Evolution of skeletal muscle excitation-contraction coupling and the appearance of dihydropyridine-sensitive intramembrane charge movement

Excitation-contraction (E-C) coupling was studied in the striated muscle fibres of lower chordates and invertebrates. In the agnathan Lampetra, twitches in response to electrical stimulation were unchanged after Ca²⁺ influx was blocked by the addition of Co²⁺. In the invertebrates examined (Branchiostoma, Oikopleura, Doliolum, Diphyes, Patinopecten and Sagitta), Ca²⁺ influx was necessary to cause a twitch. Whole-cell voltage clamp on muscle fibres of Lampetra revealed that intramembrane charge movement was partly blocked by 5 μM nifedipine, a dihydropyridine (DHP) derivative. Charge movement in fibres from Branchiostoma and Patinopecten was insensitive to nifedipine, although the Ca²⁺ current was 75% blocked by 5 μM nifedipine. The results suggest that the vertebrate type of E-C coupling was acquired between cephalochordates and agnathans, and was accompanied by the appearance of the DHP-sensitive component of intramembrane charge movement.