Hypercalcemia fails to suppress elevated serum parathyroid hormone concentrations during lactation in rats.

We have previously reported that increased serum immunoreactive parathyroid hormone (iPTH) in the lactating (L) rat is generally accompanied by hypocalcemia when diets containing 0.4% calcium (Ca) or less are fed. However, instances were also observed in which elevated iPTH levels did not coincide with a hypocalcemic signal. To test the hypothesis that iPTH levels can remain elevated even in the presence of hypercalcemia in lactation, a diet containing 1.2% Ca and 0.4% phosphorus (P) was fed to lactating rats in three experiments (A, B, and C) to achieve serum ionized calcium (ICa) levels approximately 10% above levels for nonmated (NM) controls. The serum ICa of NM controls fed the 1.2% Ca diet was slightly, but significantly, elevated, and serum iPTH (determined by an N-terminal specific assay) was significantly suppressed compared with NM controls fed a 0.4% Ca diet. In experiment A, L rats fed a 1.2% Ca diet had 81% higher serum iPTH levels than NM controls fed the same diet in spite of a mean (+/- SEM) ICa level of 1.77 +/- 0.05 mM for L rats versus 1.46 +/- 0.01 mM for NM controls; NM controls fed a 0.4% Ca diet had serum ICa of 1.37 +/- 0.01 mM. This novel finding of significantly higher iPTH and ICa in L compared with NM rats fed a 1.2% Ca and 0.4% P diet was confirmed in experiment B with eight rats in each group of L or NM rats fed either the 1.2% or the 0.4% Ca diet.(ABSTRACT TRUNCATED AT 250 WORDS)