EFFECT OF CHRONIC PARATHYROIDECTOMY ON CALCIUM-METABOLISM IN THE LACTATING RAT

This study examined the effect of chronic parathyroidectomy (PTX) in rats on calcium homeostasis in milk and serum and on litter growth rate throughout lactation. Parathyroidectomy was carried out by excision 8 weeks before mating and any animals with evidence of parathyroid regeneration over this time were discarded. Maternal serum and milk samples were collected at days 6, 12 and 20 of lactation and serum samples from the pups were obtained at day 20. Litter growth was measured by weighing the pups daily. Significantly lower concentrations of ionized and total calcium and significantly higher concentrations of phosphate in the serum were found in the PTX animals compared with the sham-operated controls (P<0.001). The ratio of ionized to total calcium in serum was significantly lower in the PTX group. The mean total calcium concentration in milk was significantly higher at day 6 in the PTX group (P<0.05) but was not different from the sham-operated group at days 12 and 20. The ratio of milk calcium to percentage milk solids was the same in both groups of animals at all three sampling periods. Litter growth rate was not significantly different in the two groups up to day 5 or from days 12 to 20. It was lower in the PTX group (P<0.001) between days 5 and 12. The total concentration of calcium in serum from the young was not significantly different in the two groups. It was concluded that parathyroid hormone (PTH) is required for normal maternal calcium homeostasis in serum during lactation and therefore its absence may affect milk homeostasis indirectly for part of lactation. In contrast the concentration of calcium in milk and litter growth rate can be maintained at normal levels in late lactation suggesting that PTH is not an essential hormone in the regulation of calcium levels in milk.