THE PTH-CALCIUM RELATIONSHIP DURING A RANGE OF INFUSED PTH DOSES IN THE PARATHYROIDECTOMIZED RAT

To establish the PTH dosage that maintains normal mineral homeostasis in the PTX rat, a series of doses of rat 1-34 PTH were infused via a s ubcutaneously implanted miniosmotic pump. The doses were 0, 0.011, 0.0 22, 0.044, and 0.11 mu g/100 g/hour. After 48 hours, serum calcium ran ged from 5.56 +/- 0.02 to 16.29 +/- 0.25 mg/dl, ANOVA P < 0.001, and s erum phosphorus from 12.49 +/- 0.03 to 5.33 +/- 0.34 mg/dl, ANOVA P < 0.001. By post hoc test, the serum calcium level was different (P < 0. 05) at every PTH dose; the serum phosphorus level was different (P < 0 .05) at every PTH dose except between the two highest doses. The PTH d osage that produced a normal serum calcium (10.09 +/- 0.10 mg/dl) and phosphorus (6.90 +/- 0.18 mg/dl) was 0.022 mu g/ 100 g/hour. The relat ionship between increasing doses of PTH and both serum calcium and pho sphorus was curvilinear and the calcium-phosphorus product was remarka bly constant from a serum calcium of 7-13 mg/dl. The increase in serum calcium and the decrease in serum phosphorus were more rapid at lower than at higher PTH doses so that for both, an asymptote was reached. At the highest serum calcium values, the calcium-phosphorus product in creased and in individual rats, an increase in serum phosphorus was as sociated with a decrease in serum calcium. Ln summary, this study show s that (1) for rat 1-34 PTH, the normal replacement dose in the PTX ra t with normal renal function on a normal diet is 0.022 mu/100 g/hour; (2) the relationship between PTH and both serum calcium and phosphorus is curvilinear, and an asymptote is reached for both; and (3) the cal cium-phosphorus product is remarkably constant as the serum calcium in creases from 7 to 13 mg/dl and only increased during marked hypercalce mia when serum phosphorus did not decrease further or even tended to i ncrease.