Limb development in chick embryos: Cyclic AMP‐dependent protein kinase activity, cyclic AMP, and prostaglandin concentrations during cytodifferentiation and morphogenesis

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Abstract
The effects of prostaglandin E2 (PGE2) on cyclic AMP (cAMP) concentrations of chick limb bud cells obtained from limbs at various stages of development were investigated. In addition, endogenous concentrations of PGE2 were examined in whole limbs from comparable stages. Prior to either chondrogenesis or myogenesis (stages 20–23), cells were more responsive to PGE2, in terms of cAMP levels, than those of differentiated phenotypes, obtained at stages 25–28. This greater responsiveness to PGE2 of undifferentiated cells was correlated with endogenous concentrations of PGE2 which were significantly higher in undifferentiated limbs than in limbs containing differentiated cartilage and muscle. Cyclic AMP‐dependent protein kinase (PKA) activity was detectable in cell homogenates at each stage examined and did not appear to change in cAMP dependency at any stage. The majority (80–85%) of total enzyme activity was localized in soluble fractions of cell homogenates while the residual activity was localized to membrane‐enriched, particulate fractions. The results demonstrate that both responsiveness of limb mesenchyme to PGE2 and endogenous concentrations of PGE2 are maximal prior to cytodifferentiation of limb tissues. The presence of cAMP‐dependent protein kinase in these undifferentiated cells supports a regulatory role for both PGE2 and a cAMP‐protein phosphorylation system in the differentiation of limb tissues.