Cardiomyocyte sarcomere length variability: Membrane fluorescence versus second harmonic generation myosin imaging

Cardiomyocyte sarcomere length (SL) variability assessed by second-harmonic generation (SHG) was significantly less than that by ANEPPS confocal fluorescence. We conclude that SHG-derived SL variability may reflect the true relaxed myocyte sarcomeric ultrastructure. Sarcomere length (SL) and its variation along the myofibril strongly regulate integrated coordinated myocyte contraction. It is therefore important to obtain individual SL properties. Optical imaging by confocal fluorescence (for example, using ANEPPS) or transmitted light microscopy is often used for this purpose. However, this allows for the visualization of structures related to Z-disks only. In contrast, second-harmonic generation (SHG) microscopy visualizes A-band sarcomeric structures directly. Here, we compared averaged SL and its variability in isolated relaxed rat cardiomyocytes by imaging with ANEPPS and SHG. We found that SL variability, evaluated by several absolute and relative measures, is two times smaller using SHG vs. ANEPPS, while both optical methods give the same average (median) SL. We conclude that optical methods with similar optical spatial resolution provide valid estimations of average SL, but the use of SHG microscopy for visualization of sarcomeric A-bands may be the gold standard for evaluation of SL variability due to the absence of optical interference between the sarcomere center and non-sarcomeric structures. This contrasts with sarcomere edges where t-tubules may not consistently colocalize to Z-disks. The use of SHG microscopy instead of fluorescent imaging can be a prospective tool to map sarcomere variability both in vitro and in vivo conditions and to reveal its role in the functional behavior of living myocardium.

Automated summary

Cardiomyocyte sarcomere length variability can be accurately assessed using second-harmonic generation microscopy, which provides a direct visualization of A-band sarcomeric structures. This method allows for a more precise evaluation of sarcomere length variability compared to confocal fluorescence microscopy. It has the potential to be a valuable tool for studying sarcomere variability in both in vitro and in vivo conditions, and its role in the functional behavior of living myocardium.