Slow amphibian muscle fibres become less sensitive to Ca2+ with increasing sarcomere length.

Here we report that mechanically skinned muscle preparations from slow amphibian muscle fibres become less sensitive to Ca2+ when the sarcomere length is increased beyond the value corresponding to optimum filament overlap. This effect is opposite to that observed in all other studies on vertebrate myofibrillar preparations from twitch (Endo, 1972a; 1972b; Moisescu & Thieleczek, 1979; Stephenson & Williams, 1982) and cardiac muscles (Fabiato & Fabiato, 1978). This finding shows that stretch-induced increased in Ca2+ sensitivity is not a general property of the contractile apparatus and suggests that differences in the ultrastructure between these muscle fibre types could be responsible for the opposite length effects. Furthermore, these results allow a more detailed understanding of the processes behind the stretch effects observed in intact slow and twitch amphibian muscle fibres (Lännergren, 1975).