Properties of junctional and extrajunctional acetylcholine-receptor channels in organ cultured human muscle fibres.

1. Current noise, obtained during steady ionophoretic application of acetylcholine (ACh) to voltage-clamped human fibres has been analysed to derive properties of end-plate channels and also extrajunctional ACh-activated channels which are present at the muscle-tendon junction of normal fibres. In addition, ACh-receptor channels present at the end-plate and tendon region in organ cultured muscles have been compared with those in fresh muscles.2. Extrajunctional channels in the tendon region of fresh fibres have a longer mean life-time, tau, and a smaller single channel conductance, gamma, than the junctional channels. tau was 1.71 +/-0.11 msec and gamma was 25.05 +/- 1.18 pS for junctional channels; tau was 3.16 +/- 0.33 msec and gamma was 12.76 +/- 1.29 pS for extrajunctional channels.3. Properties of channels in the end-plate and tendon region were unchanged during short-term (< 7 days) organ culture at 23 or 36 degrees C. The voltage sensitivity of the mean channel life-time was similar at junctional and extrajunctional sites.4. In muscles organ cultured for 7 days at 36 degrees C, double component noise spectra were obtained at some end-plates. The fast and slow time constants underlying the noise appeared to correspond to simultaneous activation of junctional and extrajunctional channels.5. After organ culture for 3-4 weeks at 23 degrees C the mean life-time of the end-plate channels was prolonged while their single channel conductance was unchanged, tau = 3.58 +/- 0.16 msec; gamma = 22.11 +/- 0.83 pS (V(m) = -80 mV, T = 21 degrees C).6. As end-plate channel properties were unchanged in short-term cultures it was possible to assess post-junctional sensitivity by comparing miniature end-plate current (m.e.p.c.) amplitudes in both normal and myasthenia gravis affected muscles. No evidence was found for a change in post-synaptic sensitivity during organ culture at 23 degrees C of normal or myasthenia gravis muscle fibres, which seems to rule out reversible block of receptors by anti-receptor antibody as playing an important role in myasthenia gravis.