Reduced K+ build-up in t-tubules contributes to resistance of the diaphragm to myotonia

Patients with myotonia congenita suffer from slowed muscle relaxation caused by hyperexcitability. The diaphragm is only mildly affected in myotonia congenita; discovery of the mechanism underlying its resistance to myotonia could identify novel therapeutic targets. Intracellular recordings from two mouse models of myotonia congenita revealed the diaphragm had less myotonia than either the EDL or the soleus muscles. A mechanism contributing to the resistance of the diaphragm to myotonia was reduced depolarisation of the interspike membrane potential during repetitive firing of action potentials, a process driven by the build-up of K+ in small invaginations of muscle membrane known as t-tubules. We explored differences between diaphragm and EDL that might underlie the reduction of K+ build-up in diaphragm t-tubules. A smaller size of diaphragm fibres, which promotes the diffusion of K+ out of t-tubules was identified as a contributor. Intracellular recording revealed slower repolarization of action potentials in the diaphragm suggesting reduced Kv conductance. Higher resting membrane conductance was identified suggesting increased Kir conductance. Computer simulation found reduction of Kv conductance had little effect on K+ build-up whereas increased Kir conductance lessened build-up, although the effect was modest. Our data and computer simulation suggest the opening of K+ channels during action potentials has little effect on K+ build-up whereas the opening of K+ channels during the interspike interval slightly lessens K+ build-up. We conclude activation of K+ channels may lessen myotonia by opposing depolarisation to action potential threshold without worsening K+ build-up in t-tubules.

Methods
Intracellular and force recordings recordings were performed. Data was collected using a CED 1401 A to D board using Spike2 software (Cambridge Electronic Design Limited). No filtering was applied to the signal.