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

Abstract

SUMMARY1. 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, r, and a smaller single channel conductance, y, than the junctional channels. T was 1-71 + 0-11 msec and y was 25-05+ 1-18 pS for junctional channels; r was 3-16+0-33 msec and y 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 0C. 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 0C, 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 TC the mean life-time of the end-plate channels was prolonged while their single channel conductance was unchanged, r = 3-58 + 0-16 msec; y = 22-11 + 0-83 pS (Vm = -80 mV, T = 21 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 0C 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.

Acetylcholine receptor channels on adult mouse skeletal muscle are functionally identical in synaptic and nonsynaptic membrane.

“…Thus, in two different species, we find no evidence for topographical segregation of AcCho receptor channel types on innervated muscle. The demonstration that nonsynaptic channels are functionally identical to those at the synapse now explains earlier findings of "junctional-type" channels in nonsynaptic regions of both amphibian (14,(16)(17)(18) and mammalian (6,19) (5). Noise analysis of AcCho-activated currents on human intercostal muscle suggested that synaptic channels had a higher conductance and briefer mean open time than channels at the myotendinous junction.…”

“…This finding led to the idea that channels at synaptic regions of innervated muscle are converted to a functionally distinct "mature" form (6,7). The idea of topographical segregation of channel types on innervated muscle was supported by a noise-analysis study that compared synaptic receptor channels to those at the myotendinous ends of the muscle (5). In that study, the receptor channels at the nonsynaptic endings of human intercostal fibers were found to be functionally distinct from those present at the synapse.…”

“…Synapse-specific acetylcholine (AcCho) receptor channel function was first suggested by the observation that the channel type in the synaptic membrane of denervated muscle had a higher conductance and briefer mean open time than the channel type in the nonsynaptic membrane (1)(2)(3)(4)(5)(6). This finding led to the idea that channels at synaptic regions of innervated muscle are converted to a functionally distinct "mature" form (6,7).…”

Formation of cholinergic synapse-like specializations at developing murine muscle spindles

“…This opens the possibility that ACh released from the sensory terminal might activate intrafusal fiberassociated AChRs in the equatorial region. While our studies favor a synapse-like role of AChRs at sensory nerve terminals, it remains possible that the AChRs have an (unknown) non-synaptic role, similar to the AChRs and associated proteins present at the myotendinous junctions (Fertuck and Salpeter, 1976;Cull-Candy et al, 1982;Chen et al, 1990;Bernheim et al, 1996). Clearly further studies are needed to clarify the role of the AChRs in muscle spindle function.…”

Differential regulation of AChR clustering in the polar and equatorial region of murine muscle spindles

“…This activation could lead to a contraction of the central part of the intrafusal fiber, potentially reversing the lengthening of the central region due to c-motoneuron activity. Alternatively, AChRs at the central region of muscle spindles might have an unknown function, similar to the myotendinous junctions where AChRs also accumulate, but the functional significance of the AChR aggregates is unknown (Fertuck & Salpeter, 1976;Cull-Candy et al, 1982;Chen et al, 1990;Bernheim et al, 1996).…”