Interaction of Parathyroid Hormone and Calcitonin

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  • ORIMO H.
    Department of Geriatrics, Faculty of Medicine, University of Tokyo
  • FUJITA T.
    Department of Geriatrics, Faculty of Medicine, University of Tokyo

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Other Title
  • 副甲状腺 2.副甲状腺副甲状腺ホルモンとカルチトニン
  • 副甲状腺ホルモンとカルチトニン
  • フクコウジョウセン ホルモン ト カルチトニン

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Since parathyroid hormone increases serum calcium mainly through the stimulation of bone resorption and calcitonin is known to antagonize this effect, interaction of these two hormones attracted widespread attention. In an attempt to study this interaction further, several experiments were conducted in rat. Firstly, the possibility of parathyroid hormone stimulation of calcitonin secretion was studied by measuring the calcitonin content in the thyroid gland of parathyroid hormone treated rat. No significant change in calcitonin content of the thyroid gland was found after the administration of parathyroid hormone, thus suggesting the failure of parathyroid hormone stimulation of calcitonin secretion. Subsequently the possibility of calcitonin stimulation of parathyroid hormone secretion was explored by measuring the plasma parathyroid hormone level in calcitonin treated rats. No significant change in the immunoreactive parathyroid hormone was noted after the administration of calcitonin in the absence of hypocalcemia. Since bone is a target organ of both parathyroid hormone and calcitonin, the interaction of these two hormones on bone was subsequently studied. In view of the current concept that the increase in lysosomal enzymes are actively involved in parathyroid hormone induced bone resorption, it is of great interest to study whether calcitonin might decrease the lysosomal enzyme activities. Both parathyroid hormone and calcitonin was found to cause an increase in lysosomal enzyme activity through an unknown mechanism. Simultaneous administration of parathyroid hormone and calcitonin further increased these enzyme activity, thus suggesting the synergistic effect of these two hormones. Pyrophosphatase, which hydrolyse inorganic pyrophosphate, has evoked special interest in view of the influence on bone metabolism through the inhibitory effect on calcification. According to Fleisch et al., increase in pyrophosphatase activity stimulates bone formation through the destruction of pyrophosphate covering the crystal surface of bone. On the other hand, decrease in this enzyme activity inhibits bone formation through the accumulation of pyrophosphate on the bone surface. Using the highly sensitive assay method of pyrophosphatase involving hydrolysis of 32PPi, two distinct pyrophosphatase activities were found at pH 2.5 and pH 7.5 in normal rat bone homogenate. Calcitonin was found to increase pyrophosphatase activity at pH 7.5, while parathyroid hormone significantly decreased this enzyme activity. These results are consistent with the previously recognized physiological action of these two hormones. Calcitonin might increase bone formation through the increase in pyrophosphatase activity in bone and parathyroid hormone might inhibit bone formation through the decrease in this enzyme activity. It is suggested that calcitonin counteracts the action of parathyroid hormone on bone through inducing a change in pyrophosphatase activity.

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