Figure 5.

The pH sensitivity of native motoneuronal currents, together with the isoflurane sensitivity of ruthenium red-resistant currents, indicates a contribution from TASK channel heterodimers. Whole-cell voltage-clamp recordings were obtained at -60 mV from hypoglossal motoneurons in brainstem slices from rat pups to determine effects of bath pH and a discriminating concentration of isoflurane (0.8 mm) on membrane current. A, Relative to control conditions, pH 7.3, bath acidification to pH 5.9 decreased motoneuronal outward current, whereas bath alkalization to pH 8.4 increased outward current, with the difference representing the total pH-sensitive current. Isoflurane caused a robust increase in outward current typical for effects on TASK-3 or TASK channel heterodimers. B, After defining the pH-sensitive current, ruthenium red (10 μm) was applied to block TASK-3 channels; application of ruthenium red invariably increased outward current, suggesting effects on multiple conductances. In the continuous presence of ruthenium red, 0.8 mm isoflurane caused a large increase in membrane current. C, The averaged isoflurane-sensitive current (±SEM) in the absence (control, filled circles; n = 5) and presence (shaded circles; n = 8) of ruthenium red was plotted against membrane potential and was well fitted by using the Goldman-Hodgkin-Katz current equation indicating open-rectifier K⁺ conductance. Inset, The isoflurane-sensitive conductance was normalized to the total pH-sensitive conductance in each cell and averaged (±SEM) for control conditions and in the presence of 10 μm ruthenium red. ^*p < 0.05, t test.