Ca²⁺-regulated-cAMP/PKA signaling in cardiac pacemaker cells links ATP supply to demand

Rationale: In sinoatrial node cells (SANC), Ca²⁺ activates adenylate cyclase (AC) to generate a high basal level of cAMP-mediated/protein kinase A (PKA)-dependent phosphorylation of Ca²⁺ cycling proteins. These result in spontaneous sarcoplasmic-reticulum (SR) generated rhythmic Ca²⁺ oscillations during diastolic depolarization, that not only trigger the surface membrane to generate rhythmic action potentials (APs), but, in a feed-forward manner, also activate AC/PKA signaling. ATP is consumed to pump Ca²⁺ to the SR, to produce cAMP, to support contraction and to maintain cell ionic homeostasis. Objective: Since feedback mechanisms link ATP-demand to ATP production, we hypothesized that (1) both basal ATP supply and demand in SANC would be Ca²⁺-cAMP/PKA dependent; and (2) due to its feed-forward nature, a decrease in flux through the Ca²⁺-cAMP/PKA signaling axis will reduce the basal ATP production rate. Methods and results: O₂ consumption in spontaneous beating SANC was comparable to ventricular myocytes (VM) stimulated at 3Hz. Graded reduction of basal Ca²⁺-cAMP/PKA signaling to reduce ATP demand in rabbit SANC produced graded ATP depletion (r²=0.96), and reduced O₂ consumption and flavoprotein fluorescence. Neither inhibition of glycolysis, selectively blocking contraction nor specific inhibition of mitochondrial Ca²⁺ flux reduced the ATP level. Conclusions: Feed-forward basal Ca²⁺-cAMP/PKA signaling both consumes ATP to drive spontaneous APs in SANC and is tightly linked to mitochondrial ATP production. Interfering with Ca²⁺-cAMP/PKA signaling not only slows the firing rate and reduces ATP consumption, but also appears to reduce ATP production so that ATP levels fall. This distinctly differs from VM, which lack this feed-forward basal cAMP/PKA signaling, and in which ATP level remains constant when the demand changes.