Abstract
Different C-terminal fragments of parathyroid hormone (PTH)-(1–84) in blood participate in the regulation of calcium homeostasis by PTH-(1–84), and an antagonizing effect for the large carboxyl-terminal parathyroid hormone (C-PTH)-fragment (7–84) on calcium release has been described in vivo and in vitro. In this study the smaller C-PTH-fragment (53–84) and mid-regional PTH fragment (28–48), which represent discrete areas of activity in the PTH-(7–84) molecule, were assayed for their effects on calcium release and alkaline phosphatase (ALP) activity in a chick bone organ culture system. Neither PTH-(28–48) nor PTH-(53–84) had any effect on calcium release into the medium and both fragments stimulated ALP activity in the bone tissue, suggesting that the cAMP/PKA signalling pathway was not affected by these fragments. However they suppressed the calcium release induced by PTH-(1–34) and attenuated the down regulation of ALP activity caused by PTH-(1–34), suggesting that the effect on the cAMP/PKA signalling pathway may be indirectly. In conclusion, the study shows that the PTH-fragments (53–84) and (28–48) antagonize the PTH-(1–34) induced effects on calcium release and inhibition of ALP activity in a chick bone organ culture system.
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