Imaging Microdomain Ca 2+ in Muscle Cells

Abstract

Ca²⁺ ions passing through a single or a cluster of Ca²⁺-permeable channels create microscopic, short-lived Ca²⁺ gradients that constitute the building blocks of cellular Ca²⁺ signaling. Over the last decade, imaging microdomain Ca²⁺ in muscle cells has unveiled the exquisite spatial and temporal architecture of intracellular Ca²⁺ dynamics and has reshaped our understanding of Ca²⁺ signaling mechanisms. Major advances include the visualization of “Ca²⁺ sparks” as the elementary events of Ca²⁺ release from the sarcoplasmic reticulum (SR), “Ca²⁺ sparklets” produced by openings of single Ca²⁺-permeable channels, miniature Ca²⁺ transients in single mitochondria (“marks”), and SR luminal Ca²⁺ depletion transients (“scraps”). As a model system, a cardiac myocyte contains a 3-dimensional grid of 10⁴ spark ignition sites, stochastic activation of which summates into global Ca²⁺ transients. Tracking intermolecular coupling between single L-type Ca²⁺ channels and Ca²⁺ sparks has provided direct evidence validating the local control theory of Ca²⁺-induced Ca²⁺ release in the heart. In vascular smooth muscle myocytes, Ca²⁺ can paradoxically signal both vessel constriction (by global Ca²⁺ transients) and relaxation (by subsurface Ca²⁺ sparks). These findings shed new light on the origin of Ca²⁺ signaling efficiency, specificity, and versatility. In addition, microdomain Ca²⁺ imaging offers a novel modality that complements electrophysiological approaches in characterizing Ca²⁺ channels in intact cells.