The ionic dependence of voltage-activated inward currents in the pharyngeal muscle of Caenorhabditis elegans.

The pharynx of Caenorhabditis elegans consists of a syncytium of radially orientated muscle cells that contract synchronously and rhythmically to ingest and crush bacteria and pump them into the intestine of the animal. The action potentials that support this activity are superficially similar to vertebrate cardiac action potentials in appearance with a long, calcium-dependent plateau phase. Although the pharyngeal muscle can generate action potentials in the absence of external calcium ions, action potentials are absent when sodium is removed from the extracellullar solution (Franks et al. 2002). Here we have used whole cell patch clamp recordings from the pharynx and show low voltage-activated inward currents that are present in zero external calcium and reduced in zero external sodium ions. Whilst the lack of effect of zero calcium when sodium ions are present is not surprising in view of the known permeability of voltage-gated calcium channels to sodium ions, the reduction in current in zero sodium when calcium ions are present is harder to explain in terms of a conventional voltage-gated calcium channel. Inward currents were also recorded from egl-19 (n582) which has a loss of function mutation in the pharyngeal L-type calcium channel and these were also markedly reduced in zero external sodium. Despite this apparent dependence on external sodium ions, the current was partially blocked by the divalent cations, cadmium, barium and nickel. Using single-channel recordings we identified a cation channel for which the open-time duration was increased by depolarisation. In inside-out patches, the single-channel conductance was highest in symmetrical sodium solution. Further studies are required to determine the contribution of these channels to the pharyngeal action potential.