Abnormal Ca ²⁺ Release, but Normal Ryanodine Receptors, in Canine and Human Heart Failure

Abstract

Sarcoplasmic reticulum (SR) Ca 2 transport proteins, especially ryanodine receptors (RyR) and their accessory protein FKBP12.6, have been implicated as major players in the pathogenesis of heart failure (HF), but their role remain controversial. We used the tachycardia-induced canine model of HF and human failing hearts to investigate the density and major functional properties of RyRs, SERCA2a, and phospholamban (PLB), the main proteins regulating SR Ca 2 transport. Intracellular Ca 2 is likely to play a role in the contractile dysfunction of HF because the amplitude and kinetics of the [Ca 2 ] i transient were reduced in HF. Ca 2 uptake assays showed 448% reduction of V max in canine HF, and Western blots demonstrated that this reduction was due to decreased SERCA2a and PLB levels. Human HF showed a 305% reduction in SERCA2a, but PLB was unchanged. RyRs from canine and human HF displayed no major structural or functional differences compared with control. The P o of RyRs was the same for control and HF over the range of pCa 7 to 4. Subconductance states, which predominate in FKBP12.6-stripped RyRs, were equally frequent in control and HF channels. An antibody that recognizes phosphorylated RyRs yields equal intensity for control and HF channels. Further, phosphorylation of RyRs by PKA did not appear to change the RyR/FKBP12.6 association, suggesting minor-adrenergic stimulation of Ca 2 release through this mechanism. These results support a role for SR in the pathogenesis of HF, with abnormal Ca 2 uptake, more than Ca 2 release, contributing to the depressed and slow Ca 2 transient characteristic of HF. (Circ Res. 2002;91:1015-1022.) Key Words: heart failure ryanodine receptor sarcoplasmic reticulum protein kinase A

The Ca2+ leak paradox and “rogue ryanodine receptors”: SR Ca2+ efflux theory and practice

“…Even though circulating levels of catecholamines are constitutively higher than in healthy persons, beta-adrenergic receptors appear to be down-regulated, and the robust responsiveness to agents such as isoproterenol seen in healthy heart cells is severely attenuated (Marks, 2001;Chien et al, 2003). Biochemical studies have indicated that this abnormal, sustained "hyperadrenergic" state leads to excess phosphorylation of RyRs (Marx et al, 2000), although this result remains controversial (Jiang et al, 2002). Complementary planar lipid bilayer studies of RyR gating have predicted that this hyperphosphorylation will lead to increased RyR open probability.…”

Cardiac ryanodine receptor phosphorylation: target sites and functional consequences

“…As attractive as this working hypothesis is, the attention it has received and its extensive testing by this group, it remains controversial. This is because numerous laboratory groups have found results that do not support different aspects of this hypothesis [1,[13][14][15][16][17]. These include findings that RyR2 is not hyperphosphorylated by PKA in HF, that PKA-dependent phosphorylation does not cause FKBP12.6 release, that PKA activation does not increase basal RyR2 activation, that there are additional PKA sites on RyR2 and that Ser-2809 can also be phosphorylated by CaMKII or PKG (protein kinase G).…”

Genetic inhibition of PKA phosphorylation of RyR2 prevents dystrophic cardiomyopathy

“…In particular, it has been proposed that increased protein kinase A (PKA)-mediated phosphorylation of RyR2 at serine 2808 (S2808) is an important signaling event in the development of heart failure (12,13). However, other studies in a canine model of heart failure disagreed with these findings (14). Genetic inactivation of RyR2-S2808 phosphorylation in S2808A knock-in mice was associated with normalized RyR2 activity and improved cardiac contractility following myocardial infarction (15).…”