Frequency-dependent Changes in Contribution of SR Ca2+to Ca2+Transients in Failing Human Myocardium Assessed with Ryanodine

We tested the influence of blocking sarcoplasmic reticulum (SR) function with ryanodine (1μm) on stimulation rate-dependent changes of intracellular Ca2+transients and twitch force in failing human myocardium. Isometrically contracting, electrically stimulated muscle strips from ventricles of 10 end-stage failing human hearts were used. Muscles were loaded with the intracellular Ca2+indicator aequorin. At stimulation rates from 0.5–3 Hz, intracellular Ca2+transients and twitch force were simultaneously recorded before and after ryanodine exposure (37°C). Ryanodine significantly reduced twitch force at 1 Hz by 46±9% and aequorin light by 57±10% in failing human myocardium (P<0.05). The blunted or inverse aequorin light– and force–frequency relation became positive after ryanodine: in failing human myocardium, twitch force and aequorin light before ryanodine did not increase with increasing frequency and force decreased significantly at 3 Hz (P<0.05). After ryanodine, twitch force (P<0.05) and aequorin light increased with increasing stimulation frequency and were maximum at 2 Hz. The data indicate that inhibition of SR function significantly reduces twitch force and Ca2+transients in failing human myocardium, but converts the blunted or inverse Ca2+– and force–frequency relation into a positive one. We infer that Ca2+responsible for 50% of twitch force is derived from the SR and 50% from sarcolemmal Ca2+influx in failing human myocardium. This sarcolemmal component increases at higher stimulation frequencies.