RGS2 overexpression or G_i inhibition rescues the impaired PKA signaling and slow AP firing of cultured adult rabbit pacemaker cells

Freshly isolated adult rabbit sinoatrial node cells (f-SANC) are an excellent model for studies of autonomic signaling, but are not amenable to genetic manipulation. We have developed and characterized a stable cultured rabbit SANC (c-SANC) model that is suitable for genetic manipulation to probe mechanisms of spontaneous action potential (AP) firing.After 48h in culture, c-SANC generate stable, rhythmic APs at 34±0.5°C, at a rate that is 50% less than f-SANC. In c- vs. f-SANC: AP duration is prolonged; phosphorylation of phospholamban at Ser¹⁶ and type2 ryanodine receptor (RyR2) at Ser²⁸⁰⁹ are reduced; and the level of type2 regulator of G-protein signaling (RGS2), that facilitates adenylyl cyclases/cAMP/protein kinase A (PKA) via G_i inhibition, is substantially reduced. Consistent with the interpretation that cAMP/PKA signaling becomes impaired in c-SANC, acute β-adrenergic receptor stimulation increases phospholamban and RyR2 phosphorylation, enhances RGS2-labeling density, and accelerates the AP firing rate to the similar maximum in c- and f-SANC. Specific PKA inhibition completely inhibits all β-adrenergic receptor effects. Adv-RGS2 infection, or pertussis toxin treatment to disable G_i-signaling, each partially rescues the c-SANC spontaneous AP firing rate.Thus, a G_i-dependent reduction in PKA-dependent protein phosphorylation, including that of Ca²⁺ cycling proteins, reduces the spontaneous AP firing rate of c-SANC, and can be reversed by genetic or pharmacologic manipulation of PKA signaling.