Determination with high resistance micropipettes of acetylcholine sensitivity in frog slow muscle fibres.

1. Acetylcholine (ACh) sensitivity was determined in slow muscle fibres of Rana temporaria by ionophoretic application of ACh from high resistance micropipettes. 2. The amplitude of ACh responses decreased with the third power of the distance between fibre surface and tip of the pipette; latency and time-to-peak of ACh responses increased with the second power of the distance. 3. ACh sensitivity values obtained by application of ACh at short distance (approx. 10 micrometers) were generally of the order of 150 mV/nC, but larger values (greater than 1000 mV/nC) pipette the ACh response was often completely lost or markedly reduced in amplitude. The latency of ACh responses was generally longer, but occasionally as short as observed in twitch fibres; time-to-peak and rate of decay were always longer than in twitch fibres. 4. It is concluded that ACh sensitive membrane areas in slow muscle fibres are generally of small diameter. There sensitivity is, however, similar to that of junctional areas in twitch muscle fibres.