{"doi":"10.1101/2022.06.20.496874","title":"Adenosine Reduces Sinoatrial Node Cell AP Firing Rate by uncoupling its Membrane and Calcium Clocks","abstract":"A bstract The spontaneous action potential (AP) firing rate of sinoatrial nodal cells (SANC) is regulated by a system of intracellular Ca 2+ and membrane ion current clocks driven by Ca 2+ -calmodulin-activated adenylyl cyclase-protein kinase A (PKA) signaling. The mean AP cycle length (APCL) and APCL variability inform on the effectiveness of clock coupling. Endogenous ATP metabolite adenosine (ado) binds to adenosine receptors that couple to G i protein-coupled receptors, reducing spontaneous AP firing rate via G βγ signaling that activates an membrane-clock outward current, I KACh . Ado also inhibits adenylyl cyclase activity via G iα signaling, impacting cAMP-mediated PKA-dependent protein phosphorylation and intracellular Ca 2+ cycling. We hypothesize that in addition to I KAdo activation, ado signaling impacts Ca 2+ via G iα signaling and that both effects reduce AP firing rate by reducing the effectiveness of the Ca 2+ and membrane clock coupling. To this end, we measured Ca 2+ and membrane potential characteristics in enzymatically isolated single rabbit SANC. 10 µM ado substantially increased both the mean APCL (on average by 43%, n=10) and AP beat-to-beat variability from 5.1±1.7% to 7.2±2.0% (n=10) measured via membrane potential and 5.0±2.2 to 10.6±5.9 (n=40) measured via Ca 2+ (assessed as the coefficient of variability, CV=SD/mean). These effects were mediated by hyperpolarization of the maximum diastolic membrane potential (membrane clock effect) and suppression of diastolic spontaneous, local Ca 2+ releases (LCRs) (Ca 2+ clock effect): as LCR size distributions shifted from larger to smaller values, the time of LCR occurrence during diastolic depolarization (LCR period) became prolonged, and the ensemble LCR Ca 2+ signal became reduced. The tight linear relationship of coupling between LCR period to the APCL in the presence of ado “drifted” upward and leftward, i.e. for a given LCR period, APCL was prolonged, becoming non-linear indicating clock uncoupling. An extreme case of uncoupling occurred at higher ado concentrations (>100 µM): small stochastic LCRs of the Ca 2+ clock failed to self-organize and synchronize to the membrane clock, thus creating a failed attempt to generate an AP resulting in arrhythmia and cessation of AP firing. Thus, the effects of ado to activate G βγ and I KACh, Ado and to activate G iα , suppressing adenylyl cyclase activity, both contribute to the ado-induced increase in the mean APCL and APCL variability by reducing the fidelity of clock coupling and AP firing rate.","journal":"bioRxiv (Cold Spring Harbor Laboratory)","year":2022,"id":309407,"datarank":0.0,"base_score":0.0,"endowment":0.0,"self_citation_contribution":0.0,"citation_network_contribution":0.0,"self_endowment_contribution":0.0,"citer_contribution":0.0,"corpus_percentile":null,"corpus_rank":null,"citation_count":0,"citer_count":0,"citers_with_citation_signal":0,"citers_with_endowment":0,"datacite_reuse_total":0,"is_dataset":false,"is_dataset_confidence":0.9504,"is_data_producer":false,"deposit_databanks":null,"is_oa":true,"file_count":0,"downloads":0,"has_version_chain":false,"published_date":"2022-01-01","fair_score":null,"fair_percentile":null,"algorithm_id":"datarank_citation_only_1hop_v6","ranking_scope":"data_only","authors":[{"id":286636,"name":"Kenta Tsutsui","orcid":"0000-0002-5121-6974","position":1,"is_corresponding":false},{"id":286638,"name":"Victor A. Maltsev","orcid":"0000-0002-3832-791X","position":2,"is_corresponding":false},{"id":55700,"name":"Edward G. Lakatta","orcid":"0000-0002-4772-0035","position":3,"is_corresponding":false},{"id":407318,"name":"Ashley Wirth","orcid":"0000-0001-5085-6921","position":0,"is_corresponding":true}],"reference_count":39,"raw_metadata":{"citation_network_status":"fetched"},"created_at":"2026-07-19T00:33:11.579763Z","pmid":null,"pmcid":null,"fwci":null,"citation_percentile":null,"influential_citations":0,"oa_status":null,"license":null,"views":0,"total_file_size_bytes":0,"version_count":0,"fair_f":null,"fair_a":null,"fair_i":null,"fair_r":null,"fair_zscore":null,"fair_rationale":null,"fair_model":null,"fair_agent_version":null,"fair_fulltext_source":null,"fair_has_llm":null,"fair_computed_at":null,"clinical_trials":[],"software_tools":[],"db_accessions":[],"linked_datasets":[],"topics":[]}