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SUMMARY1. A study was carried out to investigate the effects of ionic strength and monovalent cations on isometric, Ca2+-activated force and rigor responses in mechanically skinned muscle fibres. Three types of skeletal muscle fibres were used: rat fast-and slow-twitch fibres and toad twitch fibres.2. The contractile apparatus of rat slow-twitch fibres was affected differently from that of rat fast-twitch and amphibian twitch fibres when changing the ionic strength (expressed either in terms of ionic equivalents as I or formally as F/2) and [K+]. Thus, the apparent sensitivity to Ca2+ decreased substantially more in slow-twitch fibres (by a factor of 20) than in the other fibre types (by a factor of 12) when I and [K+] were increased from 94 to 354 mm and from 56 to 316 mm respectively. Maximum Ca2+-activated force, however, declined only by a factor of 2-2 in slow-twitch fibres compared with 4-2 in the other fibre types, when I was increased from 154 to 354 mm.3. In slow-twitch fibres the force oscillations of myofibrillar origin were found to increase substantially in amplitude, duration and frequency at low values of I and almost disappeared at high ionic strength. At low values of I, it was also discovered that ca. 50 % of the fast-twitch fibres responded with myofibrillar force oscillations when submaximally activated. The characteristics of these oscillations were different from those of slow-twitch fibres.4. Rigor force levels were found to decline markedly with increasing I and [K+] in all fibre types. Unexpectedly, once rigor force was established in a certain ionic environment, the level of force was stable regardless of further changes in ionic strength and monovalent cation concentration. These results indicate that the rigor cross-bridges can be formed in different stable positions and that the probability of attachment in certain positions (rather than the total number of cross-bridges that can be formed) is influenced by the ionic conditions. 5. Further experimental evidence provided in this study shows that the increase in [K+] is mainly responsible for the decrease of the Ca2+-sensitivity of the contractile apparatus and that ionic strength (expressed as I rather than F/2) influences markedly (i) the maximal Ca2+-activated force, (ii) the maximum steepness of the pCa-force relations and (iii) the oscillatory processes of myofibrillar origin.

SUMMARY1. Single intact muscle fibres were enzymatically isolated from the skeletal muscles of the dystrophic mouse 129/ReJ dy/dy and were subjected to a range of physiological interventions.2. Electrophysiological measurements, diffusion of injected dyes (Lucifer Yellow), microdissection and general appearance in the light microscope have shown that the majority of skeletal fibres isolated from the soleus and extensor digitorum longus (EDL) of adult dystrophic mice (10-14 weeks old) had gross morphological abnormalities. These abnormalities ranged from simple branching of the fibre to interconnections of many fibre branches which form a complex syncitium.3. Segments from fibres of normal appearance and from fibres with morphological deformities were chemically skinned with Triton X-100 and activated in Ca2+-and Sr2+-buffered solutions. The different characteristics of the Ca2+-and Sr2+-activation curves were also used to identify the fibre type.4. Gross morphological abnormalities were observed both in fibres which had predominantly slow-twitch and fast-twitch characteristics. 5. A new group of fibres was found to exist in the soleus mnuscle of dystrophic animals and represented about 18 % of the entire soleus fibre population. This group of fibres had predominantly fast-twitch characteristics and some of these fibres were also grossly malformed.6. The activation characteristics of individual branches from the same complex syncitium were similar, indicating that the contractile and regulatory proteins were of one type in one syncitium.7. Chemically skinned segments from malformed fibres which included a major deformity between the points of attachment were generally unable to sustain nearmaximal forces. 8. The proportion of malformed fibres which remained intact decreased markedly after prolonged tetanical stimulation of the intact muscle. This strongly suggests that malformed fibres are also functionally weak and prone to progressive damage when stimulated within the intact muscle. 9. The presence in large proportions of fibres with gross morphological ab-MS 7621

