Action Potentials In Sinoatrial Node Cells Research Articles

Ca 2+-regulated-cAMP/PKA signaling in cardiac pacemaker cells links ATP supply to demand

Rationale In sinoatrial node cells (SANC), Ca 2+ activates adenylate cyclase (AC) to generate a high basal level of cAMP-mediated/protein kinase A (PKA)-dependent phosphorylation of Ca 2+ cycling proteins. These result in spontaneous sarcoplasmic-reticulum (SR) generated rhythmic Ca 2+ 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 Ca 2+ 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 Ca 2+-cAMP/PKA dependent; and (2) due to its feed-forward nature, a decrease in flux through the Ca 2+-cAMP/PKA signaling axis will reduce the basal ATP production rate. Methods and results O 2 consumption in spontaneous beating SANC was comparable to ventricular myocytes (VM) stimulated at 3 Hz. Graded reduction of basal Ca 2+-cAMP/PKA signaling to reduce ATP demand in rabbit SANC produced graded ATP depletion (r 2 = 0.96), and reduced O 2 consumption and flavoprotein fluorescence. Neither inhibition of glycolysis, selectively blocking contraction nor specific inhibition of mitochondrial Ca 2+ flux reduced the ATP level. Conclusions Feed-forward basal Ca 2+-cAMP/PKA signaling both consumes ATP to drive spontaneous APs in SANC and is tightly linked to mitochondrial ATP production. Interfering with Ca 2+-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.

Ca 2+-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. This results in spontaneous sarcoplasmic reticulum-generated rhythmic Ca2+ oscillations during diastolic depolarization, that not only ignite the surface membrane to generate rhythmic action potentials (APs), but, in a feed-forward manner, also activate AC/PKA signaling. ATP is consumed to produce cAMP, to pump Ca2+ and to contract. Objective: Since nature efficiently links 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 level. Methods and Results: Graded reduction of basal Ca2+-cAMP/PKA signaling in rabbit SANC, produced graded ATP depletion (r2=0.96), and reduced O2 consumption and flavoprotein fluorescence. Neither inhibition of glycolysis, nor selectively blocking contraction reduced the ATP level. Specific inhibition of mitochondrial Ca2+ flux was without effect, indicating that the cAMP/PKA component rather than Ca2+ directly, links the Ca2+/cAMP-PKA signaling to ATP production. 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 to reduce the SANC ATP demand also “pulls the plug” on SANC ATP supply. This distinctly differs from ventricular myocytes, which lack this feed-forward basal cAMP/PKA signaling, and in which ATP level remains constant when the demand changes.

Local control of Ca2+-induced Ca2+ release in mouse sinoatrial node cells

Emerging evidence from large animal models implicates Ca2+ regulation, particularly intracellular sarcoplasmic reticulum (SR) Ca2+ release, as essential for sinoatrial node (SAN) automaticity. However, despite the apparent importance of SR Ca2+ release to SAN cell function it is uncertain how SR Ca2+ release is controlled in SAN cells from mouse. Understanding mouse SAN SR Ca2+ release mechanism will allow improved understanding of results in studies on SAN from genetic mouse models of Ca2+ homeostatic proteins. Here we investigated the functional relationship between sarcolemmal Ca2+ influx and SR Ca2+ release at the level of single SAN cell, using simultaneous patch-clamp current recording and high resolution confocal Ca2+ imaging techniques. In mouse SAN cells, both Ca2+ channel currents and triggered SR Ca2+ transients displayed bell-shaped, graded function with the membrane potential. Moreover, the gain function for Ca2+-induced Ca2+ release (CICR) displayed a monotonically decreasing function with strong voltage dependence, consistent with a “local control” mechanism for CICR. In addition, we observed numerous discrete Ca2+ sparks at the voltage range of diastolic depolarization, in sharp contrast to the much lower frequency of sparks observed at resting potentials. We concluded that the “local control” mechanism of CICR is responsible for both local Ca2+ release during diastolic depolarization and the synchronized Ca2+ transients observed during action potential in SAN cells.