Increase of cyclic AMP levels induced by isoproterenol in cultured and non-cultured chick embryonic hearts.

Cyclic AMP levels were determined in young (day 4) and old (day 16) embryonic chick hearts, fresh and cultured. The 4-day hearts contained 33.6±2.2 pmol/mg protein, and isoproterenol (5 × 10^?5m) elevated this to 118.9±1.5. When they were organ-cultured for 7 days, the cyclic AMP level decreased markedly to about 6.0 pmol/mg protein. Isoproterenol caused a large rise to 37.7±4.7. When cells were separated by trypsin and cultivated in vitro for days, their cyclic AMP levels were lowered to 5.6±1.4, and isoproterenol raised the level to 28.8±2.0 pmol/mg protein. The 16-day hearts contained 11.7±1.5 pmol/mg protein, and isoproterenol elevated this to 22.0±1.5. When old hearts were organ-cultured, their cyclic AMP level decreased somewhat to about 5.3 pmol/mg protein, and isoproterenol raised it to 17.9±2.0. When cells were separated by trypsin and placed into cell culture for 7 days, their cyclic AMP levels were low, about 4.4 pmol/mg protein, and isoproterenol raised the level to about 23.6. The results indicated that: (a) Young hearts in situ have a high cyclic AMP level. (b) Young and old hearts placed into organ and cell culture have low cyclic AMP levels, and isoproterenol is capable of producing very large increases. (c) Beta-adrenergic receptors are present in the young hearts. (d) Beta-adrenergic receptors of young and old hearts are retained in culture. (e) There seems to be no relationship between basal intracellular cyclic AMP levels and membrane electrical properties.