Publications (3)12.47 Total impact
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ABSTRACT: A reduced sinoatrial node (SAN) functional reserve underlies the age-associated decline in heart rate acceleration in response to stress. SAN cells function involves an oscillatory coupled-clock system: the sarcoplasmic reticulum (SR), a Ca(2+) clock and the electrogenic-sarcolemmal membrane clock. Ca(2+)-activated-calmodulin-AC/CaMKII-cAMP/PKA-Ca(2+) signaling regulated by phosphodiesterase (PDE) activity drives SAN cells automaticity. SR-generated local calcium releases (LCRs), regulated by coupled clock functions, activate NCX in the membrane clock, which initiates the action potential (AP). We hypothesize that SAN cell dysfunctions accumulate with age. We found a reduction in single SAN cell AP firing in aged (20-24 months) vs adult (3-4 months) mice. The sensitivity of the SAN beating rate responses to both muscarinic and adrenergic receptor activation become decreased in advanced age. Additionally, age-associated coincident dysfunctions occur stemming from compromised clock functions, including a reduced SR-Ca(2+) load as well as a reduced size, number and duration of spontaneous LCRs. Moreover, the sensitivity of SAN beating rate to a cAMP stress induced by PDE inhibitor is reduced, as are the LCR size, amplitude and number in SANC from aged vs. adult mice. These functional changes coincide with decreased expression of crucial SR Ca(2+)-cycling proteins, including SERCA2, RyR2, and NCX1. Thus, a deterioration in intrinsic Ca(2+)-clock kinetics in aged SANC, due to deficits in intrinsic SR-Ca(2+) cycling and its response to a cAMP-dependent stress is involved in age-associated reduction in intrinsic resting AP-firing rate, and may also underlie the age-associated reduction in the acceleration of heart rate in response to stress.
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ABSTRACT: Recent perspectives on sinoatrial nodal cell (SANC)(*) function indicate that spontaneous sarcoplasmic reticulum (SR) Ca(2+) cycling, i.e. an intracellular "Ca(2+) clock," driven by cAMP-mediated, PKA-dependent phosphorylation, interacts with an ensemble of surface membrane electrogenic molecules ("surface membrane clock") to drive SANC normal automaticity. The role of AC-cAMP-PKA-Ca(2+) signaling cascade in mouse, the species most often utilized for genetic manipulations, however, has not been systematically tested. Here we show that Ca(2+) cycling proteins (e.g. RyR2, NCX1, and SERCA2) are abundantly expressed in mouse SAN and that spontaneous, rhythmic SR generated local Ca(2+) releases (LCRs) occur in skinned mouse SANC, clamped at constant physiologic [Ca(2+)]. Mouse SANC also exhibits a high basal level of phospholamban (PLB) phosphorylation at the PKA-dependent site, Serine16. Inhibition of intrinsic PKA activity or inhibition of PDE in SANC, respectively: reduces or increases PLB phosphorylation, and markedly prolongs or reduces the LCR period; and markedly reduces or accelerates SAN spontaneous firing rate. Additionally, the increase in AP firing rate by PKA-dependent phosphorylation by β-adrenergic receptor (β-AR) stimulation requires normal intracellular Ca(2+) cycling, because the β-AR chronotropic effect is markedly blunted when SR Ca(2+) cycling is disrupted. Thus, AC-cAMP-PKA-Ca(2+) signaling cascade is a major mechanism of normal automaticity in mouse SANC.
National Institute on Aging
Baltimore, Maryland, United States
- Laboratory of Cardiovascular Science (LCS)