Bile acids are recognised as bioactive signalling molecules. While they are known to influence arrhythmia susceptibility in cholestasis, there is limited knowledge about the underlying mechanisms. To delineate mechanisms underlying fetal heart rhythm disturbances in cholestatic pregnancy, we used FRET microscopy to monitor cAMP release and contraction measurements in isolated rodent neonatal cardiomyocytes. The unconjugated bile acids CDCA, DCA and UDCA and, to a lesser extent, CA were found to be relatively potent agonists for the GPBAR1 (TGR5) receptor and elicit cAMP release, whereas all glyco- and tauro- conjugated bile acids are weak agonists. The bile acid-induced cAMP production does not lead to an increase in contraction rate, and seems to be mediated by the RI isoform of adenylate cyclase, unlike adrenaline-dependent release which is mediated by the RII isoform. In contrast, bile acids elicited slowing of neonatal cardiomyocyte contraction indicating that other signalling pathways are involved. The conjugated bile acids were found to be partial agonists of the muscarinic M2, but not sphingosin-1-phosphate-2, receptors, and act partially through the Gi pathway. Furthermore, the contraction slowing effect of unconjugated bile acids may also relate to cytotoxicity at higher concentrations.

Aims Guanylyl cyclase-B (GC-B; natriuretic peptide receptor-B, NPR-B) stimulation by C-type natriuretic peptide (CNP) increases cGMP and causes a lusitropic and negative inotropic response in adult myocardium. These effects are not mimicked by NPR-A (GC-A) stimulation by brain natriuretic peptide (BNP), despite similar cGMP increase. More refined methods are needed to better understand the mechanisms of the differential cGMP signaling and compartmentation. The aim of this work was to measure cGMP near proteins involved in regulating contractility to understand compartmentation of cGMP signaling in adult cardiomyocytes. Methods and Results We constructed several fluorescence resonance energy transfer (FRET)-based biosensors for cGMP subcellularly targeted to phospholamban (PLB) and troponin I (TnI). CNP stimulation of adult rat cardiomyocytes increased cGMP near PLB and TnI, whereas BNP stimulation increased cGMP near PLB, but not TnI. The phosphodiesterases PDE2 and PDE3 constrained cGMP in both compartments. Local receptor stimulation aided by scanning ion conductance microscopy (SICM) combined with FRET revealed that CNP stimulation both in the t-tubules and on the cell crests increases cGMP similarly near both TnI and PLB. In ventricular strips, CNP stimulation, but not BNP, induced a lusitropic response, enhanced by inhibition of either PDE2 or PDE3, and a negative inotropic response. In cardiomyocytes from heart failure rats, CNP increased cGMP near PLB and TnI more pronounced than in cells from sham-operated animals. Conclusions These targeted biosensors demonstrate that CNP, but not BNP, increases cGMP near TnI in addition to PLB, explaining how CNP, but not BNP is able to induce lusitropic and negative inotropic responses. Translational Perspective Although best known as heart failure biomarkers, natriuretic peptides (ANP, BNP and CNP) are important signaling molecules in the heart and other organs through increasing cyclic GMP (cGMP). Treatment preventing their degradation improves heart failure prognosis. To better understand their cardiac signaling, we employed fluorescent cGMP biosensors targeted to troponin I and phospholamban and found that BNP and CNP increase cGMP differently around these proteins in both normal and failing cardiomyocytes. This may explain the different effects of BNP and CNP on cardiac contractility and relaxation, with possible implications for understanding and treatment of heart failure.

Natriuretic peptides (NPs) increase cGMP, show beneficial cardiovascular effects and has been shown to regulate energy metabolism in other tissues. However, little is known about their direct effect on cardiac mitochondria and cardiomyocyte apoptosis. Here, we examined whether NPs increase cGMP around mitochondria and alter apoptosis in cardiomyocytes. Stimulating with ANP or CNP reduced apoptosis, together with reduced caspase 9 activation and cytochrome c release, suggesting that NPs decrease apoptosis through the intrinsic pathway that involves mitochondria. We engineered a novel FRET-based biosensor with high selectivity towards cGMP and targeted this to the outer mitochondrial membrane (OMM) and found that ANP and CNP increase cGMP at the OMM. Moreover, ANP and CNP increased phosphorylation of the pro-apoptotic protein Drp1 and CNP prevented fragmentation of mitochondria. We suggest that cGMP increase in the OMM microdomain inhibits the pro-apoptotic protein Drp1, leading to reduced mitochondrial fragmentation that inhibits apoptosis.