The ERG1a K+ Channel Increases Intracellular Calcium and Calpain Activity in C2C12 Myotubes

Abstract

In skeletal muscle increased intracellular calcium concentration ([Ca2+]i; uM range) is necessary for excitation‐contraction coupling; however, smaller increases in [Ca2+]i (nM range) can modulate other physiological processes in non‐contracting muscle. Indeed, fluctuations in localized calcium concentration can serve as a second messenger. In contrast, inappropriate changes in [Ca2+]i can have detrimental effects on muscle tissue and are associated with numerous skeletal muscle pathologies, for example muscular dystrophy, amyotrophic lateral sclerosis, malignant hyperthermia, cancer cachexia, and atrophy. Therefore, [Ca2+]i must be tightly regulated in terms of time, space and amplitude for cellular processes to occur in a properly coordinated fashion. In earlier reports, we showed that the ERG1a K+ channel is upregulated in atrophying skeletal muscle and contributes to increased ubiquitin proteasome proteolysis. Here, using the ratiometric Ca2+ indicator fura‐2AM, we show that ERG1a expression in C2C12 myotubes produces an increase in basal [Ca2+]i as well as a transient increase in [Ca2+]i as a consequence of depolarization. We explored this transient increase in [Ca2+]i using pharmacological agents. The data demonstrate that the ERG1a‐induced increase is not sensitive to the L‐type calcium channel blocker nifedipine, suggesting that it does not result from modulation of Cav1.1 channels. To further support this data, immunoblots reveal that there is no change in Cav1.1 channel abundance. However, the data do demonstrate that the increase in [Ca2+]i is sensitive to the SERCA blocking agent thapsigargin, suggesting that the source of the calcium is the sarcoplasmic reticulum stores. Additionally, the data reveal that ERG1a expression also increases basal calpain activity. In summary, to date the data show that ERG1a increases [Ca2+]i levels and suggest that this increase could occur, at least in part, as a result of release of calcium from sarcoplasmic reticulum stores. Additionally, the data reveal that ERG1a expression also increases calpain activity, suggesting that the increase in intracellular calcium results in increased calpain activity and possibly contributes to the increased proteolysis that occurs in atrophic skeletal muscle.Support or Funding InformationU.S. Department of Defense, PRMRP Grant# PR170326This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.