Two types of nicotinic acetylcholine receptor channels at slow fibre end-plates of the garter snake.
1. Two different types of acetylcholine receptor channels can be detected on the post-junctional membrane of slow muscle fibres in garter snakes. Here they are designated T-type and S-type channels. Only T-type channels can also be found at twitch fibre neuromuscular junctions. 2. The physiological properties of slow fibre T-type channels are similar to those of acetylcholine receptor channels in end-plates of twitch fibres in these animals. 3. S-type channels had a smaller conductance than T-type channels (32 vs. 49 pS), but a similar reversal potential near 0 mV. 4. Both S- and T-type channels were found together in most patches of slow fibre end-plate membrane, but some patches displayed just one type or the other. 5. The activity of both S- and T-type channels desensitized in the presence of micromolar concentrations of acetylcholine. S-type channels desensitized less rapidly and less completely than did T-type channels. 6. Desensitized channels of both types recovered and produced bursts of activity, then went silent again. During a burst, channels did not appear to change type. 7. The activation of channels of either type was not correlated with activity in channels of the other type. 8. The open-duration distribution of S-type channels required two exponential components to be well fitted, with time constants in the range of 1-2 ms and 3-10 ms. In contrast, the open-duration distribution of T-type channels was a single exponential with a time constant similar in magnitude to the slower S-type component. 9. Desensitization-resistant S-type acetylcholine receptor channels could allow slow muscle end-plates to retain their sensitivity to acetylcholine during periods of heavy use. Under non-desensitizing conditions, differences in the decay properties of slow fibre end-plate currents compared to those in twitch fibres can be explained by the activation kinetics of S-type channels.
Dionne
V E
VE
Department of Pharmacology, University of California San Diego, La Jolla 92093.
D000109:Acetylcholine / Q000494:pharmacology; D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D066298:In Vitro Techniques; D007473:Ion Channels / Q000502:physiology*; D007700:Kinetics; D009045:Motor Endplate / Q000502:physiology*; D009469:Neuromuscular Junction / Q000502:physiology*; D011950:Receptors, Cholinergic / Q000502:physiology*; D011978:Receptors, Nicotinic / Q000502:physiology; D012911:Snakes / Q000502:physiology*; D013997:Time Factors
D007473:Ion Channels; D011950:Receptors, Cholinergic; D011978:Receptors, Nicotinic; D000109:Acetylcholine