Angiotensin II increases nerve-evoked contractions in mouse tail artery by a T-type Ca(2+) channel-dependent mechanism.

Angiotensin II (Ang II) increases sympathetic nerve-evoked contractions of arterial vessels. Here the mechanisms underlying this effect were investigated in mouse tail artery. Isometrically mounted segments of mouse distal tail artery were used to investigate the effects of endothelium denudation, blocking Ca(2+) channels and inhibiting superoxide signalling on Ang II-induced facilitation of nerve-evoked contractions. In addition, in situ amperometry was used to assess effects of Ang II on noradrenaline release. Ang II (0.1-1nM) increased nerve-evoked contractions but did not change noradrenaline release. Losartan (Ang II type 1 receptor antagonist), but not PD 123319 (Ang II type 2 receptor antagonist), blocked the facilitatory effect of Ang II on nerve-evoked contractions. Ang II increased vascular muscle reactivity to phenylephrine and UK-14304 (α1- and α2-adrenoceptor agonists, respectively). Endothelial denudation increased nerve-evoked contractions and reduced the facilitatory effect of Ang II on these responses. Efonidipine (L- and T-type Ca(2+) channel blocker) and NNC 55-0396 (T-type Ca(2+) channel blocker) also attenuated this effect of Ang II, while nifedipine (L-type Ca(2+) channel blocker) did not. Blockers of superoxide generation/signalling did not change the facilitatory effect of Ang II on nerve-evoked contractions. The findings indicate that Ang II increases the contribution of T-type Ca(2+) channels to neural activation of the vascular muscle. In addition, Ang II appears to reduce the inhibitory influence of the endothelium on nerve-evoked contractions.

Hydrogen peroxide increases nerve-evoked contractions in mouse tail artery by an endothelium-dependent mechanism.

Reactive oxygen species contribute to regulating the excitability of vascular smooth muscle. This study investigated the actions of the relatively stable reactive oxygen species, H(2)O(2), on nerve-evoked contractions of mouse distal tail artery. H(2)O(2) (10-100 µM) increased nerve-evoked contractions of isometrically mounted segments of tail artery. Endothelium denudation increased nerve-evoked contractions and abolished the facilitatory effect of H(2)O(2). Inhibition of nitric oxide synthase with L-nitroarginine methyl ester (0.1mM) also increased nerve-evoked contractions and reduced the late phase of H(2)O(2)-induced facilitation. H(2)O(2)-induced facilitation of nerve-evoked contractions depended, in part, on synthesis of prostanoids and was reduced by the cyclooxygenase inhibitor indomethacin (1 µM) and the thromboxane A(2) receptor antagonist SQ 29548 (1 µM). H(2)O(2) increased sensitivity of nerve-evoked contractions to the α(2)-adrenoceptor antagonist idazoxan (0.1 µM) but not to the α(1)-adrenoceptor antagonist prazosin (10nM). Idazoxan and the α(2C)-adrenoceptor antagonist JP 1302 (0.5-1 µM) reduced H(2)O(2)-induced facilitation. H(2)O(2) induced facilitation of nerve-evoked contractions was abolished by the non-selective cation channel blocker SKF-96365 (10 µM), suggesting it depends on Ca(2+) influx. In conclusion, H(2)O(2)-induced increases in nerve-evoked contractions depended on an intact endothelium and were mediated by activating thromboxane A(2) receptors and by increasing the contribution of α(2)-adrenoceptors to these responses.

Pathways of Ca²âº entry and cytoskeletal damage following eccentric contractions in mouse skeletal muscle.

