The influence of insulin upon the metabolism of glucose by the brain

Extracts of frog sartorius muscles were assayed for their acetylcholine (ACh) content by means of pyrolysis-gas chromatography/mass spectrometry. Freshly dissected whole muscles contained 43 $\pm $ 3.1 (22) pmol ACh, and apart from ACh, no other related ester was detected. The ACh content varied among different animals, but was relatively independent of muscle mass. In denervated muscles the ACh content began to decrease after a delay of two days and, by the eighth day of denervation, reached a steady value of about 26% of the control. Muscles were divided into endplate free segments and segments containing endplates. ACh was localized predominantly in the endplate segment, but a small amount was found in the endplate free region. The endplate segments of denervated muscles contained ACh at the same low concentration as ACh in non-endplate segments. The ACh concentration in non-endplate segments was not affected by denervation. During incubation in the presence of diisopropylfluorophosphate muscles released 2.1 pmol/h into the medium. During the incubation the ACh content of the muscles remained constant. It is concluded that there is about 12 pmol extraneural ACh in sartorius muscle, and that about 30 pmol is in the nerve terminals. If it is assumed that one half of the neural ACh is contained in the synaptic vesicles of motor nerve terminals, then each vesicle would contain, on the average, some 8 $\times $ 10$^{3}$ molecules of ACh.