Intrasarcomere [Ca2+] Gradients in Ventricular Myocytes Revealed by High Speed Digital Imaging Microscopy

• Published in: Proceedings of the National Academy of Sciences - PNAS 1996-05, Vol.93 (11), p.5413-5418
• Main Authors: Isenberg, Gerrit, Etter, Elaine F., Wendt-Gallitelli, Maria-Flora, Schiefer, Alfred, Carrington, Walter A., Tuft, Richard A., Fay, Fredric S.
• Format: Article
• Language: English
• Subjects: Animals; Biochemistry; Calcium; Calcium - metabolism; Cell lines; Cellular biology; Depolarization; Electron Probe Microanalysis - instrumentation; Electron Probe Microanalysis - methods; Fluorescence; Fluorescent Dyes; Guinea Pigs; Heart; Heart Ventricles; Imaging; In Vitro Techniques; Kinetics; Models, Structural; Myocardium; Myocardium - metabolism; Myocardium - ultrastructure; Parallel lines; Sarcomeres; Sarcomeres - metabolism; Sarcomeres - ultrastructure; Sarcoplasmic Reticulum - metabolism; Sarcoplasmic Reticulum - ultrastructure; Three dimensional imaging; Time Factors; Wavelengths
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.93.11.5413

Cardiac muscle contraction is triggered by a small and brief Ca2+ entry across the t-tubular membranes, which is believed to be locally amplified by release of Ca2+ from the adjacent junctional sarcoplasmic reticulum (SR). As Ca2+ diffusion is thought to be markedly attenuated in cells, it has been predicted that significant intrasarcomeric [Ca2+] gradients should exist during activation. To directly test for this, we measured [Ca2+] distribution in single cardiac myocytes using fluorescent [Ca2+] indicators and high speed, three-dimensional digital imaging microscopy and image deconvolution techniques. Steep cytosolic [Ca2+] gradients from the t-tubule region to the center of the sarcomere developed during the first 15 ms of systole. The steepness of these [Ca2+] gradients varied with treatments that altered Ca2+ release from internal stores. Electron probe microanalysis revealed a loss of Ca2+ from the junctional SR and an accumulation, principally in the A-band during activation. We propose that the prolonged existence of [Ca2+] gradients within the sarcomere reflects the relatively long period of Ca2+ release from the SR, the localization of Ca2+ binding sites and Ca2+ sinks remote from sites of release, and diffusion limitations within the sarcomere. The large [Ca2+] transient near the t-tubular/junctional SR membranes is postulated to explain numerous features of excitation-contraction coupling in cardiac muscle.