Partial mechanical unloading of the healthy left ventricle has been demonstrated to reduce the regularity of t-tubular structures, increase the frequency of calcium sparks and disturb calcium homeostasis in cardiomyocytes (CMs). Mechanical unloading of failing myocardium has an opposing effect. The control of molecular functions of βAR and L-type calcium channel (LTCC) within highly localized membrane compartments, was investigated utilizing state of the art imaging techniques in cells following different scenarios of failure or unloading.
CMs were isolated from control (CTRL), control-unloading (UNL) (8-weeks), failing (HF) 20 week post-MI) and failing+unloaded ((HF+UNL) 12 week post-MI + 8 weeks unloaded) rat models. βAR stimulation in specific membrane compartments was studied within CMs using a PKA-RII based FRET sensor. Localized β2AR function was studied with combination scanning ion conductance microscopy (SICM)-FRET using an Epac2 based sensor. A smart-patch technique based upon SICM, allowed specific patching of T-tubular or non-T-tubular regions to investigate localized LTCC physiology.
CMs β1AR-cAMP responses were slightly suppressed in PKA-RII regions following UNL or HF (see table below), but were significantly suppressed following HF+UNL. β2AR-cAMP responses were slightly suppressed in UNL cells in comparison to controls. Responses were enhanced in HF and slightly reduced in comparison to HF+UNL. β2AR activity was suppressed only in T-tubule regions of unloaded CMs but enhanced in failing HF and HF+UNL in both T-tubule and crest regions. LTCC activity was suppressed following UNL. Interestingly, this was not because of the loss of channels, as forcing the channels to open using the agent BAY-K8644 demonstrated robust calcium channel functions in cellular regions.
The positive effect of mechanical device therapy may derive in part from a, paradoxically, beneficial suppression of βAR and L-type calcium channel activity in failing cells.