Dyadic Remodeling as a Cause of Impaired E-C Coupling in Early Stages of Myocardial Injury

Abstract

In failing myocytes, impaired E-C coupling is often associated with loss of t-tubules and appearance of orphaned calcium release units. Impairment of communication between Ca-channels of sarcolemma and of sarcoplasmic reticulum (SR) has been observed in non-failing stages of cardiac hypertrophy. We therefore studied the relationship between Ca-signaling and ultrastructure of dyadic junctions in early stages of myocardial injury. Myocardial injury (IMI) was induced by a single dose of isoproterenol (150 mg/kg, i.p.) in adult male rats. On day 15, isolated myocytes were studied using laser scanning fluorescence confocal microscopy: Ca-spikes were activated by whole-cell patch-clamp and t-tubules were visualized by di-8-ANEPPS. Ultrastructure of dyadic junctions was evaluated from electron-microscopic images. Our results show that remodeling of IMI myocytes transpired as increased proportion of longitudinal elements of tubular system as well as increased proportion of dyads with defective contact between the membranes of sarcolemma and SR; however, there was no actual loss of t-tubules and ultrastructure of t-tubules in dyads of IMI myocytes was similar to control (CTR). In parallel, Ca-spike activation was slower and less synchronized in IMI. In both CTR and IMI, distribution of latencies of Ca-spikes and the time profile of Ca-release flux measured from whole cells consisted of two components. The fraction of the slow component of calcium release in IMI was close to the fraction of dyads with defective contact between the membranes of sarcolemma and SR. We conclude that overload of cardiac myocytes of the working part of injured myocardium leads to damage of tubulo-reticular junctions of dyadic microdomains, which results in impairment of local calcium signaling. Support: APVV-0721-10, VEGA 2/047/14 and VEGA 2/0095/15