Three types of slow inward currents as distinguished by melittin in 3-day-old embryonic heart

Membrane slow inward currents of 3-day-old embryonic chick single heart cells were investigated using the whole-cell patch clamp technique. In a solution containing only Na+ ions and in the presence of tetrodotoxin and Mn2+, the inward current–voltage relationship presented two maxima, confirming the existence of two different voltage-dependent slow inward currents. The first type, a fast transient slow inward current (Isi (ft)), was activated from a holding potential of −80 mV and showed fast activation and inactivation. This current was highly sensitive to melittin (10−8 M) and insensitive to low concentrations of desmethoxyverapamil ((−)D888, 10−9–10−6 M). Depolarizing voltage steps from a holding a potential of −50 mV activated two components of the slow inward current, i.e., a slow and a sustained current (Isi(sts)) that showed a slow inactivation followed by a slow inactivation and a sustained component. Melittin at a high concentration (10−4 M) completely blocked the slow transient component (Isi(st)) and left unblocked the sustained component (Isi(s)). Both components (Isi(st) and Isi(s)) were blocked by verapamil (10−5 M) and low concentrations of (−)D888 (10−8–10−6 M).