Single-channel study of the cyclic AMP-regulated chloride current in guinea-pig ventricular myocytes.

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1. Properties of the cyclic AMP-regulated Cl- channel were studied in guinea-pig ventricular myocytes with the patch clamp technique. Cell-attached patch recordings were performed, while the cell was dialysed with a cyclic AMP (0.2-0.5 mM)-containing internal solution through a second patch pipette. The latter pipette was also used to monitor the whole-cell Cl- conductance. 2. The whole cell showed a large Cl- conductance for 10-15 min after the beginning of cell dialysis. The activity of single Cl- channels began to appear in some of the cell-attached patches during this time. 3. The channels showed a high open probability (0.69 +/- 0.14, mean +/- S.D., n = 12) at the time of their appearance, and the open probability did not appreciably increase thereafter, even when the whole-cell Cl- conductance increased further with time. 4. An increase in the number of active channels was observed in some patches with progression of the cell dialysis. In such cases, the newly activated channels also showed a high open probability. 5. The above results are consistent with the hypothesis that the cyclic AMP system makes the 'latent' Cl- channels available without influencing their own kinetic behaviour. The available channels may intrinsically exhibit a high open probability. 6. Chloride channel currents could also be recorded in the outside-out patches excised from the cyclic AMP-loaded cells. The I-V relation of these currents showed outward rectification under the condition of symmetrical Cl- gradients, suggesting that the channel itself or a related structure has the property of rectifying current flow. 7. The channel seemed to have at least one open state and two closed states; the open-time histograms showed one exponential component with the values of time constant scattering around 1 s, while the closed-time histograms showed two exponential components with the values of time constant scattering around 0.2 and 1 s. These time constants showed no clear voltage dependence.

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