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

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+ (KCa) channels in inside-out patches under similar ionic conditions. 6. Indo-1 fluorescence measurements showed that the stimulation protocol that led to KCa channel opening induced stepwise increases in intracellular [Ca2+] in the submicromolar range. 7. Our results provide evidence that activation of sarcolemmal KCa channels can be induced by a rise in intracellular [Ca2+] following voltage-activated sarcoplasmic reticulum Ca2+ release.