Characteristics of the adenylate cyclase system of differentiating rabbit bone marrow erythroblasts

Adenylate cyclase activity of rabbit bone marrow erythroblasts decreased continuously as the cells developed. The proerythroblasts were the richest cells in cAMP. No changes in cAMP levels were observed after the final cell division. cAMP-phosphodiesterase activity declined rapidly during the early period of erythroid cell development and remained constant but extremely low after condensation of the nucleus. Both cytosolic cAMP-binding capacity and cAMP-dependent protein kinase activity decreased in dividing erythroblasts when calculated in terms of cell number but remained constant per cell volume. Membrane-associated protein kinase was found to be cAMP-dependent only in the dividing cells. The adenylate cyclase activity of both early and late erythroblasts was stimulated by GTP and p (NH)ppG, whereas the stimulating effect of the beta-adrenergic drug L-isoprenaline was limited to the immature dividing cells. The lack of response of non-dividing erythroblasts to beta-adrenergic stimuli is not due to loss of beta-receptors, since both dividing and non-dividing cells bind 125I-iodohydroxybenzylpindolol with equal affinities. The number of beta-adrenergic receptors per cell was 2-fold higher in the dividing cells. No significant change in binding affinity for GTP and p (NH) ppG during erythroblast development was observed.