J. Arreola et al., ACTIVATION OF CALCIUM-DEPENDENT CHLORIDE CHANNELS IN RAT PAROTID ACINAR-CELLS, The Journal of general physiology, 108(1), 1996, pp. 35-47
The Ca2+ and voltage dependence of Ca2+-activated Cl- currents in rat
parotid acinar cells was examined with the whole-cell patch clamp tech
nique. Acinar cells were dialyzed with buffered free Ca2+ concentratio
ns ([Ca2+](i)) from <1 nM to 5 mu M. Increasing [Ca2+](i) induced an i
ncrease in Cl- current at all membrane potentials. In cells dialyzed w
ith [Ca2+](i) >25 nM, depolarizing test pulses activated a Cl- current
that was composed of an instantaneous and a slow monoexponential comp
onent. The steady-state current-voltage relationship showed outward re
ctification at low [Ca2+](i) but became more linear as the [Ca2+](i) i
ncreased because of a shift in Cl- channel activation toward more nega
tive voltages. The Ca2+ dependence of steady-state channel activation
at various membrane voltages was fit by the Hill equation. The apparen
t K-d and Hill coefficient obtained from this analysis were both funct
ions of membrane potential. The K-d decreased from 417 to 63 nM betwee
n -106 and +94 mV, whereas the Hill coefficient was always >1 and incr
eased to values as large as 2.5 at large positive potentials. We found
that a relatively simple mechanistic model can account for the channe
l steady-state and kinetic behavior. In this model, channel activation
involves two identical, independent, sequential Ca2+ binding steps be
fore a final Ca2+-independent transition to the conducting conformatio
n. Channel activation proceeds sequentially through three closed state
s before reaching the open state. The Ca2+ binding steps of this model
have a voltage dependence similar to that of the K-d from the Hill an
alysis. The simplest interpretation of our findings is that these chan
nels are directly activated by Ca2+ ions that bind to sites similar to
13% into the membrane electric field from the cytoplasmic surface.