Bay K 8644 enhances Ca2+ channel activities in embryonic chick heart cells without prolongation of open times.

The effects of the Ca2+ channel agonist, Bay K 8644, on the slow (L-type) Ca2+ channels was examined in young (3-day-old) embryonic chick heart cells, which naturally exhibit long-lasting openings. Bay K 8644 (5 microM) increased (a) the peak amplitude of the ensemble-averaged current by 3.9 +/- 0.9-fold (mean +/- S.E.) and (b) the maximal number of simultaneous opening from 2.6 +/- 0.4 to 4.4 +/- 0.9. Bay K 8644 had no effect on the unitary conductance (27 pS in control), and relatively little effect in the open-close kinetic analysis. The mean open times were 4.2 ms and 5.2 ms, in control and Bay K 8644, respectively. These results suggest that the agonistic effect of Bay K 8644 involves a mechanism other than open-time prolongation, such as activation of silent channels.