CHARACTERIZATION OF THE POTASSIUM CHANNEL FROM FROG SKELETAL-MUSCLE SARCOPLASMIC-RETICULUM MEMBRANE

1. The sarcoplasmic reticulum (SR) membrane of skeletal muscle contain s potassium channels which are thought to support charge neutralizatio n during calcium release by providing a permeability pathway for count er-ion movement. To describe the behaviour of the SR K+ channel under physiological conditions, single channel activity was recorded from ex cised patches of SR membrane. Patches were made from membrane blebs ex truded 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 r elationship of the channel was linear over most of this voltage range (slope conductance equal to 60 pS in 130 mM potassium), but showed rec tification at voltages below -50 mV. 3. The activity of the channel (n umber of state transitions per unit time) was greater at positive volt ages than at negative voltages. Analysis of dwell-time distributions s howed that the time spent in the open state is best fitted by a double Gaussian, suggesting that the channel possesses both a long (1)- and a short (s)-lived open state with identical conductances. The dwell ti mes for the two states were T-s=0.3 ms and T-l=2.6 ms at +90 mV and T- s=0.1 ms and T-l=15.1 ms at -40 mV. Thus, positive voltage decreased t he long open time significantly which was consistent with the observed increase in channel activity at positive potentials. 4. The permeabil ity 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 reduc ed when the patch was perfused with 1,10-bis-guanidino-n-decane (BisG1 0), 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 mu M at -40 mV and 338.2 mu M at +50 mV. The zero voltag e dissociation constant (K-d) was calculated to be 48 mu M. 6. Pharmac ological 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 blo cking the channel than BisG10. Physiological calcium concentrations (p Ca = 8.0 and 3.0) did not affect channel behaviour. 7. The high open p robability at 0 mV (P-o=0.9) and relative insensitivity of P-o to volt age and calcium are consistent with the idea that the SR K+ channel is a major pathway for counter-ion movement during Ca2+ release.