DEVELOPMENTAL-CHANGES IN LONG-OPENING BEHAVIOR OF L-TYPE CA-2+ CHANNELS IN EMBRYONIC CHICK HEART-CELLS

In the early (3-day) stage of development, long-lasting openings of the L-type Ca2+ channels (mode 2) occur in embryonic chick heart cells. Since mode-2 behavior is infrequently observed in adult heart cells of other species, in the present study, developmental change in behavior of the Ca2+ channel was examined in young (3 -day) and old (17-day) embryonic chick heart cells. In the whole-cell voltage clamp, the L-type Ca2+ current carried by Ca2+ ions was smaller in amplitude and had a faster inactivation in 17-day cells than in 3-day cells. The peak current density was 8.1+/-0.2-mu-A/cm2 (mean+/-SEM, n=5) and 5.1+/-0.3-mu-A/cm2 (n=5) in 3-day and 17-day cells, respectively. When the charge carrier was Ba2+, the L-type Ca2+ channel current density was also smaller in 17-day cells (22.7+/-1.8-mu-A/cm2) than in 3-day cells (28.3+/-2.1-mu-A/cm2). In single-channel recordings, the mode-2 behavior was infrequent in 17-day cells compared with 3-day cells. High-open probability sweeps (with an open probability of >0.25), reflecting mode-2 behavior, accounted for 20.2% and 3.7% in 3-day and 17-day cells, respectively. The ensemble-averaged currents in 17-day cells was 37% of that current in 3-day cells. In addition, decay of the averaged current appeared to be faster in 17-day cells than in 3-day cells. All data from the single-channel analysis agreed with the data from the whole-cell voltage clamp. The results suggest that the long-lasting opening (mode-2) behavior of the L-type Ca2+ current frequently occurs in the early stage of embryonic life and is diminished during development and that it is eventually replaced by a bursting behavior (mode-1) characteristic of adult myocytes around the time of hatching (day 21).