Cytoskeletal control of rectification and expression of four substates in cardiac inward rectifier K+ channels

Cardiac inward rectifiers may have a three-barrel channel structure, based on evidence for three substates in single-channel recordings. However, some reports indicate four substates, a feature more compatible with the four-subunit structure for which there is evidence in cloned voltage-activated K+ channels. Here we show that although the fourth is easily missed, inward rectifier channels have four substates whose expression is controlled by intracellular Ca(2+) ions. Fourth substate openings also appear after rectification loss in intracellular divalent caution-free solution. We find that this process is accelerated by cytochalasin, a microfilament disrupter. Cytochalasin also abolishes Ca(2+), but not Mg(2+),-induced rectification by restoring fourth substate openings. Thus, cytoskeletal elements control Ca(2+)-dependent substate expression and rectification in native inwardly rectifying K+ channels.