BLOOD, INTESTINAL AND SKELETAL CALCIUM DYNAMICS DURING EGG FORMATION

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 muCi of Ca-45 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 Cr-15, were reinjected into the same bird from which it was collected. At 0.5 1, 2, 4 and 6 h post-intubation wit Ca-45, 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 Cr-51 activity, and plasma, bones and shells were assayed for Ca-45 activity and Ca concentration. Another five hens were injected in the right brachial vein with 15 muCi of Ca-45 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-Ca-45 activity decreased quadratically (P < 0.05) with time for all four periods assayed. Regression analysis showed that the biological half-life of plasma Ca-45 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 Ca-45 the half-life of plasma Ca-45 was 0.1 16 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 Ca-45 during the first 6 h of the cycle. There was no difference (P > 0.05) in Ca-45 activity of either BE or MB over time in period 2. Only BE experienced a significant (P < 0.05) loss in Ca-45 during period 4. There were no differences (P > 0.05) in total shell ash. shell Ca or Ca-45 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 Ca-45 confirmed that the transfer of Ca was greatest during period 4 of the ovulatory cycle. Using Ca-45 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.