Post-Tetanic Calcium Transients In Adult Skeletal Muscle Fibers Are Frequency-Dependent And Fiber Type Specific

Abstract

Isolated adult Flexor digitorum brevis fibers from 4-6 weeks-old mice, loaded with Fluo-3 were stimulated with trains of 270, 0.3 ms pulses at different frequencies. We observed a fast calcium tetanus (associated with contraction) and a second, slower signal, similar to those previously described in cultured myotubes. The slow signal (more than the fast one) was inhibited by 25 micro M nifedipine, suggesting a role for DHPR in its onset and by the IP3R inhibitor Xestospongin-C (5 micro M). The amplitude of post-tetanic calcium transients depended on both stimulus train frequency and duration; a bell shaped curve frequency was obtained with a maximum at 10-20 Hz. Likewise, signal amplitude was proportional to stimulus train duration. Fibers isolated from soleus muscle completely lack slow calcium transients. Using immunofluorescence, we have found that all three IP3R isoforms are present in adult muscle at different levels and that IP3R-1 is differentially expressed (with a mosaic pattern) in different types of muscle fibers, being higher in a subset of fast-type fibers. ERK 1/2 phosphorylation of adult muscle fibers after tetanic stimulation appears to relate slow calcium signals to transcription-related events. These results support the idea that different calcium kinetics for the slow signals mediated by IP3R may exist in different types of muscle fibers and participate in the activation of specific transcriptional programs of slow and fast phenotype.FONDAP 15010006, Bicentenario-PSD24, FONDECYT 1080120