Control of parathyroid hormone secretion in the lactating ra

Regulation of parathyroid hormone (PTH) secretion was investigated in the lactating rat because of the extremely high calcium (Ca) demand for milk production. A significant increase in serum immunoreactive PTH (iPTH) was demonstrated in lactating rats fed a diet with 0.4% phosphorus and either a low (0.1%) or adequate (0.4%) Ca content. Serum iPTH was higher in dams suckling large litters ($>$10 pups) compared with small litters (3 pups), serum iPTH was highest during mid and late lactation when milk production is maximal, and serum iPTH returned to control levels within 24-48 hours after removal of litters during peak lactation. Although a significant negative correlation between serum iPTH and serum Ca was found, serum iPTH was also elevated even when hypocalcemia was not observed. For example, when lactating rats were fed a diet with 1.2% Ca and 0.4% phosphorus, serum iPTH was still significantly higher than in controls despite serum ionized Ca levels 15-20% higher than those of nonlactating controls. Injection of Ca raised serum Ca even higher but serum iPTH still remained elevated. The higher serum PTH concentration cannot be explained by a decreased metabolic clearance of PTH. A theoretical analysis of the relationship between Ca and PTH secretion indicated that the continued elevation of serum iPTH after high Ca diet and Ca injection was most consistent with a higher level of nonsuppressible PTH secretion. Some factors that may influence the secretion of PTH during lactation were examined. The protein content of parathyroid glands of lactating and nonlactating rats were not different. Suckling increased serum iPTH, but the effect was seen for less than 20 minutes. Corticosterone, elevated during lactation, increased PTH accumulation when present in rat parathyroid gland incubations; however, the role of corticosterone, if any, in increased serum iPTH in the lactating rat remains to be determined. In conclusion, these studies have shown that the control of PTH secretion is altered during lactation since serum iPTH is elevated even in hypercalcemic, lactating rats. The elevated PTH is likely important for maintenance of serum Ca during the large Ca demands of lactation.