Vl. Schultz et al., ALTERED REGULATION OF PARATHYROID-HORMONE SECRETION BY CALCIUM IN PREGNANT AND LACTATING RATS, Journal of bone and mineral research, 12(6), 1997, pp. 903-908
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.