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 mu M) 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 Wt, intracellular Ca2+ transients and twitch force were simultaneously recorded before and after ryanodine exposure (37 degrees 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 similar to 50% of twitch force is derived from the SR and similar to 50% from sarcolemmal Ca2+ influx in failing human myocardium. This sarcolemmal component increases at higher stimulation frequencies. (C) 1998 Academic Press.