Microdomain (Ca(2)) near ryanodine receptors as reported by L-type Ca(2) and Na+/Ca(2) exchange currents.

摘要

During Ca(2) release from the sarcoplasmic reticulum triggered by Ca(2) influx through L-type Ca(2) channels (LTCCs), [Ca(2)] near release sites ([Ca(2)]nrs) temporarily exceeds global cytosolic [Ca(2)]. [Ca(2)]nrs can at present not be measured directly but the Na+/Ca2+ exchanger (NCX) near release sites and LTCCs also experience [Ca(2)]nrs. We have tested the hypothesis that ICaL and INCX could be calibrated to report [Ca(2)]nrs and would report different time course and values for local [Ca(2)]. Experiments were performed in pig ventricular myocytes (whole-cell voltage-clamp, Fluo-3 to monitor global cytosolic [Ca(2)], 37C). [Ca(2)]nrs-dependent inactivation of ICaL during a step to +10 mV peaked around 10 ms. For INCX we computationally isolateda current fraction activated by [Ca(2)]nrs; values were maximal at 10 ms into depolarization. The recovery of [Ca(2)]nrs was comparable with both reporters (>90% within 50 ms). Calibration yielded maximal values for [Ca(2)]nrs between 10 and 15 mumol l(1) with both methods. When applied to a step to less positive potentials (-30 to -20 mV), the time course of [Ca(2)]nrs was slower but peak values were not very different. In conclusion, both ICaL inactivation and INCX activation, using a subcomponent analysis, can be used to report dynamic changes of [Ca(2)]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 [Ca(2)]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.