Long-lasting openings of single slow (L-type) Ca2+ channels in chick embryonic heart cells

Single-channel currents were recorded in cultured embryonic chick (3-day-old) cardiomyocytes in cell-attached patch-clamp experiments. The patch electrode contained 50 mM Ba2+. The cell was bathed in an external solution containing 150 mM K+ (pH 7.4) at room temperature. Depolarizing pulses above -30 mV, from a holding potential of -80 mV, elicited inward unitary currents. The conductance of the channel for this unitary current was 26 pS. The activity of these channels was completely blocked by nifedipine (3 microM). These results indicate that the channel is a slow (L-type) Ca2+ channel. The channels exhibited long-lasting openings, in addition to conventional brief openings. These long openings resembled the long openings produced by the dihydropyridine Ca2+ agonist BAY K 8644 and resultant mode 2 behavior (Hess et al., Nature Lond. 311: 538-544, 1984). The long-lasting openings were observed in 25 patches out of a total of 29 patches in which single-channel activity was present. High open-state probability (Po) sweeps (with Po greater than 0.65), which mainly contain long-lasting openings, accounted for 20.7% of all sweeps. The open-time histogram for the Ca2+ channels was fitted by two exponential components. The time constants of the two components were 0.45 ms (fast) and 6.30 ms (slow). These kinetic properties were similar to those of the previous reports using BAY K 8644. Thus the slow (L-type) Ca2+ channels in young embryonic chick heart cells naturally produce many long-lasting openings in the absence of any added dihydropyridine Ca2+ agonist.