Characteristics of functioning of electromechanical coupling in striated muscles of higher and lower vertebrates

A comparative pharmacological analysis of relative contributions of different signal transduction pathways in the activation of contraction (excitation-contraction coupling, ECC) in intact fast striated muscles of frog and lamprey was performed. It was found that the major mechanism responsible for the ECC in muscles of both animals is Ca2+ release from the sarcoplasmic reticulum through the ryanodine-sensitive channels. However, the ECC in lamprey muscle displays some important differences in the units of electromechanical coupling, which precede the calcium release from sarcoplasmic reticulum. The maximum contraction force in frog muscle develops during caffeine-induced contracture, which indicates that all Ca2+ stored in sarcoplasmic reticulum is released through ryanodine-sensitive channels. In contrast, in lamprey muscle, the maximum force develops not in response to high caffeine concentration, but in response to repetitive electrical stimulation. Hence, in addition to stores liberated by ryanodine-sensitive channels, some other sources of calcium ions should exist, which contribute to the contraction activation. A source of this additional Ca2+ ions can be external medium, because acetylcholine contracture is abolished in a calcium-free medium. In frog muscle, the acetylcholine contracture was abolished in a Na(+)-free solution. It was concluded that in frog muscle ECC can be triggered by changes in the transmembrane potential (depolarization-induced calcium release), while in lamprey muscle the entry of calcium ions into myoplasm as the trigger in ECC (calcium-induced calcium release). The lamprey muscle was found to be more resistant to tetrodotoxin and tetracaine, which is indicative of a role in the activation of contraction of tetrodotoxin-resistant Na+ and/or Ca2+ channels. It was concluded, that ECC mechanism in striated muscles of low vertebrates is not limited by the generally accepted scheme of depolarization-induced calcium release but can include some other schemes, which require the Ca2+ influx into the cell.