In this study we have shown that the skeletal muscle fibres from adult (older than 26 weeks) mdx mice have gross structural deformities. We have characterized the onset and age dependence of this feature in mdx mice. The three dimensional structure of these deformities has been visualized in isolated fibres and the orientation of these deformities was determined within the muscle by confocal laser scanning microscopy. We have also shown that the occurrence of morphologically abnormal fibres is greater in muscles with longer fibres (extensor digitorum longus (EDL) and soleus, 6-7.3 mm long), than in muscles with shorter fibres (flexor digitorum brevis (FDB), 0.3-0.4 mm long). A population of post-degenerative fibres, with both central and peripheral nuclei coexistent along the length of the fibre, has also been identified in the muscles studied. We showed that a mild protocol of lengthening (eccentric) contractions (the muscle was stretched by 12% during a tetanic contraction) caused a major reduction in the maximal tetanic force subsequently produced by mdx EDL muscle. In contrast, maximal tetanic force production in normal soleus, normal EDL and mdx soleus muscles was not altered by this protocol. We suggest that the deformed fast glycolytic fibres which are found in adult mdx EDL but not in adult mdx soleus muscles are the population of fibres damaged by the lengthening protocol.

We report observations of tunneling anisotropic magnetoresitance (TAMR) in vertical tunnel devices with a ferromagnetic multilayer-(Co/Pt) electrode and a non-magnetic Pt counter-electrode separated by an AlOx barrier. In stacks with the ferromagnetic electrode terminated by a Co film the TAMR magnitude saturates at 0.15% beyond which it shows only weak dependence on the magnetic field strength, bias voltage, and temperature. For ferromagnetic electrodes terminated by two monolayers of Pt we observe order(s) of magnitude enhancement of the TAMR and a strong dependence on field, temperature and bias. Discussion of experiments is based on relativistic ab initio calculations of magnetization orientation dependent densities of states of Co and Co/Pt model systems.PACS numbers: 85.75. Mm,75.45.+j,75.50.Cc Anisotropic magnetoresistance (AMR) sensors replaced in the early 1990s classical magneto-inductive coils in hard-drive readheads launching the era of spintronics. Their utility has, however, remained limited partly because the response of these ferromagnetic resistors to changes in magnetization orientation originates from generically subtle spin-orbit (SO) interaction effects [1]. Currently widely used giant magnetoresistance [2] and tunneling magnetoresistance (TMR) [3] elements comprising (at least) two magnetically decoupled ferromagnetic layers provided a remarkably elegant way of tying the magnetoresistance response directly to the ferromagnetic exchange splitting of the carrier bands without involving SO-coupling. Large magnetoresistances in these devices are, nevertheless, obtained at the expense of a significantly increased structure complexity, necessary to guarantee independent and different magnetization switching characteristics and spin-coherence of transport between the ferromagnetic layers.Studies of AMR effects [4,5,6,7] in ferromagnetic semiconductor tunneling devices showed that AMR response can in principle be huge and richer than TMR, with the magnitude and sign of the magnetoresistance dependent on the magnetic field orientation and electric fields. Subsequent theoretical work predicted [8] that the tunneling AMR (TAMR) effect is generic in ferromagnets with SO-coupling, including the high Curie temperature transition metal systems. A detailed investigation of the TAMR is therefore motivated both by its intricate relativistic quantum transport nature and by its potential in more versatile alternatives to current TMR devices which will not require two independently controlled ferromagnetic electrodes and spin-coherent tunneling.Experimental demonstration of the TAMR in a tunnel junction with a ferromagnetic metal electrode has recently been reported [9] in an epitaxial Fe/GaAs/Au stack. The observed TAMR in this structure is relatively small, bellow 0.5%, consistent with the weak SO-coupling in Fe. In this paper we present a study of vertical tunnel devices in which the ferromagnetic electrode comprises alternating Co and Pt films. We build upon the extensive literature [10,11,12,13,14,15] o...