PO₂-DEPENDENT HORMONAL CONTROL OF AVIAN EMBRYONIC RED CELL CARBONIC ANHYDRASE II SYNTHESIS, ORGANIC PHOSPHATE PATTERN, AND OXYGEN AFFINITY

ABSTRACT

Developmental changes of blood oxygen pressure are instrumental for the regulation of blood oxygen affinity (via ATP/2,3-diphosphoglycerate (2,3-DPG) concentration) and carbonic anhydrase (CA) synthesis in red cells of chick embryos. Hypoxia is the physiological stimulus for the coordinated onset of synthesis of CA and 2,3-DPG in late chick embryos, whereas the higher blood oxygen pressure prevailing in early—and midterm embryos suppresses CA and 2,3-DPG synthesis and increases red cell ATP. This mechanism guarantees flexible adaption of the oxygen affinity to the prevailing oxygen pressure. At least two hormonal effectors seem to be involved in the PO₂-dependent regulation: (a) a heat-labile, low molecular weight compound of embryonic chick plasma that stimulates CA and 2,3-DPG synthesis and (b) an antagonistically-acting, short-lived metabolite present in the blood of normoxic and hyperoxic embryos, whose activity/concentration should be reduced with falling PO₂. While the identity of the CAII inducer is not yet known, extracellular ATP is a likely candidate for the role of antagonist. In vitro it depresses CAII synthesis via activation of phospholipase C, and—by a different transduction pathway—stimulates net synthesis of ATP and downregulation of 2,3-DPG levels.