Subcellular Calcium (Ca2+) Redistribution During Cardiac Muscle Contraction by Electron Probe Microanalysis (EPMA)

Abstract

Substantial evidence suggests that the increase in cytosolic Ca2+which occurs upon stimulation of cardiac muscle originates from the sarcoplasmic reticulum (SR) and that its release is triggered by a transient rise in cytosolic Ca2+. The junctional SR has been proposed as a Ca2+release site, based on a number of observations, e.g. measurements by EPMA have demonstrated significant Ca2+stores in the junctional SR in resting cardiac muscle; the ryanodine-sensitive Ca2+release channels are associated with junctional SR membrane and the low affinity Ca2+binding protein, calsequestrin, is localized to the junctional SR. Although considerable evidence implicates the junctional SR as an intracellular Ca2+store in cardiac muscle,releaseof Ca2+from the junctional SR, upon muscle stimulation, hasnotbeen directly demonstrated. We therefore propose to use EPMA to directly measure the Ca2+content of junctional SR in both relaxed and contracted cardiac muscle to determine whether Ca2+is released from the junctional SR during contraction.