Demonstration of blood calcium-related parathyroid function in the diurnal variation of blood and urine calcium and phosphorus in healthy individuals

Abstract

The circadian changes in blood and urine minerals and parathyroid function assessed by nephrogenous cyclic AMP (NcAMP) and a highly sensitive midregion specific parathyroid hormone (PTH) radioimmunoassay (RIA) were investigated for 26 hours in ten healthy 23 to 53-year-old males (group 1) with normal food intakes. Also, we studied another six male and four female adults between 600 and 1200 h as fasting controls (group 2). The blood calcium (Ca) levels were the highest at 630 h in both groups and decreased thereafter, and the diurnal variation of NcAMP reflected that of serum Ca, showing a low value in the morning. There were significant negative correlations between total Ca and NcAMP in both groups 1 and 2 (r=−0.323, n=130, p<0.01; r=−0.525, n=60, p<0.01), suggesting that the blood Ca level is the major determinant of parathyroid function and that NcAMP is a good indicator of blood Ca related PTH secretion in healthy individuals. This last relationship was not seen between Ca and midregion PTH levels presumably owing to slow clearance of PTH fragments from the circulation. However, midregion PTH perhaps because of its long half life, not NcAMP, shows a significant inverse relation to urinary Ca excretion (mg/dIGF) in the diurnal variation in healthy individuals (r=−0.560, n=130, p<0.01). The circadian pattern (s) of serum phosphate (P) consisted of a minium level at 900–1100 h followed by two peaks, at 1400–1600 h and 2200–2400 h. Thus, this study conducted in healthy individuals under physiologic condition clearly demonstrated that the NcAMP level reflects blood Ca related PTH secretion and that urinary Ca excretion seems to be determined by the sum of PTH secretion.