The calcium channel agonist, Bay K-8644, antagonizes effects of diacetyl monoxime on cardiac tissues

Effects of a novel slow channel activator, Bay K-8644 (Bay K), were studied on slow action potential (APs) in young and old embryonic chick hearts, and on its antagonism of the effects of diacetyl monoxime (DAM). The slow APs of young hearts are mediated by slow Na+ channels, whereas those of old hearts are mediated by slow Ca2+ channels. In slow APs of old (13–18 days old) embryonic chick hearts superfused with a high (22 mM) K+ solution, Bay K (10−6 M) gradually increased the amplitude, maximum rate of rise [Formula: see text], and duration of the slow APs. The actions of Bay K persisted for a long time (>30 min) after washout of the drug. DAM (10 mM) depressed the [Formula: see text], duration and amplitude of the slow APs. Some of the changes in slow AP parameters produced by DAM, e.g., [Formula: see text] decrease, were antagonized by the addition of Bay K (10−6 M). In 3-day-old embryonic chick hearts, Bay K potentiated the slow APs and DAM depressed them; Bay K antagonized these effects of DAM. Thus, the actions of Bay K and DAM are likely to be produced, respectively, via the activation and depression of slow Ca2+ channels in old embryonic chick hearts. In addition, the drugs seem to influence slow Na+ channels found in young embyronic chick hearts.