Microdomain [Ca2+] near ryanodine receptors as reported by L-type Ca2+and Na+/Ca2+exchange currents

During Ca2+ release from the sarcoplasmic reticulum triggered by Ca2+ influx through L-type Ca2+ channels (LTCCs), [Ca2+] near release sites ([Ca2+](nrs)) temporarily exceeds global cytosolic [Ca2+]. [Ca2+](nrs) can at present not be measured directly but the Na+/Ca2+ exchanger (NCX) near release sites and LTCCs also experience [Ca2+](nrs). We have tested the hypothesis that I-CaL and I-NCX could be calibrated to report [Ca2+](nrs) and would report different time course and values for local [Ca2+]. Experiments were performed in pig ventricular myocytes (whole-cell voltage-clamp, Fluo-3 to monitor global cytosolic [Ca2+], 37 degrees C). [Ca2+](nrs)-dependent inactivation of I-CaL during a step to +10 mV peaked around 10 ms. For I-NCX we computationally isolated a current fraction activated by [Ca2+](nrs); values were maximal at 10 ms into depolarization. The recovery of [Ca2+](nrs) was comparable with both reporters (> 90% within 50 ms). Calibration yielded maximal values for [Ca2+](nrs) between 10 and 15 mu mol l-1 with both methods. When applied to a step to less positive potentials (-30 to -20 mV), the time course of [Ca2+](nrs) was slower but peak values were not very different. In conclusion, both I-CaL inactivation and I-NCX activation, using a subcomponent analysis, can be used to report dynamic changes of [Ca2+](nrs). Absolute values obtained by these different methods are within the same range, suggesting that they are reporting on a similar functional compartment near ryanodine receptors. Comparable [Ca2+](nrs) at +10 mV and -20 mV suggests that, although the number of activated release sites differs at these potentials, local gradients at release sites can reach similar values.