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

Dihydropyridine receptor gene expression in skeletal muscle from mdx and control mice

The expression of isoform-specific dihydropyrine receptor-calcium channel (DHPR) alpha 1-subunit genes was investigated in mdx and control mouse diaphragm (DIA) and tibialis anterior (TA). RNase protection assays were carried out with a rat DHPR cDNA probe specific for skeletal muscle and a mouse DHPR cDNA probe specific for cardiac muscle. The level of expression of the gene encoding the cardiac DHPR was very weak in TA muscle from both control and mdx mice. Compared to TA, DIA expressed mRNA for the cardiac isoform at significantly higher levels, but mdx and control mouse DIA levels were similar to one another. In contrast, mRNA expression levels for the DHPR skeletal muscle isoform were lower in control DIA than TA. However, there was a dramatic increase in the expression for the DHPR skeletal muscle isoform in mdx DIA compared with control DIA, reaching the TA expression level, whereas dystrophy did not affect TA expression. [3H]-PN200-110 binding was used to further assess DIA DHPR expression at the protein level. The density of binding sites for the probe was not significantly affected in DIA muscles of mdx vs. control mice, but it was reduced in older mdx and control mice. The increase in DHPR mRNA levels without a consequent increase in DHPR protein expression could be secondary to possible enhanced protein degradation which occurs in mdx DIA. The altered DHPR expression levels found here do not appear to be responsible for the severe deficits in contractile function of the mdx DIA.

Activation of two types of fibres in ferret, Mustela putorius furo, cremaster muscle

Some contractile, histochemical, morphological and electrophysiological properties of ferret, Mustela putorius furo, cremaster muscle have been estimated. Histochemical fibre typing revealed the presence of two types of fibres (type I 66.2%, type II 33.8%). Morphometry performed on ATPase-stained transverse sections showed that type I was composed of a large amount (40%) of small (less than 1400 microns2) cells. In mammalian Ringer two groups of fibres could be recognized on the basis of the values of resting potential (-69.7 mV and -59.1 mV) intracellular sodium activity (8.3 mmol.l-1 and 14.1 mmol.l-1, respectively). In experiments on fibre bundles, the elevation of extracellular potassium concentration to 15-200 mmol.l-1 produced contractures that consisted of a well-defined transient or phasic tension followed by a sustained or tonic tension. Properties of activation and inactivation of the tension analysed in small bundles of cut fibres (lengths 0.5-1.0 cm) were of fast- and slow-twitch type for phasic and tonic phase, respectively. In contrast to the phasic component of K contractures, the tonic phase was abolished by Ca2+ withdrawal and inhibited by Ni2+, Cd2+, Co2+, Gd3+ and gallopamil (D600). In Ca(2+)-free medium the sustained tension was restored by adding Sr2+. It is concluded that in ferret cremaster muscle the presence of slow-twitch fibres would give rise to the tonic component of the K contracture in which an extracellular source of activator Ca2+ is involved. The ability of these fibres to contract with a maintained tension for prolonged periods of time might participate in the temperature regulation of the testes.

Potassium contractures in the mouse diaphragm: regional variation in the multiphasic response

1. Submaximal potassium-induced contractures were studied in isolated mouse hemidiaphragms (HD) and compared with extensor digitorum longus (EDL) and soleus. 2. In the HD, the contracture was triphasic: an initial small, fast phase followed by a larger slow phase, and a late phase which was seen on prolonged exposure. 3. Costal segments of the HD responded to elevated potassium with a larger fast phase, while dorsal or ventral segments produced a larger slow phase. 4. In calcium-free solution, the fast phase appeared to be unaffected, while the slow phase was potentiated and its time course abbreviated. The late phase was greatly attenuated or abolished. 5. In the soleus, but not the EDL muscle, contracture amplitude in calcium-free solution was potentiated and the time course was shortened. 6. The characteristics of potassium-induced contractures of both the soleus and EDL muscles are to be seen in the response of the hemidiaphragm, but there is a regional variation in their proportional contribution which suggests a heterogeneity in diaphragm composition.

Acetylcholine-sensitivity in fast and slow twitch muscle of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice

Small bundles of muscle fibres were isolated from diaphragm, extensor digitorum longus (EDL) and soleus (SOL) muscles of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice, and their isometric tension developed in response to acetylcholine (ACh) was recorded. For each type of muscle the relationship between the maximum amplitude of the ACh-contracture and log [ACh] was similar in normal and dystrophic animals. However, this relationship was steeper for normal and dystrophic SOL than for EDL and diaphragm muscles. Dystrophy did not induce changes in the time course of the ACh-contractures, except a significant 'speeding' of dystrophic SOL that appeared in the time to peak of the contractile response. The amplitude of ACh-contractures of both normal and dystrophic diaphragm preparations increased by about 50% after perfusion for 80-90 min in physiological solution containing phospholipase C 5 mU/ml. ACh-sensitivity was measured in normal and dystrophic diaphragm preparations by iontophoretic application of ACh from high resistance pipettes. ACh-potentials were similar in time course in the two types of muscle fibres, and there was also no significant difference in the length of sensitive fibre segments and maximum sensitivity values. Extrajunctional ACh-sensitivity was absent in normal as well as in dystrophic fibres. It is concluded that the absence in dystrophic muscles of stronger ACh-contractures and of extrajunctional sensitivity can be considered as evidence against a primary neuronal involvement in murine dystrophy of the dy2J strain.