Biphasic Effect Of Cyclic Amp Axis On Cardiomyocyte Survival

Background: Persistent cardiac hypertrophy in response to pathological stimuli is results in maladaptive myocardial remodeling and cell death. Clinical evidence revealed that one of the most significantly beneficial medications for targeting heart failure with pathologic myocardial hypertrophy is beta1-adrenergic receptor (β1R) blockers. The molecular pathway of β1R is mediated by the second messenger cyclic AMP (cAMP). However, there are some debate regarding the role of cAMP in myocardial survival. We hypothesized whether there may be threshold concentration of cAMP in cell susceptibility to cardiomyocyte cell death. Methods: Male 14-week-old C57BL6 mice were subjected to the surgery of thoracic aortic constriction (TAC) to induce pressure overload. Changes in apoptosis were evaluated in each heart section and in vitro culture of neonatal cardiomyocytes using TUNEL. To elucidate the concentration-dependent distinct effect of cAMP on myocardial cell death, we tested the different concentration of cell-permeable cAMP (8-br-cAMP) at low (60 μM) and high concentration (6 mM), and receptor-mediated cAMP-stimulators (Ex4; exendin-4, ISO; isoproterenol). Results: In vitro analysis revealed that the high-cAMP and ISO exhibited marked increase in TUNEL-positivity (15.46%±3.09% for high-cAMP versus 6.71%±0.33% for ISO), which was reversed by Rp-cAMP (1.80%±0.17% and 2.05%±0.25%, respectively). Unexpectedly, the 8-p-Methoxyphenylthon-2-O-methyl-cAMP (pMe-cAMP, 50 μM), the specific activator of another cAMP-sensitive target Epac, reversed the high-cAMP-induced cell death even at a less extent compared to that observed by PKA-inhibitor Rp-cAMP (3.73%±0.70%). Serum depletion induced 3.22±0.24% of TUNEL-positive cell count of NRVM, which was reversed by pMe-cAMP, (50 μM) and Ex4 (1.74±0.18%, n=6, P<0.01), which was insensitive to PKA inhibition by Rp-cAMP (100 μM). TAC increased myocardial apoptosis. TAC-CON heart exhibited 1.66-fold decrease in cardiac cAMP concentration compared to sham-CON. Ex4 ameliorated the TAC-induced cardiac dysfunction and apoptosis by increase in cAMP. Conclusions: The cAMP-related cell death was mediated by PKA activation, which were reversed by Epac activation.