Changes in the concentrations of parathyroid hormone and ionic calcium in the plasma of laying hens during the egg cycle in relation to dietary deficiencies of calcium and vitamin D.

The concentrations of biologically active parathyroid hormone (PTH) and ionic calcium (Ca2+) were determined in the plasma of control and calcium-deficient laying hens during the egg cycle. The same measurements were made in hens that had ceased laying because of a deficiency of vitamin D and in normal hens during pauses in egg production. Changes in the concentration of Ca2+ during the egg cycle followed the same pattern and were of the same magnitude in the control and calcium-deficient birds: mean levels were high when egg shell calcification was not occurring and they were at least 0.10 mmol/litre lower during active shell formation. There was a reciprocal relationship between the concentrations of PTH and Ca2+; thus, PTH values were high during shell calcification and low before shell formation began and after oviposition. Mean concentrations of PTH in the birds given the low-calcium diet were more than twice as high as in the control birds both during shell formation at 0700 hr (12.85 and 5.95 pg/ml, respectively) and after oviposition at 1600 hr (4.25 and 0.55 pg/ml, respectively). The results of these experiments provide direct evidence that the mobilization of skeletal calcium for egg shell formation is under parathyroid control. In the non-shell-forming control and vitamin D-deficient birds there were no significant changes over the 27-hr sampling period in either PTH or ionic calcium.(ABSTRACT TRUNCATED AT 250 WORDS)