Polyamines and potassium channels: A twenty five year romance.

Potassium channels that exhibit the property of inward rectification (Kir channels) are present in most cells. Cloning of the first Kir channel genes 25 years ago led to recognition that inward rectification is a consequence of voltage-dependent block by cytoplasmic polyamines, which are also ubiquitously present in animal cells. Upon cellular depolarization, these polycationic metabolites enter the Kir channel pore from the intracellular side, blocking the movement of K+ ions through the channel. As a consequence, high K+ conductance at rest can provide very stable negative resting potentials, but polyamine-mediated blockade at depolarized potentials ensures, for instance, the long plateau phase of the cardiac action potential, an essential feature for a stable cardiac rhythm. Despite much investigation of polyamine block, where exactly polyamines get to within the Kir channel pore and how the steep voltage-dependence arises remains unclear. This review will summarize current understanding of the relevance and molecular mechanisms of polyamine block, and offer some ideas to try to help resolve the fundamental issue of the voltage-dependence of polyamine block.