Abstract
Cardiac excitation-contraction coupling relies on dyads, the intracellular calcium synapses of cardiac myocytes, where the plasma membrane contacts sarcoplasmic reticulum and where electrical excitation triggers calcium release. The morphology of dyads and dynamics of local calcium release vary substantially. To better understand the correspondence between the structure and the functionality of dyads, we estimated incidences of structurally different dyads and of kinetically different calcium release sites and tested their responsiveness to experimental myocardial injury in left ventricular myocytes of rats. According to the structure of dyads estimated in random electron microscopic images of myocardial tissue, the dyads were sorted into ‘compact’ or ‘loose’ types. The calcium release fluxes, triggered at local calcium release sites in patch-clamped ventricular myocytes and recorded by laser scanning confocal fluorescence microscopy, were decomposed into ‘early’ and ‘late’ components. ANOVA tests revealed very high correlation between the relative amplitudes of early and late calcium release flux components and the relative occurrences of compact and loose dyads in the control and in the injured myocardium. This finding ascertained the relationship between the structure of dyads and the functionality of calcium release sites and the responsiveness of calcium release sites to physical load in cardiac myocytes.
Highlights
Cardiac excitation-contraction coupling relies on dyads, the intracellular calcium synapses of cardiac myocytes, where the plasma membrane contacts sarcoplasmic reticulum and where electrical excitation triggers calcium release
The structural and the functional variability of dyads was analysed in working myocytes of control rat myocardia (CTR) and of injured rat myocardia (IMY)
We used the transmission electron microscopy to collect images of dyads, and the whole-cell patch-clamp combined with laser-scanning fluorescence confocal microscopy to collect records of calcium currents and of calcium spikes
Summary
Cardiac excitation-contraction coupling relies on dyads, the intracellular calcium synapses of cardiac myocytes, where the plasma membrane contacts sarcoplasmic reticulum and where electrical excitation triggers calcium release. Typical images show dyads sectioned randomly, since they do not have a preferential spatial orientation In such sections, the structural appearance of dyads varies substantially, retaining the key morphological determinants, namely the profile of the t-tubule, of the terminal cisterna, and of RyRs in the junctional gap. Disruption of t-tubule architecture in diseased cells of failing human heart was associated with a slight loss of RyR clusters[12] and reduced co-localization between RyRs and DHPRs27 All these facts endorse the opinion that properties of dyads are central to the quality of cardiac excitation-contraction coupling; the relationship between their structure and function is not well understood since simultaneous observation of the structure and the function of a dyad is not experimentally feasible. We expected to observe differences in the dyadic structure that would correlate with differences in characteristics of calcium release of the two myocardial groups
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