Alteration of sarcoplasmic reticulum Ca 2+ release termination by ryanodine receptor sensitization and in heart failure

Many physiological processes and pharmacological agents modulate the ryanodine receptor (RyR), the primary sarcoplasmic reticulum (SR) Ca 2+ release channel in the heart. However, how such modulations translate into functional effects during cardiac excitation–contraction coupling (ECC) is much less clear. Using a low dose (250 μ m ) of caffeine we sensitized the RyR and examined SR Ca 2+ release using dynamic measurements of cytosolic Ca 2+ ([Ca 2+ ] i ) and free Ca 2+ within the SR ([Ca 2+ ] SR ). In field stimulated (1 Hz) rabbit ventricular myocytes, application of 250 μ m caffeine caused an initial 33% increase in SR Ca 2+ release, which was followed by a decrease in SR Ca 2+ load (28%) and steady‐state SR Ca 2+ release (23%). To investigate the effects of caffeine on local SR Ca 2+ release, we measured [Ca 2+ ] SR from individual release junctions during ECC as well as during spontaneous Ca 2+ sparks. In intact myocytes during ECC, caffeine increased global fractional SR Ca 2+ release by decreasing the [Ca 2+ ] SR level at which local release terminated by 21%. Similarly, in permeabilized myocytes during spontaneous Ca 2+ sparks, caffeine decreased the [Ca 2+ ] SR level for release termination by 12%. Finally, we examined if Ca 2+ release termination was changed in myocytes from failing hearts, where remodelling processes lead to altered RyR function. In myocytes from failing rabbit hearts, the [Ca 2+ ] SR termination level for Ca 2+ sparks was 13% lower than that of non‐failing myocytes. Collectively, these data suggest that altering the termination level for local Ca 2+ release may represent a novel mechanism to increase SR Ca 2+ release and contractility during ECC.