Depolarization increases the single-channel conductance and the open probability of crayfish glutamate channels

We have studied the voltage sensitivity of glutamate receptors in outside-out patches taken from crayfish muscles. We found that single- channel conductance, measured directly at the single-channel level, increases as depolarization rises. At holding potentials from -90 mV to ~20 mV, the conductance is 109 pS. At holding potentials positive to 20 mV, the conductance is 213 pS. This increase in single-channel conductance was also observed in cell-attached patches. In addition, desensitization, rise time, and the dose-response curve were all affected by depolarization. To further clarify these multifaceted effects, we evaluated the kinetic properties of single-channel activity recorded from cell-attached patches in hyperpolarization (membrane potential around -75 mV) and depolarization (membrane potential ~105 mV). We found that the glutamate dissociation rate constant (k_-) was affected most significantly by membrane potential; it declined 6.5-fold under depolarization. The rate constant of channel closing (k(c)) was also significantly affected; it declined 1.8-fold. The rate constant of channel opening (k(o)) declined only 1.2-fold. The possible physiological significance of the depolarization-mediated changes in the above rate constants is discussed.