Roles Of I(f) And Intracellular Calcium Release In Pacemaking Of Murine Ventricular Cardiomyocytes During Embryonic Development

Introduction:Pacemaking,as the core issue of cardiophysiology,has beeninvestigated a lot.While up to now the mechanism of pacemaking during cardiogenesis isstill not thoroughly clear.Methods and Results:Here using path clamp,calcium imaging and semi-quantitativeRT-PCR techniques,we provided experimental evidences that (1) hyperpolarization-activated current (I(f)) participated in the regular phase 4 diastolic depolarization (DD) inembryonic ventricular cardiomyocytes especially at early development stage (EDS),andblockade of I(f) by 2 mM Cs⁺ induced a strong chronotropic effect.This influence startedfrom the earlier phase of DD.(2) In contrast intracellular Ca²⁺ release mediated throughryanodine receptor (RyR) and inositol 1,4,5-trisphosphate receptor (IP3R) played a key rolein pacemaking by participating in the later phase of DD.Abolishment of intracellularcalcium release by coapplication of ryanodine (Ry) and 2-aminoethyl diphenyl borate(2-APB) or application of caffeine,brought the action potentials (APs) into a complete haltin ventricular cells at both early and late development stages (LDS),though RyR mediatedCa²⁺ release contributed more to the spontaneous activities than IP3R especially at LDS andindividual application of Ry or 2-APB at LDS,showed much weaker or even no influenceon APs in ventricular cells as compared to that at EDS.(3) NCX was activatied byintracellular Ca²⁺ release,this resulted in I_NCX and paritipate in the late phase of the DD ofAP. Conclusion:Altogether we conclude that the I(f) and intracellular Ca²⁺ releasecontribute to the early and late phase of DD of APs respectively.I(f) and spontaneous Ca2+oscillations often reappear in failure heart with arrhythmogenesis.Thus our study mayprovide experimental evidence for the treatment of these diseases.