The activation of contraction and extracellular calcium in striated muscle of the lamprey.

The slow voltage dependent calcium channels have been described in skeletal muscle fibres of adult frogs and rats (Sanchez and Stefani 1978; Donaldson and Beam 1983). Some data have suggested that they may participate in excitationcontraction coupling (ECC) (Beam et al. 1986; Ildefonse et al. 1985; Rios et al. 1986). However, some other investigations have shown the independence of ?"'' tion of contraction on the slow calcium current (for a review see Caille et al. 1985). Our experiments were made on the suction muscle of lamprey, which is a typical striated twitch muscle (Samosudova et al. 1987). A characteristic feature of this muscle is its extrasynaptic sensitivity to acetylcholine (ACh) (Skorobovichuk and Itina 1968). Experiments were performed on segments of thin bundles of muscle fibres (80—120/mi in diameter), dissected from m. longitudinalis linguae of the lamprey (Lampetra fluviatilis). The membrane currents were recorded under voltage clamp conditions using the double sucrose gap method. Isotonic K2S04 (K2S04 80; TRIS 10) and isotonic CaBr2 or BaBr2 (CaBr2 or BaBr2 80; TRIS 10) (in mmol/1) were used as the external solution. Both ends of the segments were immersed in internal solution containing either KC1 (KC1 115; NaCl 5; EGTA 1; TRIS 10) or TEABr (TEABr 120; EGTA 1 —5; TRIS 10). In other experiments the conventional microelectrode technique was used to record transmembrane potentials. The tension of muscle bundles was recorded using a force transducer (6MX2B, USSR). The solutions contained (in mmol/1): Ringer: NaCl 115; KC1 2.5; CaCl2 2; TRIS 10; the calcium free solution: NaCl 115; KC1 2.5; MgCl2 5; EGTA 2; TRIS 10; the sodium solution: sucrose 240; KC1 2.5; CaCl2 0—10; TRIS 10. Experiments were performed during the winter, at 18—20 °C.