EFFECT OF RYANODINE ON SARCOPLASMIC-RETICULUM CA2+ ACCUMULATION IN NONFAILING AND FAILING HUMAN MYOCARDIUM

Background The purpose of this study was to determine whether abnormal Ca2+ release through ryanodine-sensitive Ca2+ channels in the sarcoplasmic reticulum might contribute to the abnormal [Ca2+](i) homeostasis that has been described in failing human myocardium. Methods and Results Occupancy of low-affinity ryanodine binding sites on ryanodine-sensitive Ca2+ channels stimulates oxalate-supported, ATP-dependent Ca2+ accumulation in sarcoplasmic reticulum-derived microsomes by inhibiting concurrent Ca2+ efflux through these channels. We examined the effects of 0.5 mmol/L ryanodine on Ca-45(2+) accumulation in microsomes prepared from nonfailing (n=8) and failing (n=10) human left ventricular myocardium. In the absence of ryanodine, Ca-45(2+) accumulation reached similar levels in microsomes from nonfailing and failing hearts. Incubation with 0.5 mmol/L ryanodine caused a 52.2+/-6.5% increase in peak Ca-45(2+) accumulation in microsomes from nonfailing hearts and a 24.3+/-4.1% increase in microsomes from failing hearts. The density of high-affinity ryanodine binding sites and the inhibition of [H-3]ryanodine dissociation from these sites by 0.1 mmol/L ryanodine were similar in microsomes from nonfailing and failing hearts. Conclusions These results, which demonstrate a diminished stimulation of Ca2+ accumulation by ryanodine in sarcoplasmic reticulum-derived microsomes from failing human myocardium that could be explained by an uncoupling of the occupancy of low-affinity ryanodine binding sites from the reduction in the open probability of these channels or by concurrent Ca2+ efflux through a ryanodine-insensitive mechanism, are evidence that increased efflux of Ca2+ from the sarcoplasmic reticulum may contribute to the abnormal [Ca2+](i) homeostasis described in failing human myocardium.