The relationship between the actions of vitamin D, parathyroid hormone and calcitonin
The effect of PTH and CT upon hydroxyproline and electrolyte excretion, plasma calcium and phosphate, calcium balance, and the excretion of radiocalcium was examined in vitamin D-deficient thyroparathyroidectomized rats. The results were compared to those obtained in D-fed animals. An increase in urinary hydroxyproline, which persisted for many hours, occurred in D-deficient animals approximately 30 h after thyroparathroidectomy, but was not seen in D-fed animals. It was suppressed by high calcium infusion. Infusion of PTH or CT increased the hydroxyprolinuria even further in D-deficient animls. PTH increased hydroxyprolinuria in D-fed animals, but CT caused a significant decrease. In the D-deficient animals both PTH and CT caused a decrease in urinary calcium excretion, and an increase of the already positive calcium balance. In D-fed animals CT had similar effects, but PTH caused a significant increase in calcium excretion and a negative calcium balance. Young animals injected with45Ca while on a D-deficient diet were studied 3 weeks later at a time when they had developed D-deficiency. When these animals were thyroparathyroidectomized and maintained on a constant glucose and electrolyte perfusion, the excretion of calcium was relatively constant and the specific activity of urnary calcium declined only slightly over a 28 h period. When PTH was given, both total and radioactive calcium excretion diminished initially, leading to a significant decrease in specific activity. The specific activity declined for the first 3 h of hormone infusion, then rose gradually and became greater than normal during the last 4 h of hormone infusion. When CT was given with PTH, total calcium excretion fell and remained below the control rate throughout the experiment. Specific activity rose markedly during the first 2 h, then fell rapidly to values below the control levels for the remainder of the experiment.
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