Patch Voltage Clamp Technique

Cell-attached (single-channel) voltage clamp Patch clamp technique Patch voltage clamp technique Voltage clamp... 

The opening of masses of ion channels in nematode muscle membranes may be detected using the two-microelectrode voltage-clamp technique. In contrast, the opening of single ion channels may be recorded using the vesicle preparation and patch-clamp technique. These techniques are both described below. 

This technique allows the study of single-ion channels as well as whole-cell ion channel currents. Essentially, the patch-clamp technique is an improved and refined version of the voltage-clamp technique. It requires a low electrical noise borosiUcate glass electrode, also known as a patch electrode or patch pipette, with a relatively large tip (>1 pm) that has a smooth surface rather than a sharp tip as with the conventional microelectrodes. This is a major difference between the patch electrode and the sharp electrode used to impale cells directly through the cell membrane (Figure 16.20). 

In a different context, a micropipette has been applied to monitor the current through a single-ion channel in a biological membrane. The patch-clamp technique invented by Sackmann and Neher [119] led to their Nobel Prize in medicine. The variations in channel current with voltage, concentration, type of ions, and type of channels have been explored. While the functions of specific channels, in particular their ionic selectivity, have been well known, only a handful of channels have the internal geometry and charge distribution determined. The development of a theory to interpret the mass of channel data and to predict channel action is still lacking. 

The electrophysiological experiments reported here and done with patch-clamp techniques support this idea. The external application of MTX to isolated cardiac myocytes caused a sustained inward current which was carried by Ca . MTX did not increase the voltage-dependent Ca channel current, and both the time dependence and voltage dependence of the MTX-induced current were clearly different from those of the usual Ca channel current. These results suggest that the MTX-induced steady current is different from the usual voltage-dependent Ca channel current, and that this is possibly a current which flows through a new type of Ca -permeable channel. Tbe steady current described here may be responsible for the highly enhanced Ca influx induced by MTX and could account for the excitatory action of MTX on smooth and cardiac muscles. 

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. 

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