Upper Limits of in Normal Adult Plasma

Lower and Upper Limits of [Calcium] [Phosphate] in Normal Adult Plasma 

The apparently reciprocal relationship between plasma calcium and phosphate which is seen in clinical practice is largely accounted for by the opposite effects of parathyroid activity upon the calcium and phosphate concentrations. Thus, hyperparathyroidism is associated with high calcium and low phosphate concentrations and hypoparathyroidism with the reverse this is not a biochemical but a physiological reciprocity. A true biochemical or physicochemical reciprocity exists above the solubility product of tricalcium phosphate and presumably explains the irreversible depression of serum calcium by phosphate in renal failure (see Section 4.2). 

No similar reciprocity can exist below the bone-mineralizing product where the tissue fluid must be undersaturated with respect to bone salt and inorganic calcium phosphate. Within the normal range of products, reciprocity with respect to bone as the solid phase is probably masked, as already explained, by varying bone solubility. However, if parathyroid activity regulates bone solubility, then states of fixed parathyroid activity, 

In support of this suggestion, it is known that high phosphate feeding may produce tetany and that low phosphate feeding or Benemid administration may relieve it in hypoparathyroidism (P2). Conversely, Albright (A2) has demonstrated that in primary hyperparathyroidism 

The secretion of the parathyroid glands is probably controUed by die concentration of ionic calcium concentration in the tissue fluid. Artificial elevation of plasma calcium suppresses the parathyroids (H12, K5, N5), and parathyroid size varies inversely with the level of plasma calcium (HI, S6). Part and Luckhardt (P3) demonstrated an increase in parathyroid hormone secretion in response to the perfusion of parathyroid glands with fluids of low calcium concentration. Talmage s osteoclast count, which is believed to measure parathyroid activity, rises in response to rapid removal of calcium from the extracellular fluid of intact rats (T2). The precise ionic calcium levels which stimulate and suppress the parathyroids are not known, but calcium infusions suggest that parathyroid suppression occurs when total plasma calcium rises above about 11 mg/100 ml. Stimulation probably occurs when the concentration falls below about 9 mg/100 ml. 

---