Calcium-related defects in cardiac and skeletal muscles of dystrophic mice

Alterations in dihydropyridine receptors in dystrophin-deficient cardiac muscle

The deficiency of dystrophin, a critical membrane stabilizing protein, in the mdx mouse causes an elevation in intracellular calcium in myocytes. One mechanism that could elicit increases in intracellular calcium is enhanced influx via the L-type calcium channels. This study investigated the effects of the dihydropyridines BAY K 8644 and nifedipine and alterations in dihydropyridine receptors in dystrophin-deficient mdx hearts. A lower force of contraction and a reduced potency of extracellular calcium (P < 0.05) were evident in mdx left atria. The dihydropyridine agonist BAY K 8644 and antagonist nifedipine had 2.7- and 1.9-fold lower potencies in contracting left atria (P < 0.05). This corresponded with a 2.0-fold reduction in dihydropyridine receptor affinity evident from radioligand binding studies of mdx ventricular homogenates (P < 0.05). Increased ventricular dihydropyridine receptor protein was evident from both radioligand binding studies and Western blot analysis and was accompanied by increased mRNA levels (P < 0.05). Patch-clamp studies in isolated ventricular myocytes showed no change in L-type calcium current density but revealed delayed channel inactivation (P < 0.05). This study indicates that a deficiency of dystrophin leads to changes in dihydropyridine receptors and L-type calcium channel properties that may contribute to enhanced calcium influx. Increased influx is a potential mechanism for the calcium overload observed in dystrophin-deficient cardiac muscle.

Resting calcium concentrations in isolated skeletal muscle fibres of dystrophic mice

1. Single, intact muscle fibres were dissociated enzymatically from skeletal muscles of phenotypically normal (+/?) and dystrophic mice (129/ReJ dy/dy: Dystrophia muscularis), and resting Ca2+ levels were measured by image analysis of intracellular Fura-2 fluorescence in distinct parts of the fibres. 2. Fura-2 was introduced into fibres by ionophoresis with glass microelectrodes to concentrations of between 50 and 200 microM. Over this concentration range there was no apparent buffering of intracellular Ca2+ by Fura-2. 3. Fibres isolated from the soleus, flexor digitorum brevis (FDB) and extensor digitorum longus (EDL) muscles of normal animals maintained resting [Ca2+] of 106 +/- 2 nM. Ca2+ distributions within individual fibres were homogeneous. 4. Fibres from dystrophic animals maintained [Ca2+] that was elevated two- to fourfold in comparison to normal fibres. 5. The population of skeletal fibres from dystrophic mice which displayed morphology similar to that of fibres of normal animals were found to have Ca2+ levels that averaged 189 +/- 2 nM. The distribution of Ca2+ within these fibres appeared uniform. 6. The population of dystrophic fibres that possessed morphological abnormalities maintained even higher Ca2+ concentrations (368 +/- 3 nM). Several fibres from this morphological group displayed obvious heterogeneity in Ca2+ distribution with distinct, localized areas of higher Ca2+. 7. These results support the contention that Ca2+ homeostasis is markedly impaired in dystrophic muscle. The elevated Ca2+ levels are near the threshold for contraction and, together with severe morphological fibre abnormalities, are probably centrally involved in fibre necrosis apparent in muscular dystrophy.

Contractile activation and the effects of 2,3-butanedione monoxime (BDM) in skinned cardiac preparations from normal and dystrophic mice (129/ReJ)

Small cardiac myofibrillar preparations were obtained from the right ventricle of normal (129/ReJ) and dystrophic (129/ReJ dy/dy) mice and were chemically skinned in a relaxing solution by exposure to Triton X-100 (3% v/v). (2) The isometric force produced in these skinned cardiac preparations at different sarcomere lengths was measured in solutions of different [Ca2+] and ionic strength. The effect of the negative inotropic drug 2,3-butanedione monoxime (BDM), which is known to act at the myofibrillar level was also investigated. (3) The murine cardiac preparation from normal animals was found to develop 50% maximal force at a pCa (= -log10[Ca2+]) of 5.59 +/- 0.08 and 5.94 +/- 0.03 (mean +/- SD) under physiological (ionic equivalents concentration, I = 154 mM; pH 7.10; [Mg2+] 1 mM) and low ionic strength (I = 94 mM; pH 7.10; [Mg2+] 1 mM) conditions respectively. The isometric force curves were significantly shallower at low ionic strength (Hill coefficient, 1.8 +/- 0.1) than at physiological ionic strength (Hill coefficient, 2.6 +/- 0.3) and the sarcomere length effect on the force-pCa relation was markedly reduced at lower ionic strength. (4) Increasing BDM concentrations in solutions up to 100 mM reduced the maximum Ca2+-activated force of cardiac preparations from normal mice to less than 6% of the control values in a dose dependent fashion. BDM also rendered the cardiac preparations less sensitive to Ca2+ by a factor of up to 1.5 in a process which showed saturation at BDM concentrations higher than 15 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Severe mitochondrial anomaly in dystrophic mouse skeletal muscle

Mitochondrial fractions were isolated from skeletal muscle of control (C57 BL 6J dy/+) and dystrophic (C57 BL 6J dy/dy) mice, and enzymatic activities (cytochrome c oxidase, rotenone-insensitive NADH cytochrome c reductase) were determined. After electrophoretic separation, calcium-binding proteins were identified. An important anomaly was observed in the mitochondria of dystrophic muscle, i.e., a considerable reduction of a specific calcium-binding protein (61,000 Da mol. wt.).