ALTERED REGULATION OF PARATHYROID-HORMONE SECRETION BY CALCIUM IN PREGNANT AND LACTATING RATS

We have previously shown that the serum parathyroid hormone (PTH) conc entration in lactating (L) rats is not suppressed by high serum Ca2+ t o the same extent as in nonmated (NM) rats, To investigate further Ca2 + regulation of PTH secretion, parathyroid cells from NM rats and rats in late pregnancy and at peak lactation were dispersed and incubated for 2 h in medium containing 0.52-2.05 mM Ca2+. Medium PTH was assayed with a homologous immunoradiometric assay (IRMA). At the two highest Ca2+ levels (1.81 and 2.05 mM), medium PTH was significantly higher (p = 0.031) for cells from L rats than for cells from NM rats, In contra st, significantly less (p < 0.001) PTH was secreted for the L group ve rsus the NM group at medium Ca2+ values of 1.27 and 1.46 mM, Estimated set points for L and NM groups were 1.17 mM and 1.35 mM, respectively , corresponding closely to the prevailing serum Ca2+ for these two gro ups, Consistent with the present in vitro data, high serum PTH (>40 pg /ml) in L rats occurred only at serum Ca2+ values below 1.27 mM, Eleva ted serum PTH at lower serum Ca2+ levels was also seen in pregnant rat s, Dispersed parathyroid cells from 20- to 21-day pregnant rats secret ed significantly more PTH (p = 0.028) than cells from NM rats at all C a2+ levels tested (1.1-1.6 mM). In conclusion, the relationship betwee n extracellular Ca2+ and PTH secretion is altered in rats during late pregnancy and at peak lactation, perhaps as part of the adaptation to the demands for calcium for pre- and postnatal growth.