Effect of Direction and Rate of Change of Calcium on Parathyroid Hormone Secretion in Uraemia

: We have studied the control of amino-terminal parathyroid hormone (PTH) secretion in haemodialysis patients in response to slow or fast calcium infusion and during acute hypocalcaemia. In nine patients, fast calcium infusion (0.4 mmol/kg bodyweight per hour) for 15 min increased ionised calcium and reduced PTH, with an initial t 1/2 of 12.8 min. After the infusion had ceased, calcium decreased steadily, and PTH increased, mean PTH reaching baseline values when calcium was still significantly greater than pre-infusion values. During slow calcium infusion for 2.5 h (0.1 mmol/kg bodyweight per hour), parathyroid suppression was evident at 15 min, when the calcium increment was only 0.03 mM. After 60 min, PTH did not decrease further despite progressive hypercalcaemia. Hypocalcaemic haemodialysis led to rapid increases in PTH. After 15 min, the mean calcium decrement was 0.09 mM (P less than 0.01) and the mean PTH increment was 283 pg/ml (P less than 0.01). The parathyroid response was maximal at 30 min, and did not increase subsequently, despite progressive hypocalcaemia for a further 90 min. During recovery from hypocalcaemia, PTH reduced and, despite comparable hypocalcaemia, PTH during periods of increasing calcium was always lower at a given calcium concentration than while calcium was decreasing. This influence of the direction of change of calcium was not seen during hypocalcaemia. The results showed that even in-advanced renal disease, the parathyroid glands are highly responsive to small initial increments (0.03 mM) and decrements (0.09 mM) in blood calcium, though less so to further perturbation of blood calcium.(ABSTRACT TRUNCATED AT 250 WORDS)