Patch Clamps Permit Measurement of Ion Movements Through Single Channels

The technique of patch clamping enables workers to investigate the opening, closing, regulation, and ion conductance of a single ion channel. In this technique, the inward or outward movement of ions across a patch of membrane is quantified from the amount of electric current needed to maintain the membrane potential at a particular "clamped" value (Figure 7-17a, b). To preserve electroneutrality and to keep the membrane potential constant, the entry of each positive ion (e.g., a Na+ ion) into the cell through a channel in the patch of membrane is balanced by the addition of an electron

► EXPERIMENTAL FIGURE 7-17 Current flow through individual ion channels can be measured by patch-clamping technique. (a) Basic experimental arrangement for measuring current flow through individual ion channels in the plasma membrane of a living cell. The patch electrode, filled with a current-conducting saline solution, is applied, with a slight suction, to the plasma membrane. The 0.5-^m-diameter tip covers a region that contains only one or a few ion channels. The second electrode is inserted through the membrane into the cytosol. A recording device measures current flow only through the channels in the patch of plasma membrane. (b) Photomicrograph of the cell body of a cultured neuron and the tip of a patch pipette touching the cell membrane. (c) Different patch-clamping configurations. Isolated, detached patches are the best configurations for studying the effects on channels of different ion concentrations and solutes such as extracellular hormones and intracellular second messengers (e.g., cAMP). [Part (b) from B. Sakmann, 1992, Neuron 8:613 (Nobel lecture); also published in E. Neher and B. Sakmann, 1992, Sci. Am. 266(3):44. Part (c) adapted from B. Hille, 1992, Ion Channels of Excitable Membranes, 2d ed., Sinauer Associates, p. 89.]

into the cytosol through a microelectrode inserted into the cytosol: an electronic device measures the numbers of electrons (current) required to counterbalance the inflow of ions through the membrane channels. Conversely, the exit of each positive ion from the cell (e.g., a K+ ion) is balanced by the withdrawal of an electron from the cytosol. The patch-clamping technique can be employed on whole cells or isolated membrane patches to measure the effects of different substances and ion concentrations on ion flow (Figure 7-17c).

Intracellular electrode

Device to maintain constant voltage across membrane and to measure current flow across membrane at tip of patch electrode

Ion channels

Intracellular electrode

Patch electrode filled with conducting salt solution

Tip of micropipette

Device to maintain constant voltage across membrane and to measure current flow across membrane at tip of patch electrode

Patch electrode filled with conducting salt solution

Ion channels

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