Patch voltage clamping with low-resistance seals: Loose patch clamp

This chapter discusses several loose-seal methods, and emphasizes the possible difficulties associated with each, describing how they can be avoided. The chapter also discusses applications in which the potential across the membrane is controlled. The basic loose-patch clamp, which employs a single loosely sealed extracellular pipette and no intracellular electrodes, is appropriate for studying rapidly activating currents, such as voltage-gated Na⁺ and K⁺ currents, in muscles and other large cells with stable resting potentials. This technique is not well suited to studying currents that are much smaller or slower, such as the voltage-gated Ca²⁺ current in skeletal muscles, because of artifacts associated with the low resistances of the loose seals. A combination of loose-seal patch recordings, tight-seal whole-cell recordings, and freeze-fracture electron micrographs indicate that the array of membrane particles seen at presynaptic active zones on hair cells are dusters of ion channels that contain a carefully regulated mixture of voltage-gated Ca²⁺ channels and Ca²⁺-activated K⁺ channels. Future uses of the loose-seal technique are likely to address a wider range of problems, such as local mechanisms of channel modulation that require preservation of the microscopic structure of the membrane or its spatial relationship to the cytoskeleton.