Effects of pH on membrane resistance in normal and dystrophic mouse skeletal muscle fibers

The proton pump inhibitor lansoprazole improves the skeletal phenotype in dystrophin deficient mdx mice

Background

In Duchenne muscular dystrophy (DMD), loss of the membrane stabilizing protein dystrophin results in myofiber damage. Microinjury to dystrophic myofibers also causes secondary imbalances in sarcolemmic ion permeability and resting membrane potential, which modifies excitation-contraction coupling and increases proinflammatory/apoptotic signaling cascades. Although glucocorticoids remain the standard of care for the treatment of DMD, there is a need to investigate the efficacy of other pharmacological agents targeting the involvement of imbalances in ion flux on dystrophic pathology.

Methodology/Principal Findings

We designed a preclinical trial to investigate the effects of lansoprazole (LANZO) administration, a proton pump inhibitor, on the dystrophic muscle phenotype in dystrophin deficient (mdx) mice. Eight to ten week-old female mice were assigned to one of four treatment groups (n = 12 per group): (1) vehicle control; (2) 5 mg/kg/day LANZO; (3) 5 mg/kg/day prednisolone; and (4) combined treatment of 5 mg/kg/day prednisolone (PRED) and 5 mg/kg/day LANZO. Treatment was administered orally 5 d/wk for 3 months. At the end of the study, behavioral (Digiscan) and functional outcomes (grip strength and Rotarod) were assessed prior to sacrifice. After sacrifice, body, tissue and organ masses, muscle histology, in vitro muscle force, and creatine kinase levels were measured. Mice in the combined treatment groups displayed significant reductions in the number of degenerating muscle fibers and number of inflammatory foci per muscle field relative to vehicle control. Additionally, mice in the combined treatment group displayed less of a decline in normalized forelimb and hindlimb grip strength and declines in in vitro EDL force after repeated eccentric contractions.

Conclusions/Significance

Together our findings suggest that combined treatment of LANZO and prednisolone attenuates some components of dystrophic pathology in mdx mice. Our findings warrant future investigation of the clinical efficacy of LANZO and prednisolone combined treatment regimens in dystrophic pathology.

Excitation contraction coupling in normal and mdx mice

The protein dystrophin is absent from patients with Duchenne muscular dystrophy and from the muscles of mdx mice. Recent studies have shown that dystrophin is located at the surface membrane and at the triadic junction, where it is associated with the transverse tubular membrane. Since the triadic junction is the site of excitation-contraction (EC) coupling, we have investigated whether intramembrane charge movement, a step in EC coupling, is modified by the absence of dystrophin. Charge movements are thought to arise from the transverse tubular membrane and to underlie the dependence of sarcoplasmic reticulum Ca2+ release on transverse tubular membrane potential. We find no differences between intramembrane charge movements or passive membrane electrical properties measured in muscles from mdx mice compared with normal mice. If dystrophin does play a role in EC coupling, that role is likely to be subsequent to the charge movement step.

Dystrophin and the integrity of the sarcolemma in Duchenne muscular dystrophy

It is suggested that in Duchenne muscular dystrophy the absence of dystrophin, which is probably a cytoskeletal protein underlying the sarcolemma, causes changes in stretch-activated cation channels rather than direct mechanical tearing of the surface membrane.

Ca-dependent slow action potentials in human skeletal muscle

Slow Ca-action potentials (CaAP) were studied in normal human skeletal muscle fibers obtained during surgery (fibers with both ends cut). Control studies also were carried out with intact as well as cut rat skeletal muscle fibers. Experiments were performed in hypertonic Cl-free saline with 10 or 84 mM Ca and K-channel blockers; muscles were preincubated in a saline containing Cs and tetraethylammonium. A current-clamp technique with two intracellular microelectrodes was used. In human muscle, 14.5% of the fibers showed fully developed CaAPs, 21% displayed nonregenerative Ca responses, and 64.5% showed only passive responses; CaAPs were never observed in 10 mM Ca. In rat muscle, nearly 90% of the fibers showed CaAPs, which were not affected by the cut-end condition. Human and rat muscle fibers had similar membrane potential and conductance in the resting state. In human muscle (22-32 degrees C, 84 mM Ca), the threshold and peak potential during a CaAP were +26 +/- 6 mV and +70 +/- 3 mV, respectively, and the duration measured at threshold level was 1.7 +/- 0.5 sec. In rat muscle, the duration was four times longer. During a CaAP, membrane conductance was assumed to be a leak conductance in parallel with a Ca and a K conductance. In human muscle (22-32 degrees C, 84 mM Ca, 40 micron fiber diameter), values were 0.4 +/- 0.1 microS, 1.1 +/- 0.7 microS, and 0.9 +/- 0.4 microS, respectively. Rat muscle (22-24 degrees C, 84 mM Ca) showed leak and K conductances similar to those found in human fibers. Ca-conductance in rat muscle was double the values obtained in human muscle fibers.

Comparison of force and stiffness in normal and dystrophic mouse muscles

Isometric force and stiffness of fast- and slow-twitch muscles of affected and normal mice of the 129/ReJ dy/dy strain were studied at rest and during active contraction at a variety of lengths. Dystrophic muscles developed less force in response to stimulation, but the resting stiffness was not reduced as much, particularly at long muscle lengths. This is consistent with the replacement of muscle fibers by connective tissue that is considerably less elastic. When second and third stimuli are superimposed on the rising phase of a twitch in a normal muscle, a less-than-linear summation of force and stiffness generation (early depression) is followed by a more-than-linear summation (later facilitation). Dystrophic muscles showed a smaller early depression and a greater later facilitation of force and active muscle stiffness. Many of these phenomena can be predicted from a simple model of Ca2+ release and binding to troponin.

Potassium and caffeine contractures in limb muscles of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice

The strength of contractures, produced by 15 to 146 mM [K]0 (as L-glutamate), was measured in isolated small bundles of muscle fibers from the fast-twitch extensor digitorum longus and from the slow-twitch soleus of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice. The analysis of the relation between the maximal amplitude of the contracture vs the membrane potential and the time constant of relaxation of the K-contractures has shown that dystrophy induced an attenuation of the differences between fast- and slow-twitch muscles. The repriming of K-contractures was more affected by changes in [Ca]0 in normal soleus than in normal extensor digitorum longus and this difference was unaffected by dystrophy. For both types of muscles, the ability of caffeine to produce contractures was reduced in dystrophic muscle and this modification was not related to a change in the fiber typing.