Spontaneous and Voltage-Activated Elementary Calcium Release Events in Intact Skeletal Muscle Fibers Expressing the Embryonic CaV1.1 Splice Variant

Abstract

The adult isoform of voltage-gated L-type calcium channel (CaV1.1a) opens slowly at strong depolarizations, its contribution to the calcium influx during an action potential is negligible in skeletal muscle. In contrast, calcium influx through the embryonic splice variant (CaV1.1e) substantially contributes to depolarization-induced calcium transients as this isoform displays an altered voltage-dependence and gating kinetics as compared to CaV1.1a. Utilizing a genetically modified mouse model (CaV1.1αE29), which exclusively expresses the embryonic CaV1.1e variant in adult muscle, spontaneous or depolarization-evoked calcium release events were recorded in enzymatically isolated, intact adult skeletal muscle fibers from the m. flexor digitorum brevis, using the fluorescent calcium probe Fluo-8 and the Zeiss Live fast confocal microscope equipped with an Axon Multiclamp 700b. The events occurred at preferred locations as the distribution of spark number in the given position was different from the theoretical Poisson distribution which represents random event occurrence. Calcium events with both short (spark-like, duration = 46.0±1.5 ms, n = 125) and long (ember-like, duratio n= 473±14 ms, n = 218) duration were present on the same cell. In addition, events with complex kinetics, including late openings within an event also appeared. Voltage-activated (500 ms long pulses to the threshold of SR calcium release) with similar properties were also observed.