Pflugers Arch 1999 May 01;4376:895-902.

Localization of sodium and potassium currents at sites of nerve-muscle contact in embryonic Xenopus muscle cells in culture.

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Macro-patch current recordings were used to determine the spatial distribution of INa, inactivating (IIK) and non-inactivating (IK) outward potassium currents on nerve-contacted Xenopus muscle cells grown in culture. Sites of nerve contact were identified under 400x magnification and macroscopic currents recorded from cell-attached membrane patches ranging in diameter from 2 microm to 6 microm. Membrane patch (n=133) recordings revealed no detectable current, or INa and/or IIK, or just IK. More than 85% of the membrane patches from which INa (n=35) could be recorded occurred within 40 microm of the position of nerve contact. In addition, more than 90% of the membrane patches which contained INa also contained IIK. The density of IIK in these membrane patches that also contained INa was significantly higher than the density of IIK or IK in membrane patches without INa. Sites of ACh receptor localization at nerve-muscle contacts, identified by labelling with rhodamine-alpha-bungarotoxin, were also found to contain both INa (4/5 cells) and IIK (5/5 cells). These results suggest that in Xenopus muscle cells in culture the channels mediating IIK along with sodium channels are clustered at sites of nerve contact and synaptic specializations.

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