Interrelationships between diet and elevated temperatures (cyclic and constant) on concentrations of progesterone, estradiol-17beta, and testosterone in blood plasma of laying hens

Adaptation of the kidney during reproduction: role of estrogen in the regulation of responsiveness to parathyroid hormone

Avian kidney function adapts during reproduction to provide the calcium required for eggshell formation. Adaptive changes in kidney function are 1) increased parathyroid hormone (PTH)-dependent adenylate cyclase activity; 2) elevated numbers of PTH receptors; and 3) increased synthesis of 1,25-dihydroxycholecalciferol. Because exogenous estrogen mimics these changes, this study explored the physiological role of estrogen in the regulation of kidney function by altering egg-laying status or levels of estradiol. In hens, treatment with the coccidiostatic drug, nicarbazin, led to cessation of egg laying with maintenance of the reproductive tract and of plasma estradiol and calcium. The PTH-dependent adenylate cyclase activity remained elevated (upregulated). However, when molting was induced by altering the photoperiod and diet, plasma estradiol, plasma calcium, and renal PTH-dependent adenylate cyclase activity all decreased. The depressed responsiveness to PTH was restored by administration of estradiol either during the molt or upon return to egg laying following the molt. When the estrogen antagonist, tamoxifen, was administered to laying hens, reproduction ceased and the PTH-dependent adenylate cyclase activity of renal membranes was decreased. In all three groups of nonlaying birds, the activity of kidney 25-hydroxycholecalciferol-1-hydroxylase was markedly decreased relative to that of laying hens irrespective of the amount of plasma estradiol. It was concluded that estrogen regulates the PTH-dependent adenylate cyclase system of avian kidney, whereas the activity of the 25-hydroxycholecalciferol-1-hydroxylase of kidney and thus, the synthesis of 1,25-hydroxycholecalciferol may be governed at least in part by the regulation of renal receptors for PTH by estrogen.

Renal cAMP and 1,25(OH)2D3 synthesis in estrogen-treated chick embryos and hens

Onset of sexual maturity in female chickens or administration of estrogen to mature males or to juveniles of either sex results in increased parathyroid hormone (PTH)-dependent adenylate cyclase activity and increased 25-hydroxyvitamin D3-1-hydroxylase activity in kidney. The relationship between estrogen-mediated alterations of these two enzyme systems was investigated in embryonic and mature, egg-laying chickens treated in vivo with 17 beta-estradiol (E2). Basal and PTH- and forskolin-stimulated adenylate cyclase activity in kidney plasma membrane preparations was not affected by E2 treatment of 19-day-old chick embryos or of 41-wk-old egg-laying females. High, possibly maximal, levels of catalytic activity in control embryos and hens may have precluded further stimulation by E2. In contrast, E2 significantly enhanced 25-hydroxyvitamin D3-1-hydroxylase activity of embryonic kidney up to 10-fold (P less than 0.005). In mature females, E2 caused cessation of egg laying accompanied by a significant reduction (P less than 0.005) of 25-hydroxyvitamin D3-1-hydroxylase activity. These results indicate that the PTH-dependent adenylate cyclase and the 25-hydroxyvitamin D3-1-hydroxylase systems of avian kidney can be regulated independently and suggest that factors in addition to estrogen are involved in their regulation.

Plasma levels of cholesterol and testosterone in White Leghorn hens that laid soft-shelled and shell-less eggs

Plasma concentrations of cholesterol and testosterone were determined in White Leghorn hens that had just laid soft-shelled (SS) or shell-less (SL) eggs and compared to those that laid hard-shelled (HS) eggs. Hens were bled at two different ages, at 22 to 34 and 66 to 74 weeks of age. Blood samples were collected in the morning hours (0600 to 1200 hr) for both age groups with an additional evening bleed (1500 to 1900 hr) for the 66 to 74-week-old hens. Both plasma constituents were higher in hens that laid SS or SL eggs when compared to those which laid HS eggs, but differences were statistically significant only for the 66 to 74-week-old hens bled in the morning hours. The results of this study indicate that elevated levels of testosterone and its precursor, cholesterol, may be related to the production of eggs with little or no shell calcification.