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 chan ges in intracellular Ca2+ concentration ([Ca2+](i)) measured in the bu lk cytoplasm with those estimated to occur near the sarcolemma. Sarcol emmal Na+-Ca2+ exchange current and [Ca2+](i) were measured in single, voltage-clamped ventricular myocytes. 2. Spontaneous Ca2+ release fro m the sarcoplasmic reticulum (SR) resulted in a transient inward curre nt. This current developed and decayed more quickly than the accompany ing changes in [Ca2+](i) (measured with indo-1) resulting in a hystere sis between [Ca2+](i) and current. A similar hysteresis was also obser ved if [Ca2+](i) was elevated with caffeine and was removed if the cur rent was low pass filtered with a time constant of 132 ms. 3. Digital video imaging (using flue-3 or calcium green-1 to measure [Ca2+](i)) a llowed measurement of [Ca2+](i) at all points in the cell during the w ave of spontaneous Ca2+ release. The hysteresis between [Ca2+](i) and current remained, even after allowing for the spatial and temporal pro perties of this wave.4. The hysteresis can be accounted for if there i s a barrier to diffusion of Ca2+ ions separating the bulk cytoplasm fr om the space under the sarcolemma (into which Ca2+ is released from th e 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 c onstant of 133 ms. 5. The subsarcolemmal space described in this paper may be equivalent to the 'fuzzy space' previously suggested to be imp ortant in controlling SR Ca2+ release.