Acetylcholine-sensitivity and local regenerative activity in denervated frog slow muscle fibres

The distribution of acetylcholine (ACh)-sensitive membrane areas was determined in 11-75 days denervated slow muscle fibres of Rana temporaria by iontophoretic application of acetylcholine. The fibres were also stimulated directly, and their electrical activity was recorded with an intra- and an extracellular electrode. During the first two weeks following denervation the size of the ACh-sensitive fibre surface was similar to that of normal slow fibres, but a spreading out of ACh-sensitivity occurred between the 13th and 20th day. The slow fibre membrane did not become homogeneously ACh-sensitive; even after long periods of denervation large local sensitivity gradients could be observed. Throughout the denervation period maximum values of ACh-sensitivity were in the same range as in normal slow fibres. Action potentials were fully developed when ACh-sensitivity started to spread out. Extracellularly recorded inward currents varied in amplitude along the fibre surface, and either one or two peaks were observed in individual fibres. The spatial relationship between inward current peaks and peaks of ACh-sensitivity was investigated in 12 fibres. Fifteen inward current peaks were located at distances of 30-640 microns from points of maximum ACh-sensitivity; only once did the centers of ACh-sensitivity and excitability coincide. It is concluded that Na channels are incorporated into nonjunctional membrane areas of denervated slow fibres; this process precedes the incorporation of ACh receptors by approximately one week.