Catecholamine-independent heart rate increases require Ca2+/calmodulin-dependent protein kinase II

Area of Science:

• Cardiology
• Molecular Biology
• Physiology

Background:

• Catecholamines regulate heart rate via the hyperpolarization-activated cyclic nucleotide-gated channel 4 pacemaker current (I(f)) and the Na(+)/Ca(2+) exchanger current (I(NCX)) through a "Ca(2+) clock" mechanism in sinoatrial nodal cells (SANCs).
• The multifunctional Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is crucial for heart rate responses to beta-adrenergic receptor (β-AR) stimulation, but its role in the Ca(2+) clock independent of β-ARs is debated.

Purpose of the Study:

• To investigate the contribution of the Ca(2+) clock and CaMKII to cardiac pacing independently of β-AR stimulation.
• To determine if CaMKII activation is necessary for the Ca(2+) clock to influence heart rate.

Main Methods:

• Utilized the L-type Ca(2+) channel agonist Bay K8644 (BayK) to activate the SANC Ca(2+) clock in wild-type and CaMKII-inhibited (AC3-I) mice.
• Measured heart rates in SANCs and Langendorff-perfused hearts.
• Assessed L-type Ca(2+) current (I(Ca)), I(f), and phospholamban phosphorylation.
• Evaluated late-diastolic intracellular Ca(2+) release and I(NCX), with and without ryanodine.

Main Results:

• BayK effectively increased heart rates in wild-type SANCs and hearts, mimicking isoproterenol's effect.
• SANCs and hearts with CaMKII inhibition (AC3-I) were resistant to rate increases induced by BayK.
• BayK activated CaMKII in control SANCs but not in AC3-I SANCs; increased I(Ca) was insufficient to raise heart rate without CaMKII activation.
• BayK did not affect cAMP/protein kinase A signaling targets (I(f), phospholamban phosphorylation).
• Late-diastolic Ca(2+) release and I(NCX) were reduced in AC3-I SANCs, and BayK's effect was blocked by ryanodine.

Conclusions:

• The Ca(2+) clock mechanism can independently support physiological fight-or-flight heart rate responses without involving β-AR stimulation or increased I(f).
• CaMKII activation is a requisite component for the full function of both the Ca(2+) clock and β-AR stimulation pathways in regulating heart rate.