A. Parathyroid Hormone
The major target organs for PTH actions are kidney and bone. PTH may also act upon the intestine, but its effects here are probably indirect and are achieved by virtue of the tropic actions of PTH in stimulating the renal biosynthesis of la,25(OH)2D3.
The secretion of PTH is stimulated in response to a lowered blood concentration of calcium (see Figure 9-9). In terms of the several forms of blood calcium summarized in Table 9-2, it is known that the biosynthesis and secretion of PTH are only responsive to the ionic and not to the protein-bound forms of calcium. The secretion of PTH can be (1) stimulated when hypocalcemia is induced by the infusion of calcitonin or the divalent metal-chelating agent, EDTA (ethylenedi-aminetetraacetic acid), or (2) decreased when hypercalcemia is induced by the infusion of calcium. Thus, there is an inverse correlation between serum calcium concentration and PTH concentration in the range of 4-10 mg of calcium /100 ml (see Figure 9-9). The most stringent control of serum calcium concentration is achieved in the range of 9-10.5 mg of Ca2+ /100 ml of serum, which is considered to be the normal physio-
7-Dehydrocholesterol provitamin D3
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