O. Hurnak et J. Zachar, CONDUCTANCE-VOLTAGE RELATIONS IN LARGE-CONDUCTANCE CHLORIDE CHANNELS IN PROLIFERATING L6 MYOBLASTS, General physiology and biophysics, 13(3), 1994, pp. 171-192
Large-conductance chloride channels (maxi-Cl channels) were studied in
cultured myoblasts (L6 rat muscle cell line); in excised (inside-out)
and in cell attached membrane patches using a conventional patch clam
p method. The incidence of maxi-Cl channels was substantially higher i
n proliferating myoballs, then in quiescent (bottom-attached) myoblast
s (90% and 50% percent of examined cells, respectively). The maxi-Cl c
hannels in myoballs were present both in cell attached and excised pat
ches. The channel conductance at symmetric [Cl] = 150 mmol/l was 359 /- 42 pS (n = 74) in quiescent cells and 439 +/- 10 pS (n = 6) in prol
iferating myoballs respectively. The conductance of the channel in qui
escent cells increased with chloride concentration in symmetric NaCl r
ich solutions according to Michaelis-Menten curve with the saturation
limiting conductance of about 640 pS (g(max)) and K-m = 112 mmol/l. Th
e shift of the reversal potential upon increasing the pipette concentr
ation of NaCl from 150 to 250 mmol/l was consistent with P-Na/P-Cl = 0
.1. Neither the conductance nor the activation of the channel were dep
endent on the presence of calcium ions. The bell-shaped steady state c
hannel conductance-voltage relationship is asymmetric and can be fitte
d by two Boltzmann equations with different V-h and k constants; -25.6
mV and -6.8 mV, respectively, for the negative side and +49.6 mV and
+13.7 mV for the positive side in quiescent cells. The corresponding v
alues in proliferating myoballs were as follows: -15.5 mV and -2.4 mV,
respectively, for the negative side and +31.4 mV and +6.8 mV for the
positive side. From the maximum slopes of the P-open versus V curves a
n estimate was made of the charges for the gates that close at negativ
e (3.5) or positive (1.7) potentials, respectively, in quiescent cells
. The corresponding values in myoballs were 10.6 and 3.7, respectively
. The probability of one gate to be open was dependent on the state of
activation of the opposite gate as determined by prepulses of the opp
osite polarity. The channel showed multiple (up to six) conductance le
vels that may develop in a step-like manner. The onset of the full-gro
wn maxi-Cl channel is fairly abrupt; it might, however, be preceded by
a small conductance unit activity. It is supposed that the difference
s between the quiescent myoblasts and proliferating myoballs might ref
lect increased expression of maxi-Cl channels in myoballs to perform a
s yet unknown role in the cell cycle and/or proliferation of the myobl
asts.