Adrenergic Regulation of Cardiac Contractility Does Not Involve Phosphorylation of the Cardiac Ryanodine Receptor at Serine 2808

Abstract

Abstract-The sympathetic nervous system is a critical regulator of cardiac function (heart rate and contractility) in health and disease. Sympathetic nervous system agonists bind to adrenergic receptors that are known to activate protein kinase A, which phosphorylates target proteins and enhances cardiac performance. Recently, it has been proposed that protein kinase A-mediated phosphorylation of the cardiac ryanodine receptor (the Ca 2ϩ release channel of the sarcoplasmic reticulum at a single residue, Ser2808) is a critical component of sympathetic nervous system regulation of cardiac function. This is a highly controversial hypothesis that has not been confirmed by several independent laboratories. The present study used a genetically modified mouse in which Ser2808 was replaced by alanine (S2808A) to prevent phosphorylation at this site. The effects of isoproterenol (a sympathetic agonist) on ventricular performance were compared in wild-type and S2808A hearts, both in vivo and in isolated hearts. Isoproterenol effects on L-type Ca 2ϩ current (I CaL ), sarcoplasmic reticulum Ca 2ϩ release, and excitation-contraction coupling gain were also measured. Our results showed that isoproterenol caused significant increases in cardiac function, both in vivo and in isolated hearts, and there were no differences in these contractile effects in wild-type and S2808A hearts. Isoproterenol increased I CaL , the amplitude of the Ca 2ϩ transient and excitation-contraction coupling gain, but, again, there were no significant differences between wild-type and S2808A myocytes. These results show that protein kinase A phosphorylation of ryanodine receptor Ser2808 does not have a major role in sympathetic nervous system regulation of normal cardiac function. (Circ Res. 2008;102:e65-e72.)

Phosphorylation of the ryanodine receptor mediates the cardiac fight or flight response in mice

“…Another group had reported that an independently generated RyR2-S2808A +/+ mouse exhibited a normal contractile response to Iso (2 mg/kg i.p.) (12). In the present study, the RyR2-S2808A +/+ mice exhibited a significantly blunted contractile response (WT, 13,268 ± 418 mmHg/s, n = 5; RyR2-S2808A +/+ , 9,491 ± 178 mmHg/s n = 6; P < 0.001) and heart rate response (WT, 611 ± 15 bpm, n = 5; RyR2-S2808A +/+ , 547 ± 12 bpm, n = 6; P < 0.01) to the lower Iso dose but similar responses to those of WT mice at the 1,000-fold higher Iso dose ( Figure 1, A and B).…”

“…Phosphorylation of RyR2 at Ser2808 enhances the Po of the channel by enhancing its sensitivity to cytosolic [Ca 2+ ] in planar lipid bilayers (9,10). While it is well established that increasing I Ca,L and SR Ca 2+ load enhance SR Ca 2+ release (11), it is controversial as to whether increasing RyR2 Po has similar effects (3,(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). In this study, we have used mice engineered with RyR2 that cannot be PKA phosphorylated (21) to directly address the role of PKA phosphorylation of RyR2 in regulating cardiac contractility.…”

Is ryanodine receptor phosphorylation key to the fight or flight response and heart failure?

“…Unfortunately, these new results from the Marks laboratory (25,26) are in striking contrast to studies from the groups of Valdivia and Houser in an independently generated S2808A knockin mouse line (28,29). These mice not only displayed normal heart dimensions before and after aortic banding but also normal myocyte shortening and Ca 2+ transients, normal responses to isoprenaline, and, interestingly, normal PKA-catalyzed incorporation of radioactive phosphate into RyR2, despite effective elimination of S2808 phosphorylation.…”

“…25). Indeed, the Valdivia and the Houser studies used saturating concentrations/ doses of isoprenaline in vitro (1 mmol/l) and in vivo (2 mg/kg), respectively (28,29). However, dosing alone cannot account for the strikingly different observations for two reasons.…”

“…However, dosing alone cannot account for the strikingly different observations for two reasons. First, the Houser group also tested the effects of isoprenaline on Langendorff-perfused mouse hearts at 10 nmol/l, a concentration that is clearly not saturating, and observed essentially identical effects in S2808A and WT hearts (29). Second, Figure 3 of ref.…”