Hysteresis of the PTH-calcium curve during hypocalcemia in the dog: effect of the rate and linearity of calcium decrease and sequential episodes of hypocalcemia.

Several studies have shown the presence of hysteresis of the parathyroid hormone (PTH)-calcium relationship in both normal humans and hemodialysis patients; for hypocalcemia, hysteresis is defined as a lower PTH level for the same serum calcium during the recovery from than the induction of hypocalcemia. However, some have questioned whether hysteresis is only a function of the rate and/or direction of change in calcium, and others have proposed that hysteresis is due to depletion of PTH stores. To address these issues, two groups of dogs were studied. To induce hypocalcemia, sodium EDTA (50 mg/kg) was infused either over 60 (termed slow) or 30 (termed fast) minutes; immediately after the cessation of the ethylenediamine tetracetate (EDTA) infusion, calcium chloride (0.75 mEq/kg) was infused over 60 or 30 minutes, respectively, to correct the hypocalcemia. The EDTA infusion produced a linear decrease in serum calcium by progressively increasing the infusion rate at regular intervals. A second cycle of hypocalcemia and recovery using the same protocol was started immediately after the completion of the first cycle. To determine whether a nonlinear decrease in the serum calcium affected the PTH response to hypocalcemia, a third group of dogs, termed superfast, was studied; in this group, EDTA was infused for 30 minutes at a constant rate of 50 mg/kg. The hysteretic loops of PTH produced by the two sequential slow and fast cycles and the superfast cycle during the induction of and recovery from hypocalcemia were similar. Moreover, the maximal PTH level for the two sequential slow and fast cycles and the superfast cycle was not different even though the rates of calcium decrease varied and the calcium decrease was nonlinear in the superfast cycle. In conclusion, (1) since hysteresis was reproducible and the maximal PTH was not different during two sequential cycles of hypocalcemia, hysteresis is not due to PTH depletion; (2) the PTH-calcium curve is not affected by the rate at which hypocalcemia is induced; and (3) the maximal PTH level is not influenced by either the rate or linearity of the reduction in serum calcium.