Abstract
Unidirectional fluxes of 45Ca were measured from ectoderm to endoderm (O → I) and from endoderm to ectoderm (I → O) in the normal chick chorio-allantoic membrane under both open circuit and short circuit conditions. In each case a net calcium transport from O → I could be demonstrated, and the measured flux ratios were at least three times greater than the corresponding theoretical ratios indicating an active transport process. Upon removal of the acellular inner-shell membrane from the ectodermal surface, net calcium transport was reversed. The results have been interpreted as suggesting the presence of two opposing calcium pumps. Since in vivo calcium transport does not proceed across the entire membrane, the transported calcium would undoubtedly see only one pump. The orientation of the electrical potential difference and the short-circuit current indicates the existence of at least one other actively transported ion. Ion substitution experiments suggest the possible existence of an active sodium transport mechanism.
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