The sulfonic amino acid taurine is found in high concentrations in many mammalian excitable cells and is reported to have a variety of functions including osmoregulation, modulation of neuronal excitability, antioxidation and control of Ca 2+ homeostasis (Huxtable, 1992).Taurine is also found in high levels in skeletal muscle (Chesney et al. 1986; Nieminen et al. 1988; Turner et al. 1994). The taurine content in skeletal muscle is reported to vary between muscle type and species. In the rat, the slowtwitch soleus muscle is reported to have twice the taurine content (33 µmol (g wet weight) _1 ) of the fast-twitch extensor digitorum longus (EDL) muscle (17 µmol (g wet weight)_1 ; Iwata et al. 1986). In the horse, slow-twitch type I fibres have been reported to have a high taurine content, while in fast-twitch type IIb fibres, taurine was reported to be undetectable (Dunnett et al. 1992). Recent studies suggest that the taurine content within skeletal muscle may also vary markedly between fibres in the same muscle. Wide differences in the taurine immunoreactivity of individual fibres within the same skeletal muscle have been reported for a number of species, including the rat and the cat (Quesada et al. 1993; Lobo et al. 2000). In the cat soleus muscle, the taurine immunoreactivity was reported to be high and relatively homogeneous (Quesada et al. 1993).The reason for the heterogeneity in the taurine content of mammalian skeletal muscle is unknown. Taurine is actively accumulated by most cells via a Na + -dependent high affinity taurine transporter, and this taurine transporter has been shown to be highly expressed in skeletal muscle (Ramamoorthy et al. 1994). Taurine release has also been demonstrated in many cell types, and one of the main pathways is volume activated (via hyposmotic conditions), and is thought to involve specific anion channels (Perlman & Goldstein, 1999). Hyposmotic conditions and pathological factors such as ischaemia have been shown to lead to taurine release in cardiac myocytes (Kramer et al. 1981) and neurones (Schouboe & Pasantes-Morales, 1992; Saransaari & Oja, 1998) by mechanisms that are poorly understood.In skeletal muscle, there is evidence that contractile activity may induce changes in myoplasmic taurine content. Chronic sciatic nerve stimulation (100 Hz) has been shown to increase the taurine content of rat EDL muscle, and decrease the taurine content in rat soleus muscle (Kim et al. 1986). After neural stimulation, the proportion of taurine-positive skeletal muscle fibres (determined by immunoreactivity measurements) has been reported to fall in the cat (Quesada et al. 1993). We examined the effect of taurine on depolarisation-induced force responses and sarcoplasmic reticulum (SR) function in mechanically skinned skeletal muscle fibres from the extensor digitorum longus (EDL) of the rat. Taurine (20 m) produced a small but significant (P < 0.01) decrease in the sensitivity of the contractile apparatus to Ca 2+ (increase in the [Ca 2+ ] corresponding to 50 % of maximum force...

Taurine increases force production in skeletal muscle, and taurine levels may fall during exercise. The contractile properties and fatigability of extensor digitorum longus (EDL) muscles depleted of taurine by guanodinoethane sulfonate (GES) treatment were investigated. GES treatment decreased muscle taurine levels to <40% of controls. Peak twitch force levels were 23% of controls in GES treated EDL muscles (p < 0.05), but maximal specific force was unaffected. The force-frequency relationship was examined and significantly less force was produced by the GES treated muscles compared to controls at stimulation frequencies from 50 to 100 Hz (p < 0.05). GES treated EDL muscles exhibited significantly slower rates of fatigue than controls (p < 0.05). In skinned fibres, 20 mM GES had a small but significant effect on force production, indicating that GES may have some minor taurine-like effects. In this study, a fall in taurine levels decreased force output, and increased the endurance of EDL skeletal muscles.