Sigmoidal relationship between calcitonin and calcium: studies in normal, parathyroidectomized, and azotemic rats

Calcitonin secretion is stimulated by acute hypercalcemia. Furthermore, in the rat, the calcemic response to parathyroid hormone (PTH) is decreased by calcitonin stimulation. However, in renal failure, it is not known if an increase in the serum calcium concentration within the physiologic range of serum calcium stimulates calcitonin and whether the increased calcitonin decreases the calcemic response to PTH. In the present study, four groups of pair-fed rats were evaluated: normals (N); parathyroidectomy (PTX); and two groups with renal failure (RF)--basal serum calcium less than 8.5 mg/dl (RFa) and basal serum calcium greater than 8.5 mg/dl (RFb). Hypocalcemia was induced by parathyroidectomy or in the RFa group, by a high phosphate diet. Increases in the serum calcium were produced by a 48 hour infusion of rat 1-34 PTH. In the RFa and PTX groups, stimulation of calcitonin was observed as the serum calcium increased from hypocalcemia to normal levels of calcium (P less than 0.01). In all four groups, increasing the serum calcium from normal levels to hypercalcemia increased the serum calcitonin level (P less than 0.05). The relationship between serum calcitonin and calcium was best expressed as a sigmoidal curve. In the two groups with basal hypocalcemia, PTX and RFa, the calcitonin-calcium curve was shifted to the left of the N and RFb groups.(ABSTRACT TRUNCATED AT 250 WORDS)