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 musc le fibres of lower chordates and invertebrates. In the agnathan Lampet ra, twitches in response to electrical stimulation were unchanged afte r Ca2+ influx was blocked by the addition of Co2+. In the invertebrate s 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 intramembran e charge movement was partly blocked by 5 mu M nifedipine, a dihydropy ridine (DHP) derivative. Charge movement in fibres from Branchiostoma and Patinopecten was insensitive to nifedipine, although the Ca2+ curr ent was 75% blocked by 5 mu M 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-sensi tive component of intramembrane charge movement.