The effect of contractile activity on fibrillation and extrajunctional acetylcholine-sensitivity in rat muscle maintained in organ culture.

1. The effect of contractile activity on the initiation of spontaneous action potentials (fibrillation) and on extrajunctional acetylcholine-sensitivity has been studied in single fibres in strips of previously denervated rat diaphragm maintained in organ culture for up to 10 days.2. Following removal of the diaphragm from the animal, fibrillation slowed and usually stopped altogether for about 24-36 hr. Thereafter, spontaneously active fibres were found in all cultured muscle strips.3. At any one time, about (1/4) to (1/3) of fibres impaled with micro-electrodes were active (defined as more than one action potential/10 sec), with a mean discharge frequency of 4.5/sec (range 0.1-24/sec).4. The duration of continuous activity in single fibres was, on average, 21-22 hr; a period of activity was followed by a longer inactive interval. Thus activity in single fibres is cyclical.5. Direct stimulation of fibrillating strips for 24 hr at 10/sec suppressed spontaneous activity for 1-3 days.6. Conversely, blockade of spontaneous activity with tetrodotoxin for 72 hr led to a two- to threefold increase in the number of fibrillating fibres when the drug was washed out; in some strips nearly all fibres became spontaneously active.7. The mean rate of activity of diaphragm fibres during normal breathing, determined by recording single units from the phrenic nerve in lightly anaesthetized animals, is about 18/sec.8. Direct stimulation of cultured diaphragm strips in a pattern similar to breathing for 7-8 days at an average rate of 10-12/sec (or 5/sec in some experiments), resulted in a marked reduction (about 95% in experiments at 10/sec) in extrajunctional sensitivity to ionophoretically applied ACh.9. Direct stimulation for 24 hr at 10/sec (comparable to a period of spontaneous activity) caused only a small reduction in extrajunctional ACh-sensitivity.10. We conclude that spontaneous activity in single fibres under these conditions occurs cyclically because activity, over a period of hours, inhibits the ability of the fibrillating fibre to initiate further action potentials. Repeated self-inhibition of spontaneous activity probably explains why denervated muscle fibres remain highly sensitive to extrajunctionally applied ACh.