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 ß-adrenergic receptor (ß-AR) signaling. Adenylyl cyclase (AC) is a key enzyme in the ß-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 Ca2+-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 ß-AR signaling. In contrast to atrial and ventricular myocytes, SAN cells express a diverse repertoire of ACs, with ACI as the predominant Ca2+-activated isoform. Although ACI-KO (ACI-/-) mice exhibit normal cardiac systolic or diastolic function, they experience SAN dysfunction. Similarly, SAN-specific CRISPR/Cas9-mediated gene silencing of ACI results in sinus node dysfunction. Mechanistically, hyperpolarization-activated cyclic nucleotide-gated 4 (HCN4) channels form functional microdomains almost exclusively with ACI, while ryanodine receptor and L-type Ca2+ channels likely compartmentalize with ACI and other AC isoforms. In contrast, there were no significant differences in T-type Ca2+ and Na+ currents at baseline or after ß-AR stimulation between WT and ACI-/- SAN cells. Due to its central characteristic feature as a Ca2+-activated isoform, ACI plays a unique role in sustaining the rise of local cAMP and heart rates during ß-AR stimulation. The findings provide insights into the critical roles of the Ca2+-activated isoform of AC in sustaining SAN automaticity that is distinct from contractile cardiomyocytes.

Hyperinsulinemia promotes heterologous desensitization of ß2 adrenergic receptor in airway smooth muscle in obesity.

ß-Adrenergic receptor (ß-AR) agonists are the most common clinical bronchodilators for asthma. Obesity influences asthma severity and may impair response to ß-AR agonists. Previous studies show that in obese mice, hyperinsulinemia plays a crucial role in ß-AR desensitization in the heart. We therefore investigated whether insulin promotes ß-AR desensitization in airway smooth muscle (ASM) and compromises airway relaxation responsiveness to ß-AR agonists. We found that human ASM cells and mouse airway tissues exposed to insulin exhibit impaired ß2 AR-induced cAMP accumulation and airway relaxation. This impaired relaxation is associated with insulin-induced phosphorylation and expression of phosphodiesterase 4D (PDE4D) through transactivation of a G protein-coupled receptor kinase 2 (GRK2)-dependent ß2 AR-Gi -ERK1/2 cascade. Both acute and chronic pharmacological inhibition of PDE4 effectively reversed impaired ß2 AR-mediated ASM relaxation in an obesity mouse model induced by a high fat diet. Collectively, these findings reveal that cross talk between insulin and ß2 AR signaling promotes ASM ß2 AR desensitization in obesity through upregulation of PDE4D phosphorylation and expression. Our results identify a novel pathway of asthma pathogenesis in patients with obesity/metabolic syndrome, in which the GRK2-mediated signaling can be a potential therapeutic modality to prevent or treat ß2 AR desensitization in ASM. Moreover, PDE4 inhibitors may be used as efficacious therapeutic agents for asthma in obese and diabetic subjects.