Emergence of beta-adrenergic sensitivity in the developing chicken heart.

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Abstract
Muscle cells dissociated from 5-day embryonic chicken hearts showed dose-dependent increases in chronotropic rates of contraction and cAMP levels in response to epinephrine (EPI), an effect that could be blocked by .beta.-adrenergic antagonists. At 2- to 2.5-days, 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 after several days of growth in culture. 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 the beating frequency and cAMP levels after a 48-h 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. 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. The development of .beta.-adrenergic sensitivity apparent 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 absorbed on Sephadex G-200.