Adenylyl cyclase isoform 1 contributes to sinoatrial node automaticity via functional microdomains

Sinoatrial node (SAN) cells are the heart’s primary pacemaker. Their activity is tightly regulated by β-adrenergic receptor (β-AR) signaling. Adenylyl cyclase (AC) is a key enzyme in the β-AR pathway that catalyzes the production of cAMP. There are current gaps in our knowledge regarding the dominant AC isoforms and the specific roles of Ca(2+)-activated ACs in the SAN. The current study tests the hypothesis that distinct AC isoforms are preferentially expressed in the SAN and compartmentalize within microdomains to orchestrate heart rate regulation during β-AR signaling. In contrast to atrial and ventricular myocytes, SAN cells express a diverse repertoire of ACs, with AC(I) as the predominant Ca(2+)-activated isoform. Although AC(I)-KO (AC(I)(–/–)) mice exhibit normal cardiac systolic or diastolic function, they experience SAN dysfunction. Similarly, SAN-specific CRISPR/Cas9-mediated gene silencing of AC(I) results in sinus node dysfunction. Mechanistically, hyperpolarization-activated cyclic nucleotide-gated 4 (HCN4) channels form functional microdomains almost exclusively with AC(I), while ryanodine receptor and L-type Ca(2+) channels likely compartmentalize with AC(I) and other AC isoforms. In contrast, there were no significant differences in T-type Ca(2+) and Na(+) currents at baseline or after β-AR stimulation between WT and ACI(–/–) SAN cells. Due to its central characteristic feature as a Ca(2+)-activated isoform, AC(I) plays a unique role in sustaining the rise of local cAMP and heart rates during β-AR stimulation. The findings provide insights into the critical roles of the Ca(2+)-activated isoform of AC in sustaining SAN automaticity that is distinct from contractile cardiomyocytes.