Muscles that are stretched during contraction (eccentric contractions) show deficits in force production and a variety of structural changes, including loss of antibody staining of cytoskeletal proteins. Extracellular Ca(2+) entry and activation of calpains have been proposed as mechanisms involved in these changes. The present study used isolated mouse extensor digitorum longus (EDL) muscles subjected to 10 eccentric contractions and monitored force production, immunostaining of cytoskeletal proteins, and resting stiffness. Possible pathways for Ca(2+) entry were tested with streptomycin (200 µM), a blocker of stretch-activated channels, and with muscles from mice deficient in the transient receptor potential canonical 1 gene (TRPC1 KO), a candidate gene for stretch-activated channels. At 30 min after the eccentric contractions, the isometric force was decreased to 75 ± 3% of initial control and this force loss was reduced by streptomycin but not in the TRPC1 KO. Desmin, titin, and dystrophin all showed patchy loss of immunostaining 30 min after the eccentric contractions, which was substantially reduced by streptomycin and in the TRPC1 KO muscles. Muscles showed a reduction of resting stiffness following eccentric contractions, and this reduction was eliminated by streptomycin and absent in the TRPC1 KO muscles. Calpain activation was determined by the appearance of a lower molecular weight autolysis product and µ-calpain was activated at 30 min, whereas the muscle-specific calpain-3 was not. To test whether the loss of stiffness was caused by titin cleavage, protein gels were used but no significant titin cleavage was detected. These results suggest that Ca(2+) entry following eccentric contractions is through a stretch-activated channel that is blocked by streptomycin and encoded or modulated by TRPC1.

Time to fatigue is increased in mouse muscle at 37 degrees C; the role of iron and reactive oxygen species.

Studies exploring the rate of fatigue in isolated muscle at 37 degrees C have produced mixed results. In the present study, muscle fibre bundles from the mouse foot were used to study the effect of temperature on the rate of muscle fatigue. Provided iron was excluded from the solutions, time to fatigue at 37 degrees C was increased compared to 22 degrees C (125 +/- 8% of 22 degrees C fatigue time). In contrast, when iron was present (approximately 1 microM), fatigue was accelerated (68 +/- 10%). Iron can increase reactive oxygen species (ROS), which are believed to accelerate fatigue. The addition of 25-100 microM H(2)O(2) at 22 degrees C reduced time to fatigue to 80-20% of the control, respectively. Iron was added to cultured primary skeletal muscle cells to determine if iron could increase ROS production. Neither iron entry nor ROS production were detected in non-contracting muscle cells. The addition of 8-hydroxyquinoline, which facilitates iron entry, to iron-ascorbic acid solutions caused a rapid rise in intracellular iron and ROS. Our results indicate that time to fatigue in vitro is increased at 37 degrees C relative to 22 degrees C, but the addition of ROS can accelerate fatigue. An increase in muscle iron can accelerate ROS production, which may be important during or following exercise and in haemochromatosis, disuse atrophy and sarcopenia.

Iron injections in mice increase skeletal muscle iron content, induce oxidative stress and reduce exercise performance.

Iron accelerates the production of reactive oxygen species (ROS). Excessive levels of ROS are thought to accelerate skeletal muscle fatigue and contribute to the loss of skeletal muscle mass and function with age. Patients with an iron overload disease frequently report symptoms of weakness and fatigue, which is attributed to reduced cardiac function. The contribution of skeletal muscle to these symptoms is unknown. Using a mouse model of iron overload, we determined the extent of iron accumulation in skeletal muscle and the concentrations of the iron storage protein ferritin. The level of oxidative stress, changes in antioxidant enzymes and exercise performance were also assessed. Compared with control mice, the iron overloaded mice had elevated levels of iron in the tibialis anterior muscle and a fourfold increase in ferritin light chain. The oxidative stress product malondialdehyde was increased in the iron group compared with the control group, as was the antioxidant enzyme activity of glutathione reductase and glutathione peroxidase. The iron group performed less work on an endurance test and produced less force in a strength test. Body weight and skeletal muscle weight were lower in the iron group following the intervention. Iron loading reduced the weight of the fast-twitch extensor digitorum longus muscle more than the slow-twitch soleus muscle. In summary, iron accumulation in skeletal muscle may play a significant role in the reduced exercise capacity seen in iron overload disorders and in ageing, and may play an underlying role in skeletal muscle atrophy.