Jx. Wang et Pm. Best, CHARACTERIZATION OF THE POTASSIUM CHANNEL FROM FROG SKELETAL-MUSCLE SARCOPLASMIC-RETICULUM MEMBRANE, Journal of physiology, 477(2), 1994, pp. 279-290
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.