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Department of Physiology, University of Leicester.

1. Inward unitary currents through inwardly rectifying K+ channels of myotubes derived from newborn rats or from a murine, clonal myoblast cell line were studied in the cell-attached configuration. Open-closed transitions of the channel were observed in the absence of blocking ions. 2. The single-channel conductance was 26.3 +/- 2.9 pS (mean + S.D., n = 14) with 150 mM-K+ pipette solution at room temperature (19-22 degrees C). The channel showed substates of conductance in addition to the main conductance state. A channel with a smaller conductance (8.9 +/- 2.6 pS, n = 4) was also but less frequently observed. 3. The probability of the channel being open is weakly voltage dependent: it decreased from 0.94 to 0.84 as the membrane was hyperpolarized from the resting potential (RP) + 20 mV to RP - 50 mV. 4. The lifetimes of the openings were distributed according to a single exponential. At least three exponentials were required to fit the frequency histogram of the lifetimes of all closed states. The mean open time showed a weak voltage dependence, while the mean closed times had little voltage dependence. 5. In the presence of external Na+, the open probability decreased from 0.89 to 0.43 and the mean open time decreased from 203 to 28 ms (40 mM-K+, 200 mM-Na+ pipette solution) when the patch membrane was hyperpolarized from RP - 40 mV to RP - 110 mV. The mean closed times were not different from those with 150 mM-K+, Na+-free pipette solution and showed little voltage dependence. 6. It is suggested that inactivation of the macroscopic inward currents during hyperpolarization results mainly from a voltage-dependent block by Na+ with relatively slow kinetics.

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