The synthesis of acetylcholine in skeletal muscles of the rat

Abstract

1. The synthesis of acetylcholine (ACh) has been measured in homogenates of the sciatic nerve, normal and denervated extensor digitorum longus (e.d.l.) muscles, and central (innervated) and peripheral (non-innervated) parts of the diaphragm of the rat. The synthesis proceeded under conditions accepted as optimal for the activity of choline acetyltransferase (ChAT). In view of the finding that cardiac carnitine acetyltransferase (CarAT) is able to acetylate choline (White & Wu, 1973), the possible contribution of CarAT to the synthesis of ACh in the muscles was investigated by using bromoacetylcholine (BrACh) as an inhibitor of ChAT and bromoacetylcarnitine (BrACar) as an inhibitor of CarAT.2. BrACh at a concentration of 2 mum inhibited the synthesis of ACh in nerve homogenates by 98%, in the homogenates of normal e.d.l. muscles by 53%, in denervated e.d.l. muscles by less than 5%; in the central part of the diaphragm BrACh inhibited ACh synthesis by 65%, and in the peripheral part by 13%. Comparative inefficiency of BrACh in inhibiting the synthesis of ACh in muscle homogenates was not due to its inactivation; the inhibitory effect of BrACh on the neural synthesis of ACh was preserved in the presence of muscle homogenates.3. BrACar at a concentration of 20 mum inhibited the synthesis of ACh in homogenates of the nerve by 18%, in those of normal e.d.l. muscles by 67%, in denervated e.d.l. muscles by 90%; in the central part of the diaphragm it inhibited the synthesis by 29%, and in the peripheral part by 76%.4. The inhibitory effects of BrACh and BrACar on the synthesis of ACh in muscle homogenates were roughly additive.5. Within 2 days of transection of the sciatic nerve, the BrACh-sensitive synthesis of ACh in the e.d.l. muscle diminished by 28%, whereas the BrACh-insensitive synthesis of ACh did not change. At 4 days after denervation, the rate of BrACh-sensitive synthesis decreased to 3% of control values.6. The results indicate that at least two enzymes are responsible for the synthesis of ACh in muscle homogenates is probably catalysed by CarAT. associated with intramuscular nerves, and probably corresponds to ChAT. The other enzyme is comparatively insensitive to BrACh, is sensitive to BrACar, and is probably localized in the muscle fibres. The BrACh-insensitive and BrACar-sensitive synthesis of ACh in muscle homogenates is probably catalysed by CarAT.7. Under the conditions used in the present experiments, CarAT was responsible for approximately one half of the synthesis of ACh in the homogenates of innervated e.d.l. muscles and for all ACh synthesized after denervation.8. The results provide an explanation for earlier findings of residual ACh synthesis in homogenates of denervated muscles, without resort to the idea that ChAT is localized in muscle fibres. It is proposed that CarAT catalyses some synthesis of ACh also in intact muscles, and that it is responsible for the synthesis of ACh observed during incubation of whole denervated muscles. It is not clear what is the physiological function of the synth...

Expression of the High-affinity Choline Transporter CHT1 in Rat and Human Arteries

“…However, there are several reports documenting the ability of muscle cells to synthesize ACh, either via ChAT or via the enzyme carnitine acetyltransferase. ACh-synthesizing activity and/or ACh release have been demonstrated in myoblasts (Krause et al 1995;Fu et al 1998), skeletal muscle fiber (Tucek 1982;Miledi et al 1982), and smooth muscle fibers of the human skin . Moreover, airway smooth muscle cells have been shown to display ChAT immunoreactivity .…”

Evidence for the Presence of Cholinergic Nerves in Cerebral Arteries: An Immunohistochemical Demonstration of Choline Acetyltransferase

“…It has been shown that carnitine acetyltransferase (CarAT) also acetylates choline (White and Wu, 1973;Rossier, 1977;Tucek, 1982). The syn thesis of ACh by CarAT as well as by ChAT has been shown in cardiac (White and Wu, 1973) and skeletal (Tucek, 1982) muscle. It was estimated that about one-half of the ACh synthesized from choline in innervated skeletal muscle was catalyzed by CarAT (Tucek, 1982).…”

“…The syn thesis of ACh by CarAT as well as by ChAT has been shown in cardiac (White and Wu, 1973) and skeletal (Tucek, 1982) muscle. It was estimated that about one-half of the ACh synthesized from choline in innervated skeletal muscle was catalyzed by CarAT (Tucek, 1982). Although it is not known whether vascular smooth muscles also contain CarAT, a small but significant amount of acetylated products that are not synthesized by ChAT has been demonstrated in denervated ear arteries of the rabbit (Florence et aI.…”

“…However, it should be pointed out that the method used in the biochemical estimation of ChAT activity (Florence and Bevan, 1979;Duckles, 1981) is based on the rate of synthesis of ACh from choline (Fonnum, 1975). It has been shown that carnitine acetyltransferase (CarAT) also acetylates choline (White and Wu, 1973;Rossier, 1977;Tucek, 1982). The syn thesis of ACh by CarAT as well as by ChAT has been shown in cardiac (White and Wu, 1973) and skeletal (Tucek, 1982) muscle.…”

Choline acetyltransferase detection in normal and denervated electrocyte from Electrophorus electricus (L.) using a Confocal Scanning Optical Microscopy Analysis

“…The content of ACh in tissues with cholinergic innervation is very stable. If the release of ACh is augmented as a consequence of increased functional activity of mammalian cholinergic neurons, the ACh is replaced almost immediately by new synthesis (MacIntosh & Collier 1976, Tucek 1982. However, the resynthesis is slower in piscine electric organ (Dunant et al 1974(Dunant et al , 1977.…”