Comparison of subsarcolemmal and bulk calcium concentration during spontaneous calcium release in rat ventricular myocytes

1. The aim of these experiments was to compare the time course of changes in intracellular Ca2+ concentration ([Ca2+](i)) measured in the bulk cytoplasm with those estimated to occur near the sarcolemma. Sarcolemmal Na+-Ca2+ exchange current and [Ca2+](i) were measured in single, voltage-clamped ventricular myocytes. 2. Spontaneous Ca2+ release from the sarcoplasmic reticulum (SR) resulted in a transient inward current. This current developed and decayed more quickly than the accompanying changes in [Ca2+](i) (measured with indo-1) resulting in a hysteresis between [Ca2+](i) and current. A similar hysteresis was also observed if [Ca2+](i) was elevated with caffeine and was removed if the current was low pass filtered with a time constant of 132 ms. Digital video imaging (using fluo-3 or calcium green-1. to measure [Ca2+](i)) allowed measurement of [Ca2+]1 at all points in the cell during the wave of spontaneous Ca2+ release. The hysteresis between [Ca2+]1 and current remained, even after allowing for the spatial and temporal properties of this wave. 4. The hysteresis can be accounted for if there is a barrier to diffusion of Ca2+ ions separating the bulk cytoplasm from the space under the sarcolemma (into which Ca2+ is released from the sarcoplasmic reticulum). The calculated subsarcolemmal [Ca2+] rises and falls more quickly (and reaches a higher peak) than does the bulk [Ca2+]. The delay introduced by this barrier is equivalent to a time constant of 133 ms. 5. The subsarcolemmal space described in this paper may be equivalent to the 'fuzzy space' previously suggested to be important in controlling SR Ca2+ release.