The synthesis and release of acetylcholine in normal and denervated rat diaphragms during incubation in vitro

Abstract

1. Normal and denervated rat diaphragms and neural (central) and aneural (peripheral) parts of normal diaphragms were incubated under several different conditions likely to affect the metabolism of acetylcholine (ACh), with the aim of discovering specific features of the control of neural and aneural ACh in the muscle. The concentrations of ACh in the tissue and the medium were measured at the end of the incubations using a radioenzymatic assay, and the amount of ACh synthesized during the incubations was calculated by subtracting the initial amount of ACh present in the tissue from that found in the tissue plus the medium at the end of the incubations.2. Confirming earlier results obtained with bioassays, it was found that, in a medium with 5 mM-K(+) and 2.5 mM-Ca(2+), denervated diaphragms released ACh into the medium at a rate equal to 47% of that observed in normal diaphragms; the amount of ACh released from aneural parts of normal diaphragms was 51% of that released from their neural parts. The release from normal diaphragms was increased (83%) in a Ca(2+)-dependent manner by raising the concentration of K(+) to 30 mM. In the denervated diaphragms, 30 mM-K(+) brought about a Ca(2+)-independent increase (67%) in the rate of ACh release. The elevation of K(+) was without effect on the release of ACh from aneural parts of normal diaphragms.3. The results indicate that a Ca(2+)-dependent mechanism of ACh release, known to function in the nerve terminals, is not likely to participate in the efflux of ACh from the muscle fibres. The K(+)-induced but Ca(2+)-independent enhancement of ACh release from the denervated diaphragms probably occurs by diffusion of ACh along the altered electrochemical gradient. It is suggested that the surface membranes of the muscle fibres become more permeable to ACh after denervation.4. During incubations with 30 mM-K(+) and 10 muM-hemicholinium-3 (HC-3), an inhibitor of the carrier-mediated transport of choline, the rates of ACh release and synthesis in normal diaphragms were diminished to the levels found in the denervated diaphragms, in which the concentration, release and synthesis of ACh were not affected by HC-3. The synthesis of aneural ACh thus appears to be independent of the carrier-mediated supply of choline across cell membranes.5. The release of ACh from normal diaphragms incubated with 5 mM-K(+) was increased in the presence of 100 muM-ouabain, whereas the release from denervated diaphragms was not affected. This finding suggests that the mechanism of ACh release that is activated by ouabain in the nerve cells involves, in addition to the inhibition of Na(+)-K(+)-ATPase, some other steps which are not operative in the muscle fibres.6. The results corroborate earlier evidence indicating that aneural ACh is produced, stored and released in the diaphragms. They fit the view that the aneural ACh is located in the cytoplasm of the muscle fibres and that it leaves the muscle fibres by molecular ;leakage' rather than by a specialized release mechanism. The efflux of ACh from the muscle fibres is likely to constitute about 50% of the total resting efflux (release) of ACh from normal diaphragms.