Skeletal muscle fibres contain ubiquitous and muscle-specific calcium-dependent proteases known as calpains. During normal activity, intracellular [Ca 2+ ] in muscle fibres increases to high levels (∼2-20 µM), and it is not apparent how this can be reconciled with the activation properties of the calpains. Calpains evidently do not cause widespread proteolytic damage within muscle fibres under normal circumstances, but do have a role in necrosis in dystrophic muscle fibres. In this study, we examined the in situ localization and regulation of calpains in muscle fibres in order to identify how they are attuned to normal function. The sarcolemma of individual muscle fibres of the rat was removed by microdissection (fibre 'skinning') in order to determine the compartmentalization and diffusibility of the two most Ca 2+ -sensitive calpains, µ-calpain and calpain-3, and to permit precise manipulation of cytoplasmic [Ca 2+ ] under physiological in situ conditions. Passive force production in stretched fibres, which indicates the patency of the important elastic structural protein titin, was used as a sensitive assay of the amount of diffusible proteolytic activity in individual fibre segments and in muscle homogenates at set [

Atrial contractile dysfunction after short-term atrial fibrillation is reduced by the calcium antagonist verapamil, which suggests that transsarcolemmal calcium influx contributed to this dysfunction. The calcium agonist BAY K8644 increased postfibrillation atrial contractile dysfunction. Atrial ischemia was not observed during fibrillation.

] to 10 nM, and was almost completely inhibited by 1 mM leupeptin but not by 10 µM calpastatin. Muscle homogenates preactivated by Ca 2+ exposure also evidently contained a diffusible factor that caused damage to passive force production in a Ca 2+ -dependent manner. Western blotting showed that: (a) calpain-3 was present in the skinned fibres and was activated by the Ca 2+ exposure, and (b) the Ca 2+ exposure in stretched skinned fibres resulted in proteolysis of titin. We conclude that the disruption of EC coupling occurring at elevated levels of [Ca 2+ ] is likely to be caused at least in part by Ca 2+ -activated proteases, most likely by calpain-3, though a role of calpain-1 is not excluded.

We hypothesized that in congestive heart failure (CHF) slow‐twitch but not fast‐twitch muscles exhibit decreased fatigue resistance in the sense of accelerated reduction of muscle force during activity. Experiments were carried out on anaesthetized rats 6 weeks after induction of myocardial infarction or a sham operation (Sham). Animals with left ventricular end‐diastolic pressure (LVEDP) > 15 mmHg under anaesthesia were selected for the CHF group. There was no muscle atrophy in CHF. Force generation by in situ perfused soleus (Sol) or extensor digitorum longus (EDL) muscles was recorded during stimulation (trains at 5 Hz for 6 s (Sol) or 10 Hz for 1.5 s (EDL) at 10 or 2.5 s intervals, respectively) for 1 h in Sol and 10 min in EDL at 37 °C. Initial force was almost the same in Sol from CHF and Sham rats, but relaxation was slower in CHF. Relaxation times (95–5 % of peak force) were 177 ± 55 and 131 ± 44 ms in CHF and Sham, respectively, following the first stimulation train. After 2 min of stimulation the muscles transiently became slower and maximum relaxation times were 264 ± 71 and 220 ± 45 ms in CHF and Sham, respectively (P < 0.05). After 60 min they recovered to 204 ± 60 and 122 ± 55 ms in CHF and Sham, respectively (P < 0.05). In CHF but not in Sham rats the force of contraction of Sol declined from the second to the sixtieth minute to 70 % of peak force. The EDL of both CHF and Sham fatigued to 24–28 % of initial force, but no differences in contractility pattern were detected. Thus, slow‐twitch muscle is severely affected in CHF by slower than normal relaxation and significantly reduced fatigue resistance, which may explain the sensation of both muscle stiffness and fatigue in CHF patients.