Purpose: Atrial natriuretic peptide (ANP) is a well-known vasodilatory factor and crucial hormone for adaptation to increased blood pressure. ANP plasma levels rise upon elevated cardiac afterload during aortic stenosis and, via guanylyl cyclase A, stimulate cardiomyocyte cGMP levels. Cardiac cGMP is considered as a protective second messenger which counteracts detrimental effects of the chronic β-adrenoceptor (β-AR)-mediated cAMP signaling. Previous studies analyzed cyclic nucleotide signaling mostly in advanced chronic heart disease. The purpose of this work was to investigate subcellular changes in cAMP/cGMP signaling and their interactions in early cardiac hypertrophy.
Methods: We used a compensated cardiac hypertrophy model induced by transverse aortic constriction (TAC) in mice. Heart function was evaluated by echocardiography. Plasma ANP was quantified by ELISA. Heart rates were measured from ECG recordings in intact perfused Langendorff hearts. Cardiomyocyte (CM) contractility was analyzed by sarcomere shortening. To monitor cAMP in real time, we generated transgenic mice expressing the membrane targeted Förster resonance energy transfer (FRET) biosensor pmEpac1, localized to the sarcolemma. Individual PDE activities were measured using a classical biochemical assay.
Results: 8 weeks post TAC, animals developed mild hypertrophy accompanied by a 2-fold increase in plasma ANP levels. Unexpectedly, ANP could potentiate catecholamine-induced positive chronotropic response in TAC but not sham hearts, which was accompanied by enhanced contractility in single isolated TAC CMs. FRET analysis revealed increased β1- but decreased β2-AR-cAMP signals in TAC cells without any change in β-AR densities. Interestingly, local β2-AR-associated PDE effects were dramatically altered despite unchanged whole cell PDE activities. In particular, we could uncover a switch from the PDE3- to PDE2-dependent regulation of β2-AR-cAMP signals. Since these PDEs are inversely regulated by cGMP, we tested the effect of ANP on β2-AR signaling. We found a turnaround of the cGMP/cAMP crosstalk in this microdomain, which was due to PDE2 and PDE3 redistribution and did directly translate into functional response.
Conclusions: We could show that in compensated hypertrophy, elevated levels of ANP potentiate catecholamine-stimulated inotropic and chronotropic effects. This is achieved by a switch in PDE2- and PDE3-dependent regulation of β2-and β1-AR-cAMP signaling. We propose that in early cardiac disease, ANP might transiently compensate an increased demand on cardiac output.