Blood, intestinal and skeletal calcium dynamics during egg formation

Abstract

An experiment was carried out to study the dynamics of various body-Ca pools in the hen during a single ovulatory cycle. One hundred 32-wk-old Single Comb White Leghorn hens were used. For experimental purposes the ovulatory cycle was divided into four periods: 1–6, 6–12, 12–18 and 18–24 h post-oviposition.At 0, 6, 12 and 18 h post-oviposition 25 hens each were intubated proventricularly with 15 μCi of 45Ca as CaCl2 solution. Approximately 1 mL of blood was collected from the leaf brachial vein, and the bird was subsequently placed in an individual cage. Red blood cells, labelled with 15Cr, were re-injected into the same bird from which it was collected. At 0.5, 1, 2, 4 and 6 h post-intubation with 45Ca, blood samples were taken from the right brachial vein of five hens and the birds were immediately killed, carcasses were dissected, the left femur and tibia-fibula were excised, and an egg, if present, was removed. Whole blood was assayed for 51Cr activity, and plasma, bones and shells were assayed for 45Ca activity and Ca concentration.Another five hens were injected in the right brachial vein with 15 μCi of 45Ca immediately following oviposition. Subsequently, the left brachial vein was catheterized and blood was sampled at 0.5, 1, 2, 4, 6, 12, 18 and 24 h post-injection.There were no significant (P > 0.05) differences in blood volume or plasma-Ca concentration for the four periods assayed. Following intubation, plasma-45Ca activity decreased quadratically (P < 0.05) with time for all four periods assayed. Regression analysis showed that the biological half-life of plasma 45Ca was 3.30, 2.13, 1.80 and 1.77 h for periods 1, 2, 3 and 4 respectively. When birds were injected intravenously with 45Ca the half-life of plasma 45Ca was 0.116 h.There were no significant (P > 0.05) changes over time in ash or Ca content of bone ends (BE) or medullary bone (MB). Only MB accumulated 45Ca during the first 6 h of the cycle. There was no difference (P > 0.05) in 45Ca activity of either BE or MB over time in period 2. Only BE experienced a significant (P < 0.05) loss in 45Ca during period 4.There were no differences (P > 0.05) in total shell ash. shell Ca or 45Ca with time in period 1. Shell ash increased (P < 0.05) by 188, 348 and 237 mg h−1 and shell Ca increased by 69, 128 and 82 mg h−1 in periods 2, 3 and 4, respectively. Accumulation of shell 45Ca confirmed that the transfer of Ca was greatest during period 4 of the ovulatory cycle. Using 45Ca dynamics, we estimated that over the entire ovulatory period 1716 mg of Ca was absorbed from the digestive tract and 1704 mg was secreted as shell. Key words: Calcium-45 activity, calcium-45 half-life; plasma 45Ca, bone 45Ca, shell 45Ca, shell secretion, Ca absorption