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) 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 microM) 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 microM 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.