Cav1.1 Acts as a Voltage Sensor for Two Separate Processes in Skeletal Muscle with Different Voltage Dependence

Abstract

In adult muscle fibers, tetanic electrical stimulation induces two separate calcium signals, a fast one related to contraction and dependent on Cav1.1 (DHPR) and RyRs and a slow signal, dependent on DHPR and IP3Rs. We have recently shown that the amplitude of this slow signal is dependent on frequency of stimulation, having a maximum at 20 Hz. Importantly, this signal participates in the activation of genes related to slow muscle fiber type phenotype. This signal is inhibited by nifedipine, supporting a role for the DHPR in its onset. We have stimulated adult muscle fibers with different concentrations of extracellular K+, and found calcium signals at more negative membrane potential than those described for triggering calcium release associated with RyR. Moreover, in the same conditions, we observe an increase in mRNA of the slow isoform of TroponinI, as seen in fibers stimulated with a 20Hz train of pulses. These signals have an intracellular origin, because they are still present when working in solutions with no extracellular calcium and 0.1 mM EGTA. Interestingly, they are inhibited by incubation of fibers with 20 uM XestosponginB, a specific inhibitor of IP3R. These results strongly suggest that the slow signal fires at membrane potential values more negative (Nernst potential estimated −50 mV) than the fast signal (E-C coupling related), where the fast signal threshold (−30 to −20 mV) has not yet been reached. This opens the possibility for a different part of the DHPR molecule being involved in the signal transmission of depolarization to IP3-production machinery, independently of the process of E-C coupling.FONDECYT 1080120, FONDAP 15010006.