IP₃-mediated Ca²⁺ release regulates atrial Ca²⁺ transients and pacemaker function by stimulation of adenylyl cyclases

Abstract

Inositol trisphosphate (IP3) is a Ca2+-mobilizing second messenger shown to modulate atrial muscle contraction, and is thought to contribute to atrial fibrillation. Cellular pathways underlying IP3 actions in cardiac tissue remain poorly understood, and the work presented here addresses the question whether IP3-mediated Ca2+ release from the sarcoplasmic reticulum is linked to adenylyl cyclase activity including Ca2+-stimulated adenylyl cyclases (AC1 and AC8) that are selectively expressed in atria and sino-atrial node (SAN). Immunocytochemistry in guinea pig atrial myocytes identified co-localization of type 2 IP3Rs with AC8, while AC1 was located in close vicinity. Intracellular photorelease of IP3 by UV light significantly enhanced the amplitude of the Ca2+ transient (CaT) evoked by electrical stimulation of atrial myocytes (31 ± 6 % increase 60 s post photorelease, n=16). The increase in CaT amplitude was abolished by inhibitors of adenylyl cyclases (MDL-12,330) or protein kinase A (H89), showing that cAMP signaling is required for this effect of photoreleased IP3. In mouse spontaneously beating right atrial preparations, phenylephrine, an α-adrenoceptor agonist with effects that depend on IP3 mediated Ca2+ release, increased the maximum beating rate by 14.7 ± 0.5 %, n=10. This effect was substantially reduced by 2.5 µmol/L 2-APB and abolished by a low dose of MDL-12,330, observations which are again consistent with a functional interaction between IP3 and cAMP signaling involving Ca2+ stimulation of adenylyl cyclases in the SAN pacemaker. Understanding the interaction between IP3 receptor pathways and Ca2+-stimulated adenylyl cyclases provides important insights concerning acute mechanisms for initiation of atrial arrhythmias.

Emerging Evidence for cAMP-calcium Cross Talk in Heart Atrial Nanodomains Where IP3-Evoked Calcium Release Stimulates Adenylyl Cyclases

“…(Molina et al, 2012;Van Wagoner and Lindsay, 2012). Putting these observations in the context of the observations of Capel et al (2021) discussed above showing that to the newly proposed signaling mechanism. IP 3 -evoked calcium release from SR stimulates calcium-activated adenylyl cyclases (AC8 and AC1) leading to production of cAMP and PKA, which in turn phosphorylates target proteins, L-type calcium channels (LTCC), ryanodine receptors (RyR) and phospholamban (PLB) associated with sarcoplasmic-endoplasmic reticulum calcium ATPase (SERCA).…”

“…D: An atrial myocyte in which IP 3 R2s are again labelled red while AC1 is labelled green. E: The cellular location of IP 3 R2 and AC1 at higher magnification, while F shows a plot of intensity of red and green fluorescence against distance across the cell at the line at right angles to the long axis of the cell shown in panel E. Note that there was colocalisation of IP 3 R2 and AC8, while AC1 appeared to be located on the cytosolic side of junctional SR. From Capel et al (2021).…”

“…(b) Previously proposed simple scheme showing effect of calcium release from IP 3 Rs to enhance calcium release through neighbouring RyRs without involvement of calcium-activated adenylyl cyclases. Note that in the experiments ofCapel et al (2021) shown in Figure2there was little or no effect of photoreleased IP 3 on the amplitude of CaTs when adenylyl cyclase or PKA were inhibited. (c): Scheme following on from Figure 3(a) showing…”

Cardiac Atrial Compartmentalisation Proteomics: A Modified Density Gradient Method to Analyse Endo-lysosomal Proteins

“…The value of such approaches has been demonstrated by the application of these data to define molecular changes in patients suffering from cardiovascular disease and to provide comparisons with known genomic parameters for cardiovascular disease including heart failure and atrial fibrillation (AF) 12,13 . Mishandling of Ca 2+ regulation in cardiac cells is closely linked to the pathophysiology of cardiac arrhythmias such as AF 14 and there is increasing evidence for an involvement of lysosomes in cardiac Ca 2+ handling/mishandling 15,16 . In order to understand the contribution of the organelles involved in Ca 2+ regulation (including lysosomes in addition to SR and mitochondria) of atrial function, a more detailed organelle-specific approach is required.…”

“…In order to understand the contribution of the organelles involved in Ca 2+ regulation (including lysosomes in addition to SR and mitochondria) of atrial function, a more detailed organelle-specific approach is required. The guinea pig ( Cavia porcellus ) is a common and well appreciated small mammal model used in cardiovascular research and recent work from our own group has highlighted the value of using C. porcellus tissue to study atrial Ca 2+ handling in atrial cells 14,17 . C. porcellus cardiomyocyte electrophysiology, which includes the typical long plateau phase of the action potential is closer to that of human compared to mouse or rat.…”