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Department of Pharmacology, School of Medicine, University of California, San Francisco 94143-0450.
1. Recordings of single-channel activity were made from cell-attached patches on mouse C2 muscle cells at morphologically identifiable stages of myogenesis in vitro. We have identified Ca2(+)-permeable, cation-selective channels that are gated by applying suction to the patch electrode and by changes in membrane potential and have analysed single-channel properties as well as channel expression during myogenesis. 2. Single-channel activity could be detected when the membrane was held at steady negative potentials. With monovalent cations in the electrode, the single-channel current-voltage (i-V) relations were linear. The channel is permeable to Li+, Na+, K+, Rb+ and Cs+, but is not strongly selective among the monovalent cations as judged by measurements of single-channel conductance and reversal potential. 3. With 110 mM of either CaCl2 or BaCl2 as the only inward change carrier, slope conductances were approximately 13 and 24 pS and currents reversed at approximately +22 and +17 mV, respectively. The relative permeability of Ca2+ to K+ calculated from the constant-field equation was PCa/PK = approximately 2. 4. Channel openings occurred as bursts of brief openings and closings separated by much longer closed periods. Closed-time histograms were best fitted with three exponential components, while histograms of burst duration were best fitted with two exponential components, reflecting the short and long bursts in the single-channel records. 5. Applying suction to the patch electrode while recording at steady negative membrane potentials produced channel openings to discrete current levels. Mean channel open probability depended linearly on the square of the applied pressure and was greater at positive membrane potentials. The permeability of the channel to monovalent and divalent cations was indistinguishable from the spontaneous activity recorded at steady negative potentials. 6. Channel activity recorded from cell-attached patches in the absence of applied pressure depended on membrane potential increasing approximately e-fold per 38 mV with depolarization. Analysis of the kinetics of the response to membrane potential showed that the depolarization reduced the duration of the slowest component of the closed-time distribution. 7. The lanthanide cation gadolinium (Gd) reduced the amplitude of the unitary currents in a concentration-dependent manner. The amplitudes of both inward and outward currents were reduced to the same extent suggesting block is voltage-independent. Gd produced half-maximal inhibition of the unitary current at approximately 6 microM.(ABSTRACT TRUNCATED AT 400 WORDS)
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