Abstract
Dynamic measurements of Ca within the sarcoplasmic reticulum ([Ca]SR) using low-affinity Ca indicators give critical insight into the role of [Ca]SR in Ca release termination. Here we used a low dose of caffeine to examine the effects of ryanodine receptor (RyR) sensitization on local and global SR Ca release in rabbit ventricular myocytes. In field stimulated myocytes (1 Hz), application of 250 μM caffeine caused an initial 44% increase in amplitude of action potential-induced [Ca]SR depletion. This resulted in unloading of the SR (27% decrease in steady state diastolic [Ca]SR) and a lowering of the termination level for global release (28% decrease in systolic [Ca]SR). A single stimulus protocol was used to examine the effects of caffeine on SR Ca release after varying [Ca]SR. At all [Ca]SR levels where release was observed, caffeine increased the [Ca]SR depletion amplitude by lowering the global termination level of release. We next studied the effects of caffeine on local SR Ca release events in permeabilized myocytes by simultaneously measuring cytosolic Ca sparks with associated local [Ca]SR depletions (Ca blinks). Under control conditions, Ca sparks terminated at a fixed [Ca]SR depletion threshold, irrespective of initial [Ca]SR. Application of 200 μM caffeine caused an immediate increase in Ca spark frequency (58%), amplitude (8%), duration (23%), and spatial width (13%), and decreased the Ca blink termination level below the control threshold level. Taken together, these data suggest that sensitization of the RyR produces an increase in SR Ca release by decreasing the [Ca]SR termination level for release at individual release junctions.
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