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Activation of Ca²⁺-activated K⁺ channels by an increase in intracellular Ca²⁺ induced by depolarization of mouse skeletal muscle fibres

Vincent Jacquemond and Bruno Allard

1. Ionic currents were simultaneously recorded at macroscopic and unitary level using the whole-cell and cell-attached patch-clamp procedures together on the same portion of isolated mouse skeletal muscle fibres.

2. In the presence of Tyrode solution in the patch pipette and Tyrode-TTX solution in the bath, macroscopic and unitary currents through delayed rectifier K⁺ channels were simultaneously recorded in response to depolarizing pulses of 1 s duration.

3. In five fibres, successive long-lasting incremental depolarizing levels induced, at -40 mV or -30 mV, the opening of a high conductance channel carrying an outward current superimposed on delayed rectifier K⁺ channel activity. Opening of this high conductance channel was not observed when the depolarization steps were applied in the patch pipette.

4. Using the same depolarizing protocol, activation of a high conductance channel was also observed in two fibres in the presence of a K⁺-rich solution in the pipette (145 mM K⁺) .

5. With either Tyrode or K⁺-rich solution in the pipette, unitary current amplitudes of the high conductance channel matched well with the values obtained for Ca²⁺-activated K⁺ (K_Ca) channels in inside-out patches under similar ionic conditions.

6. Indo-1 fluorescence measurements showed that the stimulation protocol that led to K_Ca channel opening induced stepwise increases in intracellular [Ca²⁺] in the submicromolar range.

7. Our results provide evidence that activation of sarcolemmal K_Ca channels can be induced by a rise in intracellular [Ca²⁺] following voltage-activated sarcoplasmic reticulum Ca²⁺ release.

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