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 (1 μM) 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.