Regulation of plasma calcium and phosphate in calcitonin-infused rats

The effects of long-term constant infusion of moderate doses (2-32 ng/h) of salmon calcitonin (sCT) on plasma Ca (and its radionuclide 45Ca), Pi, Mg, and on endogenous rat CT (rCT) metabolism were investigated in the rat. Daily variations were included. 1) The plasma concentrations of Ca and Pi fell and that of Mg increased transiently during infusion, with the duration of responses (1-3 days) depending on the sCT dose. Rats infused with 8 ng/h sCT remained sensitive to CT after 7 and 14 days, as indicated by the effects of minipump removal and of a bolus injection of exogenous sCT on plasma mineral concentrations. 2) In contrast to control rats, the well-established daily variations in plasma Ca and Pi levels were no longer observed after 7 and 14 days of sCT infusion (8 ng/h), but normal variations persisted for plasma Mg, circulating rCT, rCT mRNA, and rCT thyroid content. 3) Statistical analysis of plasma mineral data, collected at five sequential times during days 7 and 14, showed that the means were not significantly different and that the daily variations were essentially identical on days 7 and 14 in control rats. In contrast, the variability of measurements for plasma Ca and Pi, but not for Mg, increased significantly between days 7 and 14 in infused rats, and the mean differences were significantly lower in infused rats on day 7 than in control rats. These results are consistent with a transitory loss of the daily variations for Ca and Pi (day 7) and the later (day 14) spontaneous recovery of some variations in these parameters, although the individuals remain unsynchronized.(ABSTRACT TRUNCATED AT 250 WORDS)