Evidence in Favor of a Novel Excitation‐Contraction Calcium Release Mechanism in Skeletal Muscle L6 Cells

Abstract

Our laboratory has provided evidence for the first half of a model to explain both store‐operated or capacitative Ca2+ entry (CCE) as well as excitation‐contraction coupled Ca2+ release (ECC) in skeletal muscle. Here we offer evidence in favor of a model that proposes ECC acts by a relative displacement of the sarcolemmal L‐channel (DHPR) with respect to the sarcoplasmic reticulum‐localized Ryanodine Receptor (RyR). We titrated cells with ryanodine (Ry) which is known to both active (low concentrations) and inhibit (high concentrations) the RyR. We then repeated this titration after two types of treatments: conditions to depolarize the sarcolemma; and alterations in cytosolic Ca2+ levels under non‐depolarizing conditions. Depolarizing conditions, including high extracellular [K+] and field depolarization with solution electrodes, caused a shift in the Ry titration such that lower concentrations elicited both stimulation and inhibition. Addition of the DHPR agonist BAY K‐8644 caused a similar shift, although of smaller magnitude. On the other hand, altering intracellular Ca2+ through direct activation of RyR with the caffeine‐like xx did not cause a shift in Ry sensitivity. Similarly, alterations in exogenous Ca2+ concentration did not change Ry sensitivity. We suggest that these findings are consistent with our “relative displacement” hypothesis for ECC, whereby excitation moves the DHPR to unoccupied RyR, releasing Ca2+ into the cytosol from previously occluded RyR channels.