Alteration of sarcoplasmic reticulum Ca2+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) Ca2+ 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 mu m) of caffeine we sensitized the RyR and examined SR Ca2+ release using dynamic measurements of cytosolic Ca2+ ([Ca2+](i)) and free Ca2+ within the SR ([Ca2+](SR)). In field stimulated (1 Hz) rabbit ventricular myocytes, application of 250 mu m caffeine caused an initial 33% increase in SR Ca2+ release, which was followed by a decrease in SR Ca2+ load (28%) and steady-state SR Ca2+ release (23%). To investigate the effects of caffeine on local SR Ca2+ release, we measured [Ca2+](SR) from individual release junctions during ECC as well as during spontaneous Ca2+ sparks. In intact myocytes during ECC, caffeine increased global fractional SR Ca2+ release by decreasing the [Ca2+](SR) level at which local release terminated by 21%. Similarly, in permeabilized myocytes during spontaneous Ca2+ sparks, caffeine decreased the [Ca2+](SR) level for release termination by 12%. Finally, we examined if Ca2+ release termination was changed in myocytes from failing hearts, where remodelling processes lead to altered RyR function. In myocytes from failing rabbit hearts, the [Ca2+](SR) termination level for Ca2+ sparks was 13% lower than that of non-failing myocytes. Collectively, these data suggest that altering the termination level for local Ca2+ release may represent a novel mechanism to increase SR Ca2+ release and contractility during ECC.