Hindlimb unloading (HU) in rats induces severe atrophy and a slow-to-fast phenotype transition in postural slow-twitch muscles, as occurs in human disuse conditions, such as spaceflight or bed rest. In rats, a reduction of soleus muscle weight and a decrease of cross-sectional area (CSA) were observed as signs of atrophy. An increased expression of the fast-isoform of myosin heavy chain (MHC) showed the phenotype transition. In parallel the resting cytosolic calcium concentration (restCa) was decreased and the resting chloride conductance (gCl), which regulates muscle excitability, was increased toward the values of the fast-twitch muscles. Here, we investigated the possible role of taurine, which is known to modulate calcium homeostasis and gCl, in the restoration of muscle impairment due to 14-days-HU. We found elevated taurine content and higher expression of the taurine transporter TauT in the soleus muscle as compared to the fast-twitch extensor digitorum longus (EDL) muscle of control rats. Taurine level was reduced in the HU soleus muscle, although, TauT expression was not modified. Taurine oral supplementation (5 g/kg) fully prevented this loss, and preserved resting gCl and restCa together with the slow MHC phenotype. Taurine supplementation did not prevent the HU-induced drop of muscle weight or fiber CSA, but it restored the expression of MURF-1, an atrophy-related gene, suggesting a possible early protective effect of taurine. In conclusion, taurine prevented the HU-induced phenotypic transition of soleus muscle and might attenuate the atrophic process. These findings argue for the beneficial use of taurine in the treatment of disuse-induced muscle dysfunction.

Cozzoli A, Liantonio A, Conte E, Cannone M, Massari AM, Giustino A, Scaramuzzi A, Pierno S, Mantuano P, Capogrosso RF, Camerino GM, De Luca A. Angiotensin II modulates mouse skeletal muscle resting conductance to chloride and potassium ions and calcium homeostasis via the AT 1 receptor and NADPH oxidase. Am J Physiol Cell Physiol 307: C634 -C647, 2014. First published July 30, 2014; doi:10.1152/ajpcell.00372.2013.-Angiotensin II (ANG II) plays a role in muscle wasting and remodeling; however, little evidence shows its direct effects on specific muscle functions. We presently investigated the acute in vitro effects of ANG II on resting ionic conductance and calcium homeostasis of mouse extensor digitorum longus (EDL) muscle fibers, based on previous findings that in vivo inhibition of ANG II counteracts the impairment of macroscopic ClC-1 chloride channel conductance (gCl) in the mdx mouse model of muscular dystrophy. By means of intracellular microelectrode recordings we found that ANG II reduced gCl in the nanomolar range and in a concentration-dependent manner (EC 50 ϭ 0.06 M) meanwhile increasing potassium conductance (gK). Both effects were inhibited by the ANG II receptors type 1 (AT 1)-receptor antagonist losartan and the protein kinase C inhibitor chelerythrine; no antagonism was observed with the AT 2 antagonist PD123,319. The scavenger of reactive oxygen species (ROS) N-acetyl cysteine and the NADPH-oxidase (NOX) inhibitor apocynin also antagonized ANG II effects on resting ionic conductances; the ANG II-dependent gK increase was blocked by iberiotoxin, an inhibitor of calcium-activated potassium channels. ANG II also lowered the threshold for myofiber and muscle contraction. Both ANG II and the AT 1 agonist L162,313 increased the intracellular calcium transients, measured by fura-2, with a two-step pattern. These latter effects were not observed in the presence of losartan and of the phospholipase C inhibitor U73122 and the in absence of extracellular calcium, disclosing a G q-mediated calcium entry mechanism. The data show for the first time that the AT1-mediated ANG II pathway, also involving NOX and ROS, directly modulates ion channels and calcium homeostasis in adult myofibers.angiotensin II; chloride channel conductance; AT1 receptor; protein kinase C; NADPH oxidase IN FAST-TWITCH MUSCLE FIBERS, the macroscopic chloride conductance (gCl), due to the expression/activity of ClC-1 channel, accounts for ϳ80% of the total resting ionic conductance and ensures the electrical stability of myofibers (4, 9, 67, 68).In fact, the large gCl prevents membrane overexcitability due to potassium accumulation in transverse tubules during firing by reducing the length constant of electrotonic signal propagation (4, 9, 54). In spite of the important progress made in the last years in measuring ClC-1 chloride currents (25,28,48,60), macroscopic recordings of muscle gCl still provide crucial information about the function, pharmacology, and biochemical modulation of these channels in native skeletal muscle (8,19,20,22...

CLC-K blockers may represent a new class of drugs for the treatment of conditions associated with expanded extracellular volume, with a hopeful high therapeutic potential for hypertensive patients carrying ClC-K gain-of-function polymorphisms.

Hypertension is often associated with skeletal muscle pathological conditions related to function and metabolism. The mechanisms underlying the development of these pathological conditions remain undefined. Because calcium homeostasis is a biomarker of muscle function, we assessed whether it is altered in hypertensive muscles. We measured resting intracellular calcium and store-operated calcium entry (SOCE) in fast- and slow-twitch muscle fibers from normotensive Wistar-Kyoto rats and spontaneously hypertensive rats (SHRs) by cytofluorimetric technique and determined the expression of SOCE gene machinery by real-time PCR. Hypertension caused a phenotype-dependent dysregulation of calcium homeostasis; the resting intracellular calcium of extensor digitorum longus and soleus muscles of SHRs were differently altered with respect to the related muscle of normotensive animals. In addition, soleus muscles of SHR showed reduced activity of the sarcoplasmic reticulum and decreased sarcolemmal calcium permeability at rest and after SOCE activation. Accordingly, we found an alteration of the expression levels of some SOCE components, such as stromal interaction molecule 1, calcium release-activated calcium modulator 1, and transient receptor potential canonical 1. The hypertension-induced alterations of calcium homeostasis in the soleus muscle of SHRs occurred with changes of some functional outcomes as excitability and resting chloride conductance. We provide suitable targets for therapeutic interventions aimed at counterbalancing muscle performance decline in hypertension, and propose the reported calcium-dependent parameters as indexes to predict how the antihypertensive drugs could influence muscle function.

PIP2 activation of a voltage-dependent potassium channel (Kv7.1). Kv7.1 channels have a canonical Kv structure with a central pore-gate domain (PGD) surrounded by four peripheral voltage-sensing domains (VSDs). Kv channel activation is thought to involve two general steps. First, membrane depolarization is sensed by gating charges in the VSDs resulting in conformational changes within the VSDs from the resting state to the activated state. After all four VSDs have been activated, the second general step in Kv activation involves a concerted motion during which the PGD opens to allow ion permeation. In this study we ask if PIP2 regulates activation of Kv7.1 by potentiating the early VSD conformational changes, the concerted opening of the PGD, or both. Using the voltage-clamp fluorometry (VCF) technique to assay local conformation changes in the VSDs and the PGD simultaneously, we are able to show that although depletion of PIP2 eliminates the ionic current, the early conformational changes in the VSD do not require PIP2. This result indicates that the later conformational changes that couple VSD activation to the opening of the PGD are eliminated by PIP2 depletion. We continue this work by dissecting the molecular details of why PIP2 is required for opening of the PGD in response to VSD activation. These results may provide insights on the common principle of how Kv channels are modulated by PIP2.