The effect of cellular energy reserves and internal calcium ions on the potassium conductance in skeletal muscle of the frog.

The increase in K+ conductance induced by repetitive stimulation in metabolically poisoned sartorius muscle fibres of the frog was investigated, using a two-micro-electrode voltage-clamp technique. After the inhibition of creatine kinase by 0.4 mM-1-fluoro-2,4-dinitrobenzene (FDNB) and a complete and irreversible exhaustion of contractility, a nearly linear current-voltage relation was measured between -100 and 0 mV. In the presence of CN- (4 mM) an 'intermediate state' could be established by repetitive stimulation towards complete mechanical exhaustion. In this labile state, the high and potential-independent K+ conductance could be induced by repetitive voltage-clamp pulses (100 ms duration) from -85 to 0 mV membrane potential. After the pulses had ceased, fibres regained their original membrane conductance within several minutes. After the electrophoretic injection of the Ca2+-chelating agent H2EGTA2- into fibres in the intermediate state, an increase in membrane conductance by repetitive voltage-clamp pulses could no longer be induced. Fibres in the intermediate state into which H2EGTA2- -buffered Ca2+ (free Ca2+ approximately 10(-5) M) was injected, or to which external caffeine (1.5 mM) was applied, showed a spontaneous and reversible increase in membrane conductance. In metabolically poisoned and mechanically exhausted sartorius muscles the concentrations of creatine phosphate (CP) and ATP were estimated using biochemical standard methods. The concentration of CP remained basically unchanged after FDNB poisoning. In solutions containing CN- plus iodoacetate CP fell below the detectable concentration of about 0.5-1% of the normal value. ATP decreased to slightly less than 20% under both conditions. It is concluded that internal free Ca2+ promotes the activation of the K+ conductance in exhausted muscle fibres, and that a shortage of energy reserves increases the 'sensitivity' of K+ channels to Ca2+ ions.