Differential polyamine sensitivity in inwardly rectifying Kir2 potassium channels

Recent studies have shown that Kir2 channels display differential sensitivity to intracellular polyamines, and have raised a number of questions about several properties of inward rectification important to the understanding of their physiological roles. In this study, we have carried out a detailed characterization of steady-state and kinetic properties of block of Kir2.1-3 channels by spermine. High-resolution recordings from outside-out patches showed that in all Kir2 channels current-voltage relationships display a 'crossover' effect upon change in extracellular K+. Experiments at different concentrations of spermine allowed for the characterization of two distinct shallow components of rectification, with the voltages for half-block negative (V-1/2(1)) and positive (V-1/2(2)) to the voltage of half-block for the major steep component of rectification (V-1/2(0)). While V-1/2(1) and V-1/2(2) voltages differ significantly between Kir2 channels, they were coupled to each other according to the equation V-1/2(1) - V-1/2(2) = constant, strongly suggesting that similar structures may underlie both components. In Kir2.3 channels, the V-1/2(2) was similar to 50 mV positive to V-1/2(0), leading to a pattern of outward currents distinct from that of Kir2.1 and Kir2.2 channels. The effective valency of spermine block (Z(0)) was highest in Kir2.2 channels while the valencies in Kir2.1 and Kir2.3 channels were not significantly different. The voltage dependence of spermine unblock was similar in all Kir2 channels, but the rates of unblock were similar to 7-fold and similar to 16-fold slower in Kir2.3 channels than those in Kir2.1 and Kir2.2 when measured at high and physiological extracellular K+, respectively. In all Kir2 channels, the instantaneous phase of activation was present. The instantaneous phase was difficult to resolve at high extracellular K+ but it became evident and accounted for nearly 30-50% of the total current when recorded at physiological extracellular K+. In conclusion, the data are consistent with the universal mechanism of rectification in Kir2 channels, but also point to significant, and physiologically important, quantitative differences between Kir2 isoforms.