Activation of locomotor and grasping spine muscle fibres in chaetognaths: a curious paradox

Voltage-gated sodium channel gene repertoire of lampreys: gene duplications, tissue-specific expression and discovery of a long-lost gene

Studies of the voltage-gated sodium (Nav) channels of extant gnathostomes have made it possible to deduce that ancestral gnathostomes possessed four voltage-gated sodium channel genes derived from a single ancestral chordate gene following two rounds of genome duplication early in vertebrates. We investigated the Nav gene family in two species of lampreys (the Japanese lamprey Lethenteron japonicum and sea lamprey Petromyzon marinus) (jawless vertebrates-agnatha) and compared them with those of basal vertebrates to better understand the origin of Nav genes in vertebrates. We noted six Nav genes in both lamprey species, but orthology with gnathostome (jawed vertebrate) channels was inconclusive. Surprisingly, the Nav2 gene, ubiquitously found in invertebrates and believed to have been lost in vertebrates, is present in lampreys, elephant shark (Callorhinchus milii) and coelacanth (Latimeria chalumnae). Despite repeated duplication of the Nav1 family in vertebrates, Nav2 is only in single copy in those vertebrates in which it is retained, and was independently lost in ray-finned fishes and tetrapods. Of the other five Nav channel genes, most were expressed in brain, one in brain and heart, and one exclusively in skeletal muscle. Invertebrates do not express Nav channel genes in muscle. Thus, early in the vertebrate lineage Nav channels began to diversify and different genes began to express in heart and muscle.

Excitation—contraction coupling in skeletal and caudal heart muscle of the hagfishEptatretus burgeriGirard

Hagfishes are regarded as the most primitive living craniates. Excitation—contraction (E—C) coupling mechanisms were studied in skeletal and caudal heart muscle fibres of the hagfish Eptatretus burgeri. In white (fast) skeletal muscle fibres from the musculus tubulatus, force generation in response to electrical stimulation was maintained in nominally Ca2+ free artificial seawater (ASW)(0Ca2+-ASW) containing 10 mmol l-1 Co2+ (a blocker of Ca2+ currents). Similarly, in red (slow) fibres from parietal muscle bathed in 0Ca2+-ASW containing 10 mmol l-1 Co2+, force generation occurred in association with K+ depolarisation when the external K+ concentration was increased to 100 mmol l-1. Therefore, external Ca2+ is not required for muscle contraction. Hence, both white and red fibres possess the function of depolarisation-induced Ca2+-release from intracellular Ca2+ stores. This function is the same as in the skeletal muscle of all other vertebrates. In caudal heart muscle fibres,twitches in response to electrical stimuli were maintained in 0Ca2+-ASW containing 30 mmol l-1 Co2+. In fibres loaded with fluo-3 bathed in 0Ca2+-ASW containing 30 mmol l-1 Co2+, an increase in the intracellular free Ca2+ level associated with K+ depolarisation was observed after the external K+ concentration was increased to 100 mmol l-1. Thus E—C coupling in the caudal heart muscle is also of the vertebrate skeletal muscle type.

Differential sensitivity to calciseptine of L-type Ca(2+) currents in a 'lower' vertebrate (Scyliorhinus canicula), a protochordate (Branchiostoma lanceolatum) and an invertebrate (Alloteuthis subulata)

Voltage-dependent calcium currents in vertebrate (Scyliorhinus canicula), protochordate (Branchiostoma lanceolatum), and invertebrate (Alloteuthis subulata) skeletal and striated muscle were examined under whole-cell voltage clamp. Nifedipine (10 microM) suppressed and cobalt (5 mM) blocked striated/skeletal muscle calcium currents in all of the animals examined, confirming that they are of the L-type class. Calciseptine, a specific blocker of vertebrate cardiac muscle and neuronal L-type calcium currents, was applied (0.2 microM) under whole-cell voltage clamp. Protochordate and invertebrate striated muscle L-type calcium currents were suppressed while up to 4 microM calciseptine had no effect on dogfish skeletal muscle L-type calcium currents. Our results demonstrate the presence of at least two sub-types of L-type calcium current in these different animals, which may be distinguished by their calciseptine sensitivity. We conclude that the invertebrate and protochordate L-type current sub-type that we have examined has properties in common with vertebrate 'cardiac' and 'neuronal' current sub-types, but not the skeletal muscle sub-type of the L-type channel.