ACUTE HIGH ENVIRONMENTAL-TEMPERATURE AND CALCIUM-ESTROGEN RELATIONSHIPS IN THE HEN

Much is known about the effects of high environmental temperature (HT) on egg production, but very little is understood about the mechanisms that underlie them. Two experiments were conducted to examine the eff ects of acute heat stress on circulating estradiol, on calcium uptake by gut tissue, on bone resorption, and on the dynamic relationships be tween estradiol and calcium in the hen during one ovulatory cycle. In one study, hens were moved individually and randomly into a hot [HT: t emperature (T) = 35 C, relative humidity (RH) = 50%; n = 18] or a cont rol, thermoneutral (TN: T = 23 C, RH = 50%; n = 18) environment immedi ately after a mid-sequence oviposition and brachial vein cannulation. Blood samples (2 mt) were collected every 3 h for 21 h for ionized cal cium (Ca2+) and pH determinations and from which aliquots were frozen for 17 beta-estradiol (E(2)), total calcium (TCa), and inorganic P ana lysis. Excreta and urine were assayed for TCa and hydroxyproline (OHPr ), respectively. A second study was conducted to determine the effects of HT (T = 35 C, H = 50%, 12 h) vs TN (T = 23 C, RH = 50%, 12 h) on t he ability of duodenal cells to take up calcium (CaT). Blood pH and ca lcium responded to HT as expected (pH increased, Ca2+ decreased, and T Ca decreased) and the cyclic pattern of Ca2+ in blood was abolished. T he ratio of Ca2+:TCa decreased sharply at approximately the onset of s hell calcification in control hens, but in HT hens there was no clear change in the ratio at any point in the cycle. The pattern of E(2) typ ical of hens under normal conditions was significantly depressed in pl asma of HT hens. Calcium uptake by duodenal epithelial cells of I-IT h ens was lower than in TN hens. There was a clear inverse correlation b etween blood Ca2+ and urine OHPr in TN hens (r(2) = -73, P = 0.0021) b ut not in HT hens (r(2) = -27, P = 0.32). In addition to alterations i n acid-base balance and the status of Ca2+: diminished ability of duod enal cells to transport calcium may be a critical factor in the detrim ental effects of heat stress on egg production (numbers), eggshell cha racteristics, and skeletal integrity often documented in the laying he n.