The problem of detecting small Cl -channels

Small Cl -channels appear to play a pivotal role in cell volume regulation and in exocytosis in general. The existence of these channels has been difficult to prove, and it is only recently that they have been identified in very many cells. 

The main problem has been a methodological one. The patch clamp analysis of single channels views the world of channels through a very small analytical window [10]. A single channel event (opening) needs to be sufficiently long-lived and sufficiently large to be picked up within the current noise band under optimized conditions, and with the low-pass filter set to say 2 kHz. The open time needs to be close to a millisecond and the current amplitude close to 0.5 pA to permit detection. 

The current amplitude cannot be increased at free will, because the clamp voltage will usually not exceed + 100 mV. A simple calculation reveals that, given the limitations above, channels need to have a conductance of 10 pS, and time constants in the millisecond range to be detected. Any channel smaller or faster than this will escape detection in this analysis. 

Even beyond this, it should be clear that the large clamp voltage of say 100 mV may already lead to the inactivation of larger channels, and, still worse, the excision of the cell membrane itself may inactivate channels. It is not surprising then that the current literature gives a grossly distorted view of the world of Cl -channels. It is full of large and intermediate channels, but much less data are available on small channels. This analytical problem can be overcome by other patch clamp techniques. 

Neher and coworkers, for example, have used the whole-cell analysis of currents 

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