual animals. Those in each group which had phosphate retention in the blood died in acute tetany.

So also in the fasting experiments, reported in this paper, no distinctions could be drawn between normal, irradiated, and partially parathyroidectomized rabbits on the basis of their calcium and phosphorus metabolism. Blood calcium was reduced in a similar manner in all three groups of rabbits, and wide individual variations in serum phosphorus levels preclude significant deductions.

But the irradiated rabbits suffered disproportionate losses of weight during the fasting period and four of five of them died during the course of the experiment. The deaths of these rabbits are attributed to this more rapid consumption of their own tissues and possibly also to suprarenal injury caused both by fasting and by exposure to ultra-violet light.

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