Imaging Microdomain Ca 2+ in Muscle Cells

Abstract

Ca 2+ ions passing through a single or a cluster of Ca 2+ -permeable channels create microscopic, short-lived Ca 2+ gradients that constitute the building blocks of cellular Ca 2+ signaling. Over the last decade, imaging microdomain Ca 2+ in muscle cells has unveiled the exquisite spatial and temporal architecture of intracellular Ca 2+ dynamics and has reshaped our understanding of Ca 2+ signaling mechanisms. Major advances include the visualization of “Ca 2+ sparks” as the elementary events of Ca 2+ release from the sarcoplasmic reticulum (SR), “Ca 2+ sparklets” produced by openings of single Ca 2+ -permeable channels, miniature Ca 2+ transients in single mitochondria (“marks”), and SR luminal Ca 2+ depletion transients (“scraps”). As a model system, a cardiac myocyte contains a 3-dimensional grid of 10 4 spark ignition sites, stochastic activation of which summates into global Ca 2+ transients. Tracking intermolecular coupling between single L-type Ca 2+ channels and Ca 2+ sparks has provided direct evidence validating the local control theory of Ca 2+ -induced Ca 2+ release in the heart. In vascular smooth muscle myocytes, Ca 2+ can paradoxically signal both vessel constriction (by global Ca 2+ transients) and relaxation (by subsurface Ca 2+ sparks). These findings shed new light on the origin of Ca 2+ signaling efficiency, specificity, and versatility. In addition, microdomain Ca 2+ imaging offers a novel modality that complements electrophysiological approaches in characterizing Ca 2+ channels in intact cells.