Abstract
Muscle cells dissociated from 5-day embryonic chicken hearts showed dose-dependent increases in both chronotropic rates of contraction and cyclic AMP (cAMP) levels in response to epinephrine (EPI), an effect that could be blocked by beta-adrenergic antagonists. However, 2- to 2.5-day embryonic chicken myocardial cells, although similar to 5-day heart cells with respect to the organization of myofibrils, failed to respond to EPI either by increased rates of contraction or by elevated levels of intracellular cAMP. Development of beta-adrenergic sensitivity in 2- to 2.5-day cells did not occur even after several days of growth in culture. However, addition of an extract prepared from 11-day chicken embryos to 2- to 2.5-day muscle cell cultures at any point during in vitro growth resulted in the development of sensitivity to EPI as measured by increases in both the beating frequency and cAMP levels after a 48-hr incubation in embryo extract (EE). The basal level of cAMP in cells unresponsive to EPI is 5 times that in EPI-sensitive muscle cells from older hearts (5 days). The high basal level of cAMP in these cells is reduced to a level characteristic of cells from older hearts when treated with the EE. Once sensitivity was acquired, it was retained as a stable trait of the muscle cells in culture. Furthermore, EE-treated 2- to 2.5-day cells showed less reduction of positive chronotropy in response to multiple doses of EPI than did cells prepared from 5-day hearts. Fractionation of EE on Sephadex G-200 showed that the activity was present in the large-molecule fractions. It is concluded that the development of beta-adrenergic sensitivity can be induced in unresponsive heart cells cultured from 2- to 2.5-day embryos by a factor(s) in the EE that is not adsorbed on Sephadex G-200.