Key points: Regulation of ion channel function during repeated firing of action potentials is commonly observed in excitable cells. Recently it was shown that muscle activity is associated with rapid, protein kinase C (PKC)-dependent ClC-1 Cl− channel inhibition in rodent muscle. While this PKC-dependent ClC-1 inhibition during muscle activity was shown to be important for the maintenance of contractile endurance in rat muscle it is unknown whether a similar regulation exists in human muscle. Also, the molecular mechanisms underlying the observed PKC-dependent ClC-1 inhibition are unclear. Here we present the first demonstration of ClC-1 inhibition in active human muscle fibres, and we determine the changes in ClC-1 gating that underlie the PKC-dependent ClC-1 inhibition in active muscle using human ClC-1 expressed in Xenopus oocytes. This activity-induced ClC-1 inhibition is suggested to represent a mechanism by which human muscle fibres maintain their excitability during sustained activity. Abstract: Repeated firing of action potentials (APs) is known to trigger rapid, protein kinase C (PKC)-dependent inhibition of ClC-1 Cl− ion channels in rodent muscle and this inhibition is important for contractile endurance. It is currently unknown whether similar regulation exists in human muscle, and the molecular mechanisms underlying PKC-dependent ClC-1 inhibition are unclear. This study first determined whether PKC-dependent ClC-1 inhibition exists in active human muscle, and second, it clarified how PKC alters the gating of human ClC-1 expressed in Xenopus oocytes. In human abdominal and intercostal muscles, repeated AP firing was associated with 30–60% reduction of ClC-1 function, which could be completely prevented by PKC inhibition (1 μm GF109203X). The role of the PKC-dependent ClC-1 inhibition was evaluated from rheobase currents before and after firing 1000 APs: while rheobase current was well maintained after activity under control conditions it rose dramatically if PKC-dependent ClC-1 inhibition had been prevented with the inhibitor. This demonstrates that the ClC-1 inhibition is important for maintenance of excitability in active human muscle fibres. Oocyte experiments showed that PKC activation lowered the overall open probability of ClC-1 in the voltage range relevant for AP initiation in muscle fibres. More detailed analysis of this reduction showed that PKC mostly affected the slow gate of ClC-1. Indeed, there was no effect of PKC activation in C277S mutated ClC-1 in which the slow gate is effectively locked open. It is concluded that regulation of excitability of active human muscle fibres relies on PKC-dependent ClC-1 inhibition via a gating mechanism.
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