Early changes of type 2B fibers after denervation of rat EDL skeletal muscle

Abstract

Early changes of type 2B fibers after denervation of rat EDL skeletal muscle. J Appl Physiol 92: 2045-2052, 2002. First published December 21, 2001 10.1152/japplphysiol.00673.2001.-Skeletal muscle type 2B fibers normally receive a moderate level of motoneuron discharge. As a consequence, we hypothesize that type 2B fiber properties should be less sensitive to the absence of the nerve. Therefore, we have investigated the response of sarcoplasmic reticulum and myofibrillar proteins of type 2B fibers isolated from rat extensor digitorum longus muscle after denervation (2 and 7 days). Single fibers were identified by SDS-PAGE of myosin heavy chain isoforms. Electrophysiological and isometric contractile properties of the whole muscle were also analyzed. The pCa-tension relationship of type 2B single fibers was shifted to the left at 2 days and to right at 7 days after denervation, with significant differences in the Hill coefficients and pCa threshold values in 2-vs. 7-day-denervated fibers. The sarcoplasmic reticulum Ca 2ϩ uptake capacity and rate significantly decreased after 2 days of denervation, whereas both increased at 7 days. Caffeine sensitivity of sarcoplasmic reticulum Ca 2ϩ release was transitory and markedly increased in 2-day-denervated fibers. Our results indicate that type 2B fiber functional properties are highly sensitive to the interruption of nerve supply. Moreover, most of 2-day-denervated changes were reverted at 7 days. calcium sensitivity; chemically skinned muscle fibers; sarcoplasmic reticulum; extensor digitorum longus SKELETAL MUSCLES ARE COMPOSED of a variety of fiber types with different morphological, biochemical, and functional characteristics (36,42). During embryonic and postnatal developments, differentiation into the different fiber types is mainly controlled by innervation, with the pattern and amount of neuromotor impulses playing a major role (21,35,42), although the weight of neurotrophic factors cannot be excluded. Therefore, it is not surprising that denervation causes profound changes to skeletal muscle properties. Some alterations, related to sarcoplasmic reticulum properties, occur very early after denervation, whereas alterations of the myofibrillar proteins profiles need, in general, more time. We have, in fact, previously described changes in sarcoplasmic reticulum properties of the slow-twitch type 1 fibers from denervated soleus muscle showing that the maximum Ca 2ϩ uptake capacity and rate were reduced after 2 days of denervation and increased at 7 days (33). Moreover, the initial Ca 2ϩ release rate was reduced in 2-day-denervated type 1 fibers. These changes in sarcoplasmic reticulum properties were related to mechanical changes of the denervated muscle.In this study, we examined whether also the sarcoplasmic reticulum properties of fast-twitch fibers are nerve dependent and, therefore, need an intact innervation. We focused our attention to type 2B fibers from the extensor digitorum longus (EDL) muscle, i.e., to fibers that, within the continuum of fiber types (1...

Denervation produces different single fiber phenotypes in fast- and slow-twitch hindlimb muscles of the rat

“…Thus, the pattern of fiber transformation upon denervation differs not only between EDL and SOL muscles from the same rat, but also between EDL muscles of different rat strains that have a different fiber type composition before denervation. The pattern of denervation-induced changes in MHC isoform composition in the EDL muscles nevertheless follows the general pattern of denervation-induced changes in MHC composition for fast-twitch muscles: from faster to slower MHC isoforms (34), although it appears from this study and others (13,17,47) that denervated EDL muscle does not express new MHC isoforms that were not expressed in the muscle before denervation.…”

“…However, type IIB/D EDL hybrids displayed SR Ca 2ϩ loading properties that were shifted toward the slow phenotype, suggesting that the SR Ca 2ϩ loading at pCa 7.1 was closer to capacity. Evidence of increased SR Ca 2ϩ loading in fibers from denervated EDL muscles was also reported for day 7 denervated Wistar rat (13) and day 14 denervated rabbit (44) EDL muscles. Because the time course of the twitch response is slower and the height of the twitch response is greater when the C526 SR of EDL fibers is loaded with Ca 2ϩ above endogenous levels (36), the presence in denervated EDL muscles of a large population of hybrid IIB/D fibers that are more loaded with Ca 2ϩ explains why the EDL twitch becomes slower and the twitch to tetanus ratio rises post-denervation (13,22).…”

“…Evidence of increased SR Ca 2ϩ loading in fibers from denervated EDL muscles was also reported for day 7 denervated Wistar rat (13) and day 14 denervated rabbit (44) EDL muscles. Because the time course of the twitch response is slower and the height of the twitch response is greater when the C526 SR of EDL fibers is loaded with Ca 2ϩ above endogenous levels (36), the presence in denervated EDL muscles of a large population of hybrid IIB/D fibers that are more loaded with Ca 2ϩ explains why the EDL twitch becomes slower and the twitch to tetanus ratio rises post-denervation (13,22). In relation to SR loading properties, Schulte et al (39) showed that denervation caused a marked decrease in the density of the fast isoform of the sarco(endo)plasmic reticulum Ca 2ϩ -adenosinetriphosphatase (SERCA1) in EDL muscles of Wistar rats 14 days after denervation.…”

“…Evidence was also provided that only the TnC-f isoform was functional in all EDL fibers examined from control and denervated muscles, indicating that the decrease in sensitivity to Ca 2ϩ and n Ca must be due to denervation-induced changes in myofibrillar components other than TnC. Germinario et al (13) also reported a small decrease in Ca 2ϩ sensitivity (pCa 50 ) at 7-day post-denervation in Wistar EDL fibers. These results rule out the possibility that the marked shift to more negative membrane potentials of the contraction threshold (31) and of the curve relating the strength of the K ϩ contractures to the membrane potential in intact denervated EDL muscle is due to an increased sensitivity of the contractile apparatus to Ca 2ϩ (8).…”

“…In Wistar-Kyoto rats, the EDL muscles contain in addition to the predominant IIb and IId MHC isoforms/type IIB and type IID fibers, 8 -20% IIa MHC/IIA fibers, and small amounts of MHCI/ type I fibers (13,17,47). In a previous study (2), we also found that type IIA and IIA/IID hybrid fibers were present in sizeable proportions in both EDL (9.6 and 17.3%) and SOL (7.5 and 8.2%) muscles of Wistar-Kyoto rats.…”