Regulation of Myoplasmic Ca2+ During Fatigue in KATP Channel Deficient FDB Muscle Fibres

Abstract

It is known that muscles that lack KATP channel activity generate much greater unstimulated [Ca2+]i and force than normal muscles during fatigue. The increase in unstimulated force in KATP channel deficient muscles is abolished by a partial inhibition of L-type Ca2+ channels, suggesting that it is due to a Ca2+ influx through L-type Ca2+ channels and a subsequent increased myoplasmic Ca2+. However, there is also evidence that the increase in resting force is abolished by NAC, a ROS scavenger. The objective of this study was to reconcile these observations by studying the hypothesis that “the increase in resting [Ca2+]i during fatigue in KATP channel deficient muscles starts with an excess Ca2+ influx through L-type Ca2+ channels, followed by an excess ROS production that causes a further increase in resting [Ca2+]i”. To test the hypothesis, single FDB fibres were fatigued with one tetanic contraction/sec for 180 sec. KATP channel deficient fibres were obtained i) by exposing wild type muscle fibers to glibenclamide, a KATP channel blocker and ii) by using fibres from Kir6.2-/- mice, which are null mice for the Kir6.2 gene that encodes for the protein forming the channel pore. Verapamil, a L-type Ca2+ channel blocker, applied at 1 μM, significantly reduced resting [Ca2+]i during fatigue in glibenclamide-exposed wild type fibres. NAC (1 mM) also reduced resting [Ca2+]i in glibenclamide-exposed muscles. The results suggest that the increase in resting [Ca2+]i during fatigue in KATP channel deficient FDB fibres is due to an influx through L-type Ca2+ channels, and an excess ROS production.