Effects of parathyroid hormone and calcitonin on embryonic chick bone in organ culture

Calcium homeostasis: the effect of parathyroid hormone on bone membrane electrical potential difference

Although it is generally accepted that calcium ion homeostasis is performed primarily by bone, the mechanism by which this regulation is accomplished remains unclear. The recent demonstration of a metabolism-related electrical potential difference across bone membrane implies an active transport process within this layer of cells lining bone surfaces. This work presents a determination of the effect of parathyroid hormone on the measured potential difference to demonstrate whether this potential is involved in the homeostatic mechanism. A thermodynamic evaluation is performed based on a measurement of the distribution of charged and uncharged tracers between bone extra-cellular fluid and the bathing medium. For embryonic chick calvaria whose viability was assured by the measurement of oxygen consumption, parathyroid hormone at dosage levels up to pharmacological caused an increase in the measured potential difference and thus in the inferred activity level of ionic transport through the membrane. This result is shown to be consistent with an indirect regulation of Ca++ ion through a variable active transport of K+ ion into the interior of bone and thus provides support for the argument that the membrane potential is part of the calcium homeostatic mechanism.

In vitro prostaglandin E2 stimulation of 45Ca mobilization from chick bone

The ability of prostaglandin E2 (PGE2) to mobilize 45Ca from chick embryo long bones was assayed in an in vitro bone culture system. Concentrations of PGE2 tested ranged from 10(-9) to 10(-5) M. The PGE2 was effective in stimulating release of 45Ca from prelabelled bones at all concentrations tested except at 10(-9) M. In addition, stimulation of 45Ca release could be produced daily for 4 consecutive days of PGE2 culture-pulsing at what appeared to be the optimal PGE2 concentration, 10(-7) M. We conclude, as in mammals, PGE2 is a potent stimulator of calcium mobilization from avian bone. The potential involvement of prostaglandins in eggshell formation is discussed.

Release of calcium from bone in vitro by homogenates of embryonic chick parathyroid glands

Parathyroid glands were removed from chick embryos of 12 to 20 days of development and tested for their ability to stimulate calcium release from calcium-45-labeled embryonic chick bone in organ culture. All glands stimulated calcium-45 release to approximately the same degree, although the glands of 16- and 20-day embryos had a somewhat enhanced calcium-mobilizing ability. It appears that parathyroid hormone is present in the embryonic chick coincidental with the time of early development of the skeleton and is present in similar quantities throughout the remainder of embryogenesis.

Effects of 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 on embryonic chick bone in organ culture

The ability of embryonic chick bone to respond to 25 hydroxyvitamin D3 and 1,25 dihydroxyvitamin D3 was assessed in organ culture. These metabolites were added to prelabeled chick embryo long-bone explants, and the amount of calcium-45 released into the medium after 24 or 48 h of hormone exposure was measured. For each time period a significant release of calcium-45 from hormone-treated bones was observed. The response to 25 hydroxyvitamin D3 and 1,25 dihydroxyvitamin D3 was always greater at 48 h and a clear dose-dependency was established at this time as well. 1,25 dihydroxyvitamin D3 was the more potent resorbing agent at all concentrations tested. The results from this study suggest that vitamin D metabolites may be involved in bone regulation in the chick embryo.