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1. The excitatory process travelling along the T-system may be either electrotonic or regenerative. If Na+ dependent action potential is present in the tubular membranes, high frequency of stimulation might cause a Na+ depletion in the tubules sufficient to abolish this process.
2. We tested this hypothesis by recording tension in isolated muscle fibres stimulated tetanically (up to 60 shocks/sec). In low [Na+] solutions, output tension was initially similar to that in normal Ringer, but then fell smoothly to a substantially lower value.
3. The activity of individual myofibrils was recorded directly with ciné-micrographs during isotonic contractions while the fibres were stimulated at high frequencies. In low [Na+]o wavy myofibrils appeared in the centre of the fibre and spread towards the periphery, indicating failure of activation. Wavy myofibrils never appeared in normal Ringer.
4. Intracellular action potentials recorded during the tetanic stimulation indicated that the inactivated myofibrils present in low [Na+] solutions cannot be explained by the changes in size and duration of the action potential.
5. Our results strongly suggest the existence of a regenerative Na+ conductance in the tubular membrane during the inward spread of an excitatory process.
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