Abstract 10978: In Vivo Evidence that Adenylyl Cyclase and Sarcoplasmic Reticulum Pumping Regulate the Intrinsic Rate of Heart’s Pacemaker
Studies in isolated sinoatrial nodal (SAN) cells have led to a theory that a coupled-clock system drives normal automaticity: A Ca2+ clock in the sarcoplasmic reticulum (SR) generates rhythmic local Ca2+ releases that activate adenylyl cyclase (AC), i.e., Type 8, that produce cAMP, leading to protein kinase A activation. This results in phosphorylation of phospholamban (PLB), which promotes SR Ca2+ cycling, and in phosphorylation of components of the membrane clock that generates action potentials. Evidence that this coupled-clock system operates in-vivo and how its operation may change in advanced age are lacking.
We hypothesized that genetic manipulation of key components of this signaling cascade in mice, i.e., AC8 overexpression (AC8TG) or PLB knockout (PLB-/-) would increase the intrinsic heart rate (IHR). IHR is an in-vivo heart rate when SAN is isolated from autonomic neurotransmitter signaling using double pharmacologic blockade (propranolol & atropine). IHR was measured during light anesthesia in AC8TG and PLB-/- mice at ages of 3 and 24 mo and in their age-matched wild type (WT) controls (C57 and FVB, respectively). As predicted by the couple-clock theory, either increases in AC activity (AC8TG mice) or SR Ca2+ pumping (PLB-/- mice) increased the IHR above that of WT at 3 mo. However, at 24 mo, the IHR advantage in the genetically manipulated mice waned, and their IHR did not differ from their respective WT mice. In concert with these in-vivo results, the SR Ca2+ load measured in permeabilized, isolated, SAN cells was higher in PLB-/- mice compared to WT at 3 mo, and with aging, this waned by 24 mo.
Conclusion: the increase in IHR when cAMP or Ca2+ cycling are increased in mice provides evidence that the coupled-clock pacemaker system is operative in-vivo and Ca2+ clock function is associated by enhanced SR function in-vitro. That both IHR in genetically manipulated mice and SR function decline with age indicates that, with age, coupled clock functions became less robust.
- © 2013 by American Heart Association, Inc.