The role of extracellular matrix in modulation of myocyte Ca2+ signalling and contraction in the isolated rat heart
Traditionally the extracellular matrix (ECM) in the heart has been thought to provide mechanical scaffolding for the contracting myocytes. However, it was observed that the amplitude of the intracellular Ca2+ transients that underlie contraction was reduced in isolated myocytes in comparison to Ca2+ transients from multicellular trabeculae. The role of ECM in myocyte Ca2+ modulation is poorly understood. We therefore hypothesised that Ca2+ handling might be modulated by (bidirectional) signalling between myocytes and the ECM, and that this important link could be lost during the standard cell isolation by enzymatic process. The aim of this thesis was to investigate the contribution of ECM in myocyte Ca2+ handling in cardiac trabeculae. To achieve this, measurements of isometric force and intracellular Ca2+ (340/380 ratio) in electrically stimulated rat cardiac trabeculae before and after enzyme exposure that dismantle the linkages between the ECM and the myocytes was continuously measured at room temperature. Immuno-histochemical techniques were used to determine the extent of loss of key proteins linking the ECM and the myocytes during this dismantling. Experiments revealed that partial enzymatic digestion increased the resting Ca2+ and stress in trabeculae. Furthermore, this study showed that there is decreased RECA-1 labelling in partial digested trabeculae and its complete absence in single myocytes but without any loss in laminin, Cxn-43, and vinculin labelling. In addition, the evaluation of ET-1 effect show neither increase in stress nor in [Ca2+]i in partial dismantled trabeculae. Additionally, the ET-1 application in isolated single myocytes significantly increase the amplitude of [Ca2+]i. Together, these results suggest the contribution of ECM components especially endothelial cells to myocyte Ca2+ modulation. Next, this study also looked at the evidence of presence of primary cilia in adult rat hearts and found no evidence of primary cilia in either ventricular tissue sections or isolated myocytes from the adult rat hearts. The effect of novel EPAC signalling pathway which contributes to the EC-coupling and Ca2+ homeostasis was also investigated by activating with cpTOME (an EPAC agonist) in intact trabeculae. It was depicted that cpTOME do not affect trabeculae until the experimental conditions of trabeculae are matched with isolated myocytes. EPAC activation increase the myofilament Ca2+ sensitivity when Ca2+ currents were lowered by reducing the [Ca2+] to 0.5 mM which was removed by CamKII inhibition. This work indicates that EPAC contributes to Ca2+ signalling in a CamKII-dependent fashion in the cardiac tissue preparations.
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