| Abstract: | We have studied the properties of beta-adrenergic receptors and of their interaction with adenylate cyclase in the chick myocardium during embryogenesis. Between 4.5 and 7.5 days in ovo the number of receptors determined by (-)-3H]dihydroalprenolol (3H]DHA) binding is constant at approx. 0.36 pmol of receptor/mg of protein. By day 9 the density decreases significantly to 0.22 pmol of receptor/mg of protein. At day 12.5--13.5 the number was 0.14--0.18 pmol of receptor/mg of protein. This number did not change further up to day 16. The same results were obtained with guanosine 5'-beta, gamma-imido]triphosphate (pNH]ppG) added to the assay mixtures. There was no significant change in receptor affinity for the antagonist 3H]DHA between days 5.5 and 13. Despite the decrease in numbers of beta-adrenergic receptors, there was no change in basal, pNH]ppG-, isoprenaline- or isoprenaline-plus-pNH]ppG-stimulated adenylate cyclase activity between days 3 and 12 of development. We conclude that beta-adrenergic receptors and adenylate cyclase are not co-ordinately regulated during early embryonic development of the chick heart. Some of the beta-adrenergic receptors present very early in the ontogeny of cardiac tissue appear not to be coupled to adenylate cyclase since their loss is not reflected in decreased activation of the enzyme. |
