Two types of nicotinic acetylcholine receptor channels at slow fibre end‐plates of the garter snake.

Abstract

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. © 1989 The Physiological Society