No voltage change at skeletal muscle SR membrane during Ca2+ release—just Mermaids on acid

Calcium ions control multiple physiological functions by binding to extracellular and intracellular targets. One of the best-studied Ca 2+ -dependent functions is contraction of smooth and striated muscle tissue, which results from Ca 2+ ligation to calmodulin and troponin C, respectively. Ca 2+ signaling typically involves flux of the ion across membranes via specifically gated channel proteins. Because calcium ions are charged, they possess the ability to generate changes in the respective transmembrane voltage. Ca 2+ -dependent voltage alterations of the surface membrane are easily measured using microelectrodes. A well-known example is the characteristic plateau phase of the action potential in cardiac ventricular cells that results from the opening of voltage-gated L-type Ca 2+ channels. Ca 2+ ions are also released from intracellular storage compartments in many cells, but these membranes are not accessible to direct voltage recording with microelectrodes. In muscle, for example, release of Ca 2+ from the sarcoplasmic reticulum (SR) to the myoplasm constitutes a flux that is considerably larger than the entry flux from the extracellular space. Whether this flux is accompanied by a voltage change across the SR membrane is an obvious question of mechanistic importance and has been the subject of many investigations. Because the tiny spaces enclosed by the SR membrane are inaccessible to microelectrodes, alternative methods have to be applied. In a study by Sanchez et al. (2018. J. Gen. Physiol. https://doi.org/10.1085/jgp.201812035) in this issue, modern confocal light microscopy and genetically encoded voltage probes targeted to the SR were applied in a new approach to search for changes in the membrane potential of the SR during Ca 2+ release.