Electrophysiological properties of chick embryonic hearts grafted and organ cultured in vitro

Because young (2- to 3-days-old) embryonic chick hearts organ cultured in vitro for 1 to 2 weeks do not gain the fast Na + channels that they normally would during in situ development, instead retaining their high density of tetrodoxin (TTX)-insensitive slow Na + channels, we tested whether revascularization and blood perfusion allow membrane differentiation to proceed. Young hearts (3-day-old) (ventricular portion) and old hearts (16-day) (small pieces of about 1 mm 3 ) were grafted on to the chorioallantoic membrane of a host chick (6-day-old) where they were revascularized by the host circulation; for comparison, hearts were also placed into test-tube organ culture. After periods of 1 week (grafts) and 2 weeks (test-tube cultures), the tissues were removed for intracellular microelectrode recordings. The young hearts had electrical activity similar to fresh non-cultured 3-day-old hearts, i.e., the cells failed to further differentiate in culture. The cells had low resting potentials and slowly-rising (5–10 V/s) TTX-insensitive action potentials. There was no difference in results between the two culture methods. The old hearts cultured by either method had electrical activity similar to fresh non-cultured 16-day-old hearts, i.e., the cells retained their highly differentiated state. They had high resting potentials and fast-rising action potentials which were completely abolished by TTX. The action potential duration was longer in the grafted hearts (young and old) than in the test-tube hearts. Hence, young or old myocardial cells tend to retain the membrane cation channels that they originally possessed when placed into culture, and the young grafted hearts fail to gain TTX-sensitive fast Na + channels, even though blood perfused.