In the last few years, the knowledge about the molecular and structural determinants of a special class of potassium channels regulated by G proteins, the G protein-activated inwardly rectifying K+ channels (GIRK channels) has increased (Luscher et al. 1997). GIRK channels are members of a large family of inwardly rectifying K+ channels (referred also with the symbols Kir1–Kir7), which stabilize the potential (by changing the current) to maintain the equilibrium potential for K+ (EK), corresponding to the zero current level. The term “inwardly rectifying K+ channels rectification” refers to the ability of these channels to pass current into the cells most efficiently when the cell is hyperpolarized, thus allowing a large influx of potassium ions. The mechanism behind this rectification, yet not fully understood, involves the blockade by high-affinity endogenous polyamines and Mg2+,which...