9407136 Packard An avian embryo obtains 100-150 mg of calcium from its calcareous eggshell during the last half of development. This remarkable influx of calcium occurs with no observable perturbation to calcium status, an observation that has led to the conclusion that his influx of calcium must be under homeostatic control. Embryos may mediate the influx of calcium by controlling shell calcium mobilization and/or the transport of calcium into the vasculature. The mechanism and control of calcium transport during embryogenesis have been examined extensively, but efforts to elucidate the mechanism and control of shell calcium mobilization have been unsuccessful, in part because of the complexity of the in vivo system and the absence, until recently, of appropriate in vitro protocols. This research will use a new protocol for in vitro culture of the calcium-mobilizing system to examine the mechanism and control of shell calcium mobilization. The mechanism of shell calcium mobilization will be examined by exposing the calcium- mobilizing system to inhibitors of two enzymes (carbonic anhydrase and a proton ATPase) that are believed to be important mediators of this process. Control of shell calcium mobilization will be examined by culturing the system with a variety of calcium- regulating hormones that have the potential to stimulate or inhibit the removal of calcium from the eggshell. This research will answer fundamental questions about how embryos maintain calcium homeostasis during a period of intense calcium influx, in isolation from the control systems of the maternal parent, and when their own control systems are developing and differentiating. ***
