Junctional and extrajunctional acetylcholine receptors in normal and denervated frog muscle fibres

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Abstract
Ionic channel properties of acetylcholine receptors located in, in the vicinity of, or far away from a frog neuromuscular junction were investigated by noise analysis of drug induced current fluctuations. For drugs applied to the junction, in certain cases two Lorentzian curves were necessary to describe the data. It is postulated that the reason for this observation is that a contribution from perijunctional receptors was being observed. The conductance of a single channel in the junction was independent of the nature of the agonist and had an average value of 17.9 pS (temperature range 8–25°C, solution buffered with Tris). After denervation for 21 days the conductance γ was 7.5 pS at extrajunctional locations. In the close neighbourhood of the junction (perijunctional receptors) values were found between 4 and 19 pS. The mean value of the open channel life-time τ in the endplate exposed to acetylcholine was 2.4 ms at 8–11°C. This value was 0.90 ms with carbachol, 0.50 ms with succinylcholine, 0.28 ms with decamethonium and 0.45 ms with nicotine. The receptors outside the endplate exhibited τ-values which at a given temperature were 2–3 times larger than those at the endplate. Raising the temperature to 23°C reduced all τ-values by factors of 2–3. It is concluded that at least two types of ACh-receptors with different properties exist in the muscle membrane, possibly produced by ACh-receptive units in different states of aggregation.