EDS of thin Biological Specimen in the Study of Time-Dependent Physiological Processes

The contraction of muscle depends on the ability of intracellular organelles to rapidly release and take up large amounts of activator calcium. In cardiac muscle the intracellular sodium concentration is known to also modulate this Ca2+ release: Na+-Ca2+ exchange creates narrow cytosolic microdomains just internal to the sarcolemma (approximately 20 nm wide) with locally elevated concentrations of Ca2+ and Na+. The existence of such “functional” compartments with transient high ionic concentrations has been postulated for explaining the nigh efficiency of signal transmission during excitation-contraction coupling. We present results showing that electron probe microanalysis (EPMA) at the high spatial resolution of scanning transmission electron microscopy (STEM) can detect changes in sodium and calcium concentration of membrane-limited organelles and functional microdomains provided the cardiac myocytes are rapidly frozen at defined times during activation of contraction.