Action of 1,25-dihydroxyvitamin D3 and parathyroid hormone on 45calcium uptake by the yolk sac membrane of chick embryos

The effect of calcium-regulating hormones on transport of calcium across the chorioallantoic membrane of the chicken embryo

The hormonal form of vitamin D3 (1,25(OH)2D3), parathyroid hormone (PTH), or appropriate vehicle were injected into the yolk sac of eggs of domestic fowl on days 16 and 17 of incubation. The chorioallantoic membrane (CAM) and overlying inner shell membrane were removed from eggs on day 18 and mounted in a Ussing-type apparatus. Transport of calcium was assessed by monitoring movements of radiolabeled calcium. Transport of calcium from the chorionic aspect of the CAM to the allantoic aspect increased considerably with time for all treatment groups except the one receiving PTH. "Back-flux" of calcium (movement of calcium from the allantoic aspect to the chorionic) was negligible for all treatment groups at all sampling periods. PTH treatment did not affect flux of calcium from allantois to chorion but reduced flux from chorion to allantois considerably. The underlying cause of this effect has not been identified. The hormonal form of vitamin D3 did not affect flux of calcium in either direction. These data raise the possibility that control of calcium transport by the CAM may not be the primary function of the vitamin D hormone.

Ultrastructural localization of calcium in the chick yolk sac membrane endodermal cells as revealed by cytochemistry and X-ray microanalysis

The yolk sac membrane (YSM) of the chick embryo transports calcium from the yolk into the embryonic circulation during the first half of development, but the intracellular pathway of calcium transport is poorly understood. In the present study, the ultrastructural localization of calcium was investigated in cells of the YSM of 9-day chick embryos. X-ray microanalysis as well as cytochemical techniques performed on yolk sac membrane cells treated with potassium oxalate, potassium ferricyanide and potassium antimonate demonstrated accumulation of calcium in yolk granules, digested yolk products, electron-dense bodies (EDBs; 100-400 nm diameter) and electron-dense granules (EDGs; 30-50 nm diameter). When strontium ions were injected into the yolk, they were incorporated into the endodermal cells and sequestered specifically in EDGs. From these results, we propose that calcium enters the endodermal cells by endocytosis of calcium-containing yolk granules, as well as through calcium channels in the apical cell membrane. In the cytoplasm, digested yolk products, EDBs, and EDGs act as sites of sequestration and accumulation of calcium. Extrusion of intracellular calcium into the extracellular space and embryonic circulation is accomplished by exocytosis of calcium-containing material and via an ion pump in the basal cell membrane.

Effects of cytochalasin B on calcium transport by 1,25(OH)2D3- or PTH-treated chick embryonic yolk sac in vitro and in vivo

The present study was done to elucidate the mechanism(s) by which calcium is taken up or transported across the yolk sac membrane of the embryonic chick. We examined the effect of various inhibitors or experimental conditions on the uptake of calcium in vitro. Treatment with ouabain, verapamil, antimycin A and calcium ionophore A23187; substitution of choline for sodium or potassium in the buffer; or incubation of the tissue at 0 degree C had no significant effect on calcium uptake by the yolk sac membrane. Dinitrophenol (DNP) and lanthanum chloride (LaCl2) reduced 45Ca uptake from day 6 and 9 embryos by 15% and 30%, respectively. Cytochalasin B decreased the uptake of 45Ca in yolk sac membrane disks of day 6 embryos, but not in older embryos. The effects of cytochalasin B were explored further in embryos pretreated with either 1,25(OH)2D3 or PTH, both of which enhance calcium uptake. Cytochalasin B decreased calcium uptake in 9-day and 12-day vitamin D-treated embryos to about 60% of their hormone-enhanced level and also decreased PTH-stimulated 45Ca uptake into yolk sac disks by about 50% in embryos of all age groups tested. We also examined the effect of cytochalasin B on 45Ca transport across the yolk sac membrane in vivo. Cytochalasin B did not affect this transport in control (vehicle-treated) embryos. However, it significantly decreased the enhanced in vivo 45Ca transport in day-9 and -12 vitamin D-treated embryos approximately 30% and 45%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium-dependent short-circuit current across the yolk sac membrane during embryonic development in normal and shell-less cultured chicks

The transepithelial electrical characteristics of the isolated yolk sac membrane of normal in ovo or shell-less cultured chick embryos were investigated. In normal chicks the potential difference (blood side positive relative to yolk side) and short-circuit current of the membrane increased during development. Ouabain (10(-4) M) on the blood side (basolateral side, serosal side) significantly decreased potential difference and short-circuit current but was without effect on the yolk side (brush border side, mucosal side). Substitution of choline for Na+ in the bathing solutions abolished the potential difference and the short-circuit current; when Na+ replaced choline this effect was reversed. Amiloride added to both sides of the yolk sac membrane had no effect on potential difference or short-circuit current. Injection of aldosterone (50 micrograms) and T3 (10 microM) into yolk did not induce amiloride sensitivity. The short-circuit current was not altered by addition of either glucose or alanine to the bath. The short-circuit current of the yolk sac membrane of shell-less cultured embryos was significantly lower than that of normal controls. Addition of Ca2+ to the serosal bathing medium did not reverse the foregoing condition, but decreased the short-circuit current. It is concluded that the yolk sac short-circuit current is Na+ dependent and increases with developmental age in the chick embryo.

Variation during development in the response of chicken embryos to calcitriol administered via slow-release pellets

Embryonic chickens were exposed to 0, 30, or 300 pg of calcitriol per day via slow-release pellets implanted adjacent to the chorioallantoic membrane. Pellets were placed in eggs on Days 10 and 15, and eggs were sampled on Days 12 and 17, respectively. The hormone induced high mortality among embryos receiving pellets on Day 10, but not among those whose treatment was begun on Day 15. Embryos receiving hormone were hypercalcemic and hypophosphatemic on both Day 12 and Day 17, but the concentration of magnesium in plasma was not affected. Size of embryos sampled on Day 12 was not affected by hormone treatment, but embryos sampled on Day 17 showed a dose-related reduction in size. Yolk-free carcasses of the embryos sampled on Day 17 also showed dose-dependent reductions in phosphorus and magnesium, but calcium content of carcasses on Day 17 was unaffected by treatment. These results indicate that both younger and older embryos respond to very small quantities of calcitriol administered via slow-release pellets. The absence of sustained hypercalcemia in earlier studies that used this protocol to dispense hormone was not caused by exposing older embryos to subthreshold quantities of calcitriol. The fact that reduced body size in late embryos receiving calcitriol was not accompanied by a reduction also in calcium content may mean that embryos deposit calcium in the carcass in an effort to deal with the extreme hypercalcemia induced by calcitriol.