doi: 10.1161/01.RES.0000045654.34731.FF
© 2002 American Heart Association, Inc.
Editorials |
Heart Failure and the Ryanodine Receptor
Does Occam’s Razor Rule?
From the Unit of Cardiac Physiology, Manchester University, Manchester, UK.
Correspondence to D.A. Eisner, Unit of Cardiac Physiology, 1.524 Stopford Bldg, Manchester University, Oxford Rd, Manchester M13 9PT, UK. E-mail eisner{at}man.ac.uk
Key Words: heart failure • SERCA • hyperphosphorylation
In heart failure, the amplitude and rate of decay of both contraction and the underlying systolic Ca2+ transient are reduced. A current debate concerns the mechanism of these alterations of calcium handling. One theory invokes decreased activity of the sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA)1 while another focuses on alterations in the SR Ca2+ release channel (ryanodine receptor, RyR).2 A significant contribution to this debate is made by Jiang et al3 in this issue of Circulation Research.
Excitation-Contraction Coupling
Calcium that activates contraction comes from two sources: (1) the extracellular fluid, largely via the L-type Ca2+ current (ICa); and (2) the SR, by release through the RyR. Because the latter is generally larger and the amplitude of ICa is not consistently altered in failure (see review4), work has focused on release from the SR. Release occurs via calcium-induced calcium release (CICR) whereby Ca2+ entry increases the probability of opening of a closely apposed RyR (see general reviews5,6). Relaxation requires that [Ca2+]i be lowered by the combined effects of SERCA and Na+-Ca2+ exchange (NCX). The activity of SERCA is depressed by the accessory protein phospholamban and this inhibition is removed by phosphorylation, providing a mechanism whereby sympathetic stimulation can increase SR Ca2+ content and hence Ca2+ release from the SR. Importantly, depression of SERCA activity will not only decrease the amplitude of the Ca2+ transient (by decreasing SR content) but will also directly slow the rate of decay.
Phosphorylation of the RyR
The RyR can be phosphorylated,7 and there is an important interaction between an auxiliary protein (FKBP), phosphorylation, and RyR opening. Briefly, FKBP stabilizes interactions between RyRs such that they show coupled gating whereby all the RyRs in the group open and close together.8 Removal of the stabilizing effect (as occurs with the application of rapamycin or phosphorylation) results in subconducting states even at low activating Ca2+ concentrations thereby producing a diastolic Ca2+ leak that can also be seen as long-duration Ca2+ "sparks" of release from the SR.9 Removal of the coupled gating may also decrease the number of RyRs that open for a given trigger and thereby decrease the gain of CICR.
What Is the Mechanism of Decreased SR Ca2+ Release in Failure?
There are three sites at which the amount of Ca2+ release from the SR may be decreased (see Figure). (1) A decrease in ICa may result in the opening of fewer RyRs. However, as mentioned previously, there is little evidence to suggest that the amplitude of this current changes in failure. (2) A change in the properties of the RyR or its sensitivity to activation by ICa such that fewer open for a given trigger. (3) A decrease in the Ca2+ content of the SR such that less Ca2+ is released for the opening of a given number of RyRs. There are two suggested causes of decreased SR content. (1) The longest established is a decrease in the expression or activity of SERCA and/or an increase in the activity of NCX.1 (2) A more recent suggestion is that in failure the RyR is hyperphosphorylated and becomes leaky to Ca2+ ions during diastole thereby depleting the SR.10 At this point, two questions should be addressed: (1) Is the reduced Ca2+ transient due to the RyR or the SR Ca2+ content? (2) Does any reduction of SR content result from (a) decreased SERCA or (b) increased leak?
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Is the Reduced Ca2+ Transient Due to the RyR or the SR Ca2+ Content?
To the best of our knowledge, no study has measured SR Ca2+ content under conditions that exactly mimic those seen in vivo (body temperature, action potential stimulation at normal heart rates). Notwithstanding this point, several studies have reported a decreased SR Ca2+ content in failure.11–13 What is less well established, however, is whether the change of SR Ca2+ content can quantitatively account for that of the Ca2+ transient.
In contrast, other studies have reported that decreased Ca2+ release is not accompanied by a change of SR content. Suggested mechanisms include a decrease in the number of RyRs14 or depressed opening of RyRs due to diminished coupling between ICa and RyR15 (possibly due to decreased abundance of transverse tubules16). We have previously criticized these explanations on the grounds that, in the steady state, the Ca2+ efflux from the cell must equal the Ca2+ influx. If the influx is unchanged in failure, then the efflux must also be constant.6 Therefore, decreasing the number of RyRs or their activation by ICa would be expected to produce a compensatory increase in SR content with no steady-state change of the amplitude of the Ca2+ transient.
There are two other points relating to experiments that show alterations of SR Ca2+ release in the apparent absence of changes of SR Ca2+ content. (1) As mentioned previously, it is important to consider whether the SR content would also be unchanged in vivo. (2) The other problem concerns whether small changes of SR Ca2+ content can be measured. This is a particularly important point because Ca2+ release from the SR is a steep function of content17 approximating to its cube.18 Thus, a reduction of the Ca2+ transient in failure to 67% of control could be accounted for by only a 12% decrease of SR content with no need to invoke changes of the Ca2+ release mechanism. Current methods may not be sufficiently precise to reveal such a modest difference of SR content between different animals.
Our provisional conclusion is that current evidence favors a decrease of SR content rather than a primary decrease of RyR opening. However, quantitative studies are still required to check whether the decrease of content is adequate to account for that of the transient.
Does Any Reduction of SR Content Result From (a) Decreased SERCA or (b) Increased Leak?
Until recently, there was widespread agreement that the most likely explanation of a decreased SR content was a decrease in SERCA coupled to an increase of NCX as measured in many experiments.1 In support of this, overexpression of SERCA in cells from failing hearts can normalize contraction.19 A competing hypothesis is that hyperphosphorylation of the RyR causes the appearance of subconducting states and consequent diastolic leak through RyRs. This can also explain the beneficial effects of ß blockers in heart failure because these will decrease RyR phosphorylation.2,20 It is this hypothesis that has been reexamined in the study by Jiang et al,3 who found (in both humans and dogs) that the Ca2+ sensitivity of the RyR opening was unaffected by failure. Furthermore, there was no evidence that the probability of subconducting states was any higher in cells from failing hearts. These two studies with diametrically different conclusions used a similar canine model of heart failure (in addition to human tissue). Other work has also questioned some of the tenets of the hyperphosphorylation hypothesis. Recent work on skinned cells has found that the increase of Ca2+ spark frequency produced by cAMP-dependent phosphorylation is not observed in cells from phospholamban knockout mice. It was concluded that the effect of cAMP-dependent phosphorylation is mediated via changes of SR content (secondary to phospholamban phosphorylation) rather than to an effect on the RyR.
The SERCA inhibition hypothesis has the additional merit that it can obviously explain the slowed relaxation of the transient as well as the reduction in size. Although under some conditions modifiers of the RyR can also slow the transient,21 for a given decrease of amplitude, the effects on decay rate are less striking.22
In conclusion, it is important that the relative contributions of changes of SERCA and RyR to heart failure be reevaluated. In particular, we need to know not merely whether particular effects occur but, more importantly, the relative contributions to the failure phenotype.
Footnotes
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
References
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2. Marx SO, Reiken S, Hisamatsu Y, Jayaraman T, Burkhoff D, Rosemblit N, Marks AR. PKA phosphorylation dissociates FKBP12.6 from the calcium release channel (ryanodine receptor): defective regulation in failing hearts. Cell. 2000; 101: 365–376.[CrossRef][Medline] [Order article via Infotrieve]
3. Jiang MT, Lokuta AJ, Farrell EF, Wolff MR, Haworth RA, Valdivia HH. Abnormal Ca2+ release, but normal ryanodine receptors, in canine and human heart failure. Circ Res. 2002; 91: 1015–1022.
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7. Lokuta AJ, Rogers TB, Lederer WJ, Valdivia HH. Modulation of cardiac ryanodine receptors of swine and rabbit by a phosphorylation-dephosphorylation mechanism. J Physiol (Lond). 1995; 487: 609–622.
8. Marx SO, Gaburjakova J, Gaburjakova M, Henrikson C, Ondrias K, Marks AR. Coupled gating between cardiac calcium release channels (ryanodine receptors). Circ Res. 2001; 88: 1151–1158.
9. Xiao R-P, Valdivia HH, Bogdanov K, Valdivia C, Lakatta EG, Cheng H. The immunophilin FK506-binding protein modulates Ca2+ release channel closure in rat heart. J Physiol (Lond). 1997; 500: 343–354.
10. Marks AR. Cardiac intracellular calcium release channels: role in heart failure. Circ Res. 2000; 87: 8–11.
11. Lindner M, Erdmann E, Beuckelmann DJ. Calcium content of the sarcoplasmic reticulum in isolated ventricular myocytes from patients with terminal heart failure. J Mol Cell Cardiol. 1998; 30: 743–749.[CrossRef][Medline] [Order article via Infotrieve]
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14. Vatner DE, Sato N, Kiuchi K, Shannon RP, Vatner SF. Decrease in myocardial ryanodine receptors and altered excitation-contraction coupling early in the development of heart failure. Circulation. 1994; 90: 1423–1430.
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16. He J-Q, Conklin MW, Foell JD, Wolff MR, Haworth RA, Coronado R, Kamp TJ. Reduction in density of transverse tubules and L-type Ca2+ channels in canine tachycardia-induced heart failure. Cardiovasc Res. 2001; 49: 298–307.
17. Shannon TR, Ginsburg KS, Bers DM. Potentiation of fractional sarcoplasmic reticulum calcium release by total and free intra-sarcoplasmic reticulum calcium concentration. Biophys J. 2000; 78: 334–343.[Medline] [Order article via Infotrieve]
18. Trafford AW, Díaz ME, Sibbring GC, Eisner DA. Modulation of CICR has no maintained effect on systolic Ca2+: simultaneous measurements of sarcoplasmic reticulum and sarcolemmal Ca2+ fluxes in rat ventricular myocytes. J Physiol (Lond). 2000; 522: 259–270.
19. del Monte F, Harding SE, Schmidt U, Matsui T, Kang ZB, Dec GW, Gwathmey JK, Rosenzweig A, Hajjar RJ. Restoration of contractile function in isolated cardiomyocytes from failing human hearts by gene transfer of SERCA2a. Circulation. 1999; 100: 2308–2311.
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