The ryanodine binding sarcoplasmic reticulum calcium release channel in nonfailing and in failing human myocardium

Abstract

The ryanodine-sensitive Ca²⁺ release channel (RyaCRC) of the sarcoplasmic reticulum plays a key role in the intracellular Ca²⁺ handling in cardiomyocytes. Altered expression of the RyaCRC has been supposed to contribute to abnormal cellular Ca²⁺ handling and to myocardial dysfunction in dilated and ischemic cardiomyopathy. In the present study the ³H-ryanodine binding site in human myocardial homogenates was characterized and the density of the RyaCRC (which corresponds to the cardiac ryanodine receptor) was determined in nonfailing and in failing human myocardium.

Homogenates were prepared from nonfailing left ventricular myocardium from the hearts of 5 organ donors (NF) and from failing myocardium from 14 explanted hearts of transplant recipients with end-stage heart failure resulting from dilated (DCM, n = 5) or ischemic (ICM, n = 9) cardiomyopathy. Radioligand saturation binding experiments revealed a specific, high-affinity ³H-ryanodine binding site (K_d-values: NF: 0.65±0.11 nmol/l, DCM: 0.66±0.09 nmol/l, ICM: 0.88±0.18 nmol/l; n.s.) in all preparations. Specific ³H-ryanodine binding depended on the free Ca²⁺ concentration in the assay. It was maximal at 3–100 μmol/l Ca²⁺. The binding was inhibited by the RyaCRC antagonists ruthenium red (K_i-value: 0.32 [0.18–0.56] μmol/l, n = 5) and Mg²⁺ (K_i-value: 2.95 [1.23–7.11] mmol/l, n = 5). The RyaCRC density was 103.5±11.9 fmol/mg protein in nonfailing myocardium. There was no significant change in the RyaCRC density in dilated or ischemic cardiomyopathy (112.4±17.1 and 122.7±13.9 fmol/mg protein) compared to nonfailing control myocardium.

In summary, ³H-ryanodine binds specifically and with high-affinity to the RyaCRC in human myocardium. There is no change in the RyaCRC density in failing myocardium of patients with DCM or ICM in comparison to nonfailing controls.