CGMP as a New Drug Target in Atrial Fibrillation

Objectives: Cyclic guanosine 3¢,5¢-monophosphate (cGMP) serves as a second messenger molecule which regulates many cellular functions in health and disease. cGMP is generated by particulate or soluble guanylyl cyclases upon stimulation with natriuretic peptides or nitric oxide, respectively. Furthermore, the cGMP concentration is modulated by cGMP-degrading phosphodiesterases. During the last decade, it emerged that cGMP is a novel drug target for the treatment of pulmonary and cardiovascular disorders. In this respect, several drugs were developed, which are now in clinical phase studies. These new drugs act NO-independently with/without heme on soluble guanylyl cyclases or particulated guanylyl cyclases. NO-dependent soluble guanylyl cyclases drugs were rapidly discarded for treatment of cardiac diseases because these drugs have had no effect in mouse trials. However, no study has been ever performed in human myocytes to measure cGMP directly.

Methods: We isolated myocytes from the right and/or left atrium of 27 patients in sinus rhythm (SR), and with AF. Cells were transfected with adenovirus to express the cytosolic FRET-based cGMP sensor and cultured for 48 hours. Förster resonance energy transfer (FRET) was used to measure cGMP in 61 living human atrial myocytes. We stimulated cells with the C-type natriuretic peptide (CNP), and the non-selective phosphodiesterases (PDEs) inhibitor IBMX. PDE specific inhibitors for PDE2 (Bay 60-7550) and PDE3 (Cilostamide), inhibitor of the soluble guanylyl cyclase (ODQ) and NO donor (SNAP) were used.

Results: We could show that stimulation with CNP increased cGMP levels in human atrial myocytes. However, in myocytes from patients with AF, global cGMP responses to CNP and to IBMX were reduced compared to SR. Additionally, there was a difference in response to CNP and IBMX in patients with AF between the right and the left atria. Although in the right atria IBMX could further increase cGMP levels in the cell, in the left atria leaded to a reduction in cGMP levels. RT-qPCR showed a tendency of PDE3 to be reduced in AF. SNAP effect, opposite to CNP effect, tended to increase in AF, especially in the right atria.

Conclusion: PDEs contribute to cGMP signaling in the human atria and are involved in atrial pathophysiology. Our data show differences in cGMP regulation in myocytes isolated from left and right atrium from patients in AF and SR. Our results demonstrate that in human atrial myocytes, cGMP levels are regulated not only by particulated guanylyl cyclases but also by NO-dependent soluble guanylyl cyclases, thereby leading to new therapeutic concepts.