title: Microdomain [Ca²⁺] near ryanodine receptors as reported by L-type Ca²⁺ and Na+/Ca²⁺ exchange currents.
abstract: 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 ICaL and INCX 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◦C). [Ca²⁺]nrs-dependent inactivation of ICaL during a step to +10 mV peaked around 10 ms. For INCX we computationally isolateda 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 μmol 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 ICaL inactivation and INCX 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.
authors.affiliation: Lab of Experimental Cardiology, Catholic University of Leuven, Belgium.
authors.forename: Karoly
authors.initials: K
authors.lastname: Acsai
authors.forename: Gudrun
authors.initials: G
authors.lastname: Antoons
authors.forename: Leonid
authors.initials: L
authors.lastname: Livshitz
authors.forename: Yoram
authors.initials: Y
authors.lastname: Rudy
authors.forename: Karin R
authors.initials: KR
authors.lastname: Sipido
mesh_terms: D000814:Aniline Compounds / Q000008:administration & dosage; D000818:Animals; D002118:Calcium / Q000032:analysis / Q000378:metabolism / Q000502:physiology; D020746:Calcium Channels, L-Type / Q000502:physiology; D005456:Fluorescent Dyes / Q000008:administration & dosage; D021962:Membrane Microdomains / Q000502:physiology*; D008564:Membrane Potentials / Q000502:physiology; D032383:Myocytes, Cardiac / Q000502:physiology; D019837:Ryanodine Receptor Calcium Release Channel / Q000502:physiology*; D012519:Sarcoplasmic Reticulum / Q000378:metabolism / Q000502:physiology; D019831:Sodium-Calcium Exchanger / Q000502:physiology*; D013552:Swine; D016276:Ventricular Function; D014966:Xanthenes / Q000008:administration & dosage
chemical_list: D000814:Aniline Compounds; D020746:Calcium Channels, L-Type; D005456:Fluorescent Dyes; D019837:Ryanodine Receptor Calcium Release Channel; D019831:Sodium-Calcium Exchanger; D014966:Xanthenes; C059715:Fluo-3; D002118:Calcium