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Properties of both the contractile system and the sarcoplasmic reticulum (s.r.) of slow fibres from the iliofibularis muscle of Xenopus laevis were examined by using 'skinned' preparations mainly at 4 degrees C and pH 7.0. The results were compared with those of skinned fast fibres. The contractile system was activated with various concentrations of alkaline earth metal ions and it was found that the sensitivity of the contractile system of slow fibres to Sr2+ and Ba2+ was much higher than that of fast fibres while their Ca2+ sensitivity was similar. Caffeine (20-50 mM) reversibly induced appreciable active steady contraction of the slow fibre with one-third to one-half maximal isometric tension in the practical absence of Ca2+ (in the presence of 40 mM-EGTA) at relatively high temperatures (10-20 degrees C). The speed of unloaded shortening of caffeine-activated skinned slow fibres in the absence of Ca2+ was not slow but even faster than fibres activated to the same isometric tension level with Ca2+. When the s.r. of a skinned slow fibre was loaded with Ca2+ or Sr2+, 25 mM-caffeine caused transient contraction of the fibre due to a release of Ca2+ or Sr2+ from the s.r. The magnitude of such caffeine contracture was used as a quantitative indicator of the amount of Ca2+ or Sr2+ in the s.r. The dependence on [Ca2+] or [Sr2+] of the initial rate of Ca2+ or Sr2+ uptake by the empty s.r. of slow fibres was almost the same as that of fast fibres. In both types of fibres, a much higher concentration of Sr2+ than Ca2+ was required to activate the pump. When [Ca2+] outside the s.r. was sufficiently high (e.g. 10(-5) M), the maximum level to which the slow fibre s.r. could take up Ca2+ decreased, suggesting that the Ca2+-induced Ca2+ release mechanism also exists in slow fibre s.r. The rate of the Ca2+ leakage from slow fibre s.r. into the media without Ca2+ was higher than that from fast fibre s.r. In the absence of ATP, the enhancing effect of caffeine on the Ca2+-induced Ca2+ release mechanism was much weaker in slow fibres than in fast fibres, although adenosine-5'-monophosphate (AMP) enhanced it to a similar extent in both slow and fast fibres.(ABSTRACT TRUNCATED AT 400 WORDS)
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