Hypocalcemia after an acute phosphate load is secondary to reduced calcium efflux from bone

Seven patients with severe hyperparathyroidism secondary to chronic renal failure, six patients with hypoparathyroidism after remote total parathyroidectomy also with chronic renal failure, and a miscellaneous group of three patients, some of whom were in the previous two groups, were studied on 24 occasions over a 6-h period. Each test consisted of a 2-h control period followed by a 4-h phosphate (Pi) infusion period. Radioactive calcium, 45Ca, had been administered the evening before. Samples were taken every 15 min throughout the 6-h study. In all tests, ionized and total calcium fell as Pi rose. Intact parathyroid hormone levels (PTH) rose, except in the hypoparathyroid patients, in whom there was no change. The decline in 45Ca activity was not affected by the Pi infusion, the fall being -0.131 +/- 0.057 cpm/min during the control period and -0.124 +/- 0.043 during the Pi infusion. There were no changes in pH, bicarbonate, electrolytes, or vitamin D metabolites during the procedure. The mean overall fall in total calcium was -0.118 mmol/mmol rise in Pi. For ionized calcium, it was -0.067 mmol/mmol Pi or 56.8% of the total calcium. This ratio was unchanged throughout the test period. With a steady flux of calcium from extracellular fluid (ECF) to bone as measured by 45Ca, the fall in ECF calcium has to be due to a decreased flux from bone to ECF. This could be produced by the reduced dissolution of a labile pool of a calcium salt such as brushite, CaHPO4.