Figure 1.
Isolated cardiomyocyte geometry with in situ mitochondria and sarcomere structures.
(A) Schematic diagram of the coordinate system (X-Y-Z axes) of isolated myocytes and in situ mitochondria. (B) Brightfield image of cardiac myocyte showing orientation of laser line-scan imaging of in situ mitochondria along the orthogonal (C) length-x and (D) width-y axes of the cell. The cell structure, consisting of sarcomeres (∼1.9 µm structures between dark z-lines) and mitochondria (∼0.95 µm structures) is evident.
Figure 2.
Fourier transform spectra obtained from linescan imaging to determine sarcomere and mitochondrial dimensions.
(A) Transmitted optics line-scan image along cardiomyocyte long-axis (length-x) during rest and electrical stimulation (1 Hz). (B) Sarcomere length and mitochondrial length and width peaks during quiescent state determined from frequency domain analysis.
Figure 3.
Cardiomyocyte structural determinants of principal Fourier transform spectral peaks: assignment and validation.
Consecutive Fourier analysis along length (A) and width (B) directions calculated from bright field and fluorescence images of tetramethylrhodamine methyl ester (TMRM) loaded cardiomyocytes. Low concentrations of diazoxide (Dz; 30 µM) did not shift the sarcomere peak (C), but shifted the frequency of the mitochondrial peak (D) along the length-axis and (E) width-axis to the left by ∼1%.
Figure 4.
Deformation of mitochondria in response to sarcomere contraction.
(A) Sarcomere length and (B) mitochondrial length calculated from frequency domain analysis along length-x axis during electrical stimulation (1 Hz). (C) Mitochondrial width calculated from frequency domain analysis along width-y axis during electrical stimulation (1 Hz).
Table 1.
Sarcomere and mitochondria deformation parameters.
Figure 5.
Sarcomere-mitochondrial length and mitochondrial length-width relationships during cardiomyocyte contraction-relaxation cycle.
The relationship between mitochondrial length and (A) sarcomere length or (B) mitochondrial width. c is the coefficient of anisotropy (ratio of deformation magnitude between the width-y and thick-z axes) and D is the average mitochondrial diameter. (C) Mitochondrial length and width vs. time during a single contraction cycle. (D) Change in mitochondrial width and calculated thickness vs. time during a single contraction cycle. Data shown is from a representative single cardiomyocyte experiment.
Figure 6.
Cell length-width relationship during cardiomyocyte contraction-relaxation cycle.
(A) The percent change in the cell-length and cell-width vs. time during a single contraction cycle. (B) The measured relationship between percent change in cell-length vs. percent change in cell-width. c is the coefficient of anisotropy (ratio of deformation magnitude between the width-y and thick-z axes). Data shown is from a representative single cardiomyocyte experiment.
Figure 7.
Scheme of mitochondrial 3D-deformation and proposed cytoskeleton mechanical analog.
(A) Asymmetric radial expansion of mitochondria when compressed longitudinally by the sarcomeres during active contraction. While the mitochondria compress in the length-axis (x) they expand in the width-axis (y); however, the expansion in the thick-axis (z) is quite distinct from the width expansion. (B) An “equivalent spring” model of the cardiomyocyte cytoskeleton relevant to sarcomere lengths achieved during contraction, and capable of storing contractile energy during active cell shortening, then releasing this potential energy during relaxation as external work performed to relengthen the cell. The cytoskeleton is apparently stiffer in the cardiomyocyte thick-axis (shown in bold) compared to the width-axis. (C) Based on a simple analog of Hookean springs, during cardiomyocyte contraction the elastic components of the cytoskeleton store more potential energy in the cell's width-axis compared to the thick-axis (see text for details).