Spatiotemporally Non-Uniform Ca2+ Dynamics of Cardiac Purkinje Fibers in Mouse Myocardial Infarct

Surviving Purkinje fibers in myocardial infarct are regarded as an important substrate in arrhythmogenesis. However, poorly understood are functional properties of Purkinje fibers in the infarcted heart. We sought to visualize intracellular Ca²⁺ ([Ca²⁺]_i) dynamics of Purkinje fiber networks in the mouse myocardial infarct. Using 3- to 4-day-old or 7- to 9-day-old infarcted hearts after the left coronary-artery ligation corresponding, respectively, to acute or healing phase, we conducted rapid fluo4-fluorescence imaging on the endocardial surface of the left ventricular septum by macro-zoom fluorescence microscopy and rapid-scanning confocal microscopy. In contrast with the intact heart, where uniform Ca²⁺ transients propagated rapidly, the infarcted heart exhibited slow, non-uniform impulse propagations. On confocal microscopy, Purkinje fibers in the peri-infarct zone exhibited non-uniform [Ca²⁺]_i dynamics: beat-to-beat alternans of the Ca²⁺ transient amplitude in and among the individual fibers, whereas the intact fibers exhibited uniform Ca²⁺ transients. Such non-uniform [Ca²⁺]_i dynamics were more conspicuous in the acute infarcted hearts than in the healing ones. In accordance with [Ca²⁺]_i dynamics, fixed fluo4-loaded heart preparations exhibited definitive connexin-40 plaques in the peri-infarct Purkinje fibers, whereas the subjacent myocardium presented coagulative necrosis and granulation tissues, respectively. The surviving Purkinje fibers in the peri-infarct zone exhibited non-uniform [Ca²⁺]_i dynamics, which may lead to arrhythmogenesis.

Fast measurement of sarcomere length and cell orientation in Langendorff-perfused hearts using remote focusing microscopy

RATIONALE

Sarcomere length (SL) is a key indicator of cardiac mechanical function, but current imaging technologies are limited in their ability to unambiguously measure and characterize SL at the cell level in intact, living tissue.

OBJECTIVE

We developed a method for measuring SL and regional cell orientation using remote focusing microscopy, an emerging imaging modality that can capture light from arbitrary oblique planes within a sample.

METHODS AND RESULTS

We present a protocol that unambiguously and quickly determines cell orientation from user-selected areas in a field of view by imaging 2 oblique planes that share a common major axis with the cell. We demonstrate the effectiveness of the technique in establishing single-cell SL in Langendorff-perfused hearts loaded with the membrane dye di-4-ANEPPS.

CONCLUSIONS

Remote focusing microscopy can measure cell orientation in complex 2-photon data sets without capturing full z stacks. The technique allows rapid assessment of SL in healthy and diseased heart experimental preparations.

Arrhythmogenic substrates in myocardial infarct

Life-threatening ventricular tachyarrhythmias are common clinical complications in ischemic heart diseases, especially infarcted heart. Although electrophysiological mechanisms have been extensively clarified for the genesis of arrhythmias in myocardial infarct, arrhythmogenic substrates in the infarct that eventually lead to electrical derangements are not fully understood. This review focuses on the intracellular calcium ion (Ca2+) dynamics and connexin43 (Cx43) gap junctions that play pivotal roles in excitation/contraction processes and intercellular communication, respectively, in heart muscle cells. Recent development of Ca2+-sensitive fluorescent dyes as well as microscopy imaging techniques has contributed substantially to a more precise understanding of spatiotemporal aspects in the intra- and inter-cellular dynamics of Ca2+ in cardiomyocytes. Ca2+ waves, heterogeneous wave-like elevations of the intracellular Ca2+ concentrations ([Ca2+](i)) that develop under [Ca2+](i)-overloaded conditions of the injured myocardium, play an essential role in arrhythmias, especially in triggered arrhythmias. Alteration of Cx43-mediated electrical coupling, that is, gap junction remodeling that arises at myocyte-myocyte and myocyte-myofibroblast interfaces, would also be an important substrate for arrhythmias, especially re-entrant tachyarrhythmias. Clarification of these substrates would provide not only deeper insights into the upstream events of life-threatening tachyarrhythmias in the infarcted heart but also bases for new therapeutic strategies for cardiovascular diseases.

Recent advances in fluorescent labeling techniques for fluorescence microscopy

Tremendous progress in recent computer-controlled systems for fluorescence and laser-confocal microscopy has provided us with powerful tools to visualize and analyze molecular events in the cells. Various fluorescent staining and labeling techniques have also been developed to be used with these powerful instruments. Fluorescent proteins such as green fluorescent protein (GFP) allow us to directly label particular proteins of interest in living cells. This technique has been extended over a large area of cell biology, and a variety of fluorescent protein-derived techniques have been developed to visualize the functions and conditions of the molecules within living cells. In this review, we summarize the techniques for fluorescent staining and labeling for recent fluorescence microscopy.