Non-Ca2+-conducting Ca2+ channels in fish skeletal muscle excitation-contraction coupling

During skeletal muscle excitation-contraction (EC) coupling, membrane depolarizations activate the sarcolemmal voltage-gated L-type Ca²⁺ channel (Ca_V1.1). Ca_V1.1 in turn triggers opening of the sarcoplasmic Ca²⁺ release channel (RyR1) via interchannel protein-protein interaction to release Ca²⁺ for myofibril contraction. Simultaneously to this EC coupling process, a small and slowly activating Ca²⁺ inward current through Ca_V1.1 is found in mammalian skeletal myotubes. The role of this Ca²⁺ influx, which is not immediately required for EC coupling, is still enigmatic. Interestingly, whole-cell patch clamp experiments on freshly dissociated skeletal muscle myotubes from zebrafish larvae revealed the lack of such Ca²⁺ currents. We identified two distinct isoforms of the pore-forming Ca_V1.1α_1S subunit in zebrafish that are differentially expressed in superficial slow and deep fast musculature. Both do not conduct Ca²⁺ but merely act as voltage sensors to trigger opening of two likewise tissue-specific isoforms of RyR1. We further show that non-Ca²⁺ conductivity of both Ca_V1.1α_1S isoforms is a common trait of all higher teleosts. This non-Ca²⁺ conductivity of Ca_V1.1 positions teleosts at the most-derived position of an evolutionary trajectory. Though EC coupling in early chordate muscles is activated by the influx of extracellular Ca²⁺, it evolved toward Ca_V1.1-RyR1 protein-protein interaction with a relatively small and slow influx of external Ca²⁺ in tetrapods. Finally, the Ca_V1.1 Ca²⁺ influx was completely eliminated in higher teleost fishes.