Voltage‐dependent block by internal spermine of the murine inwardly rectifying K + channel, Kir2.1, with asymmetrical K + concentrations

Effects of internal spermine on outward single‐channel currents through a strongly inwardly rectifying K + channel (Kir2.1) were studied at asymmetrical K + concentrations (30 m m external and 150 m m internal K + ). The current–voltage ( I–V ) relation for the single channel was almost linear and reversed at −37 ± 3 mV ( V R ; n = 19). The channel conductance was 26.3 ± 1.3 pS ( n = 24). The open‐time and closed‐time histograms were fitted with a single exponential function. Internal spermine at a concentration of 1–100 n m reduced the open time of the outward currents in a concentration‐dependent manner and produced a blocked state. The steady‐state open probability of the outward current decreased with larger depolarizations in both the absence and presence of internal spermine. The steady‐state open probability with asymmetrical K + and symmetrical (150 m m external and internal K + ) concentrations plotted against driving force ( V − V R ) coincided with smaller depolarizations in the absence of spermine and larger depolarizations and higher spermine concentrations in the presence of spermine. The blocking rate constants and unblock rates with 30 m m and 150 m m external K + were similar at the same driving force. The dissociation constant–membrane potential relation for 30 m m external K + was shifted in the negative direction from that for 150 m m external K + by 36 mV. These results suggested that the blocking kinetics depends on driving force to produce driving force‐dependent inward rectification when the equilibrium potential for K + is altered by changing external K + and that the energy barriers and wells for blocking ions from passing or lodging are not stable but affected by external K + ions.