Characterization of the potassium channel from frog skeletal muscle sarcoplasmic reticulum membrane

1. The sarcoplasmic reticulum (SR) membrane of skeletal muscle contains potassium channels which are thought to support charge neutralization during calcium release by providing a permeability pathway for counter-ion movement. To describe the behaviour of the SR K+ channel under physiological conditions, single channel activity was recorded from excised patches of SR membrane. Patches were made from membrane blebs extruded from contracted muscle fibres whose surface membranes had been removed previously by mechanical dissection. 2. The channel was active over a large voltage range from -80 to +100 mV. The current-voltage relationship of the channel was linear over most of this voltage range (slope conductance equal to 60 pS in 130 mM potassium), but showed rectification at voltages below -50 mV. 3. The activity of the channel (number of state transitions per unit time) was greater at positive voltages than at negative voltages. Analysis of dwell-time distributions showed that the time spent in the open state is best fitted by a double Gaussian, suggesting that the channel possesses both a long (l)- and a short (s)-lived open state with identical conductances. The dwell times for the two states were Ts = 0.3 ms and Tl = 2.6 ms at +90 mV and Ts = 0.1 ms and Tl = 15.1 ms at -40 mV. Thus, positive voltage decreased the long open time significantly which was consistent with the observed increase in channel activity at positive potentials. 4. The permeability sequence of the channel to various monovalent cations was deduced from the channel reversal potential under bi-ionic conditions and was found to be: K+ > Rb+ > Na+ > Cs+ > Li+. 5. Channel activity was reduced when the patch was perfused with 1,10-bis-guanidino-n-decane (BisG10), a drug reported to block the SR K+ channel with high affinity. The drug concentration necessary to reduce the open probability (P(o)) by 50% was 19.8 microM at -40 mV and 338.2 microM at +50 mV. The zero voltage dissociation constant (Kd) was calculated to be 48 microM. 6. Pharmacological agents known to affect surface membrane K+ channels, such as 0.5 mM Ba2+ or 3.0 mM 4-aminopyridine, were much less effective in blocking the channel than BisG10. Physiological calcium concentrations (pCa = 8.0 and 3.0) did not affect channel behaviour.4