Alteration of sarcoplasmic reticulum Ca2+ release termination by ryanodine receptor sensitization and in heart failure

摘要

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 μm) 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 μm 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.