Excitation-contraction coupling in skeletal and caudal heart muscle of the hagfish Eptatretus burgeri Girard.

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 Ca(2+) free artificial seawater (ASW) (0Ca(2+)-ASW) containing 10 mmol l(-1) Co(2+) (a blocker of Ca(2+) currents). Similarly, in red (slow) fibres from parietal muscle bathed in 0Ca(2+)-ASW containing 10 mmol l(-1) Co(2+), force generation occurred in association with K(+) depolarisation when the external K(+) concentration was increased to 100 mmol l(-1). Therefore, external Ca(2+) is not required for muscle contraction. Hence, both white and red fibres possess the function of depolarisation-induced Ca(2+)-release from intracellular Ca(2+) 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 0Ca(2+)-ASW containing 30 mmol l(-1) Co(2+). In fibres loaded with fluo-3 bathed in 0Ca(2+)-ASW containing 30 mmol l(-1) Co(2+), an increase in the intracellular free Ca(2+) 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.