Functional acetylcholinesterase of rat diaphragm muscle

Abstract

Surface (external) acetylcholinesterase (AChE) and internal AChE of rat diaphragm muscle were assayed separately with radioactive acetyl-β-methylcholine (MeCh) or acetylcholine (ACh) on the intact muscle in vitro and on the muscle homogenate. Surface AChE had a K m for ACh of 1.05 mM and V max of 19.4 μM min −1 per diaphragm. External AChE activity increased 27 per cent for a doubling of muscle weight and averaged 20 per cent of total homogenate activity. Phospholine treatment (4 × 10 −7 M, 7 min, 30°) potentiated the indirectly elicited twitch response 164 per cent when inhibition of external AChE was 91 per cent and internal AChE 64 per cent, whereas DFP treatment (5 × 10 −6 M, 20 min, 30°) potentiated the twitch 257 per cent when inhibition of external AChE was 88 per cent and internal AChE 92 per cent. Inhibition by DPP was reduced by 80 per cent for surface and 32 per cent for internal AChE in the presence of edrophonium (8 × 10 − 6 M) and by 77 and o per cent respectively, in the presence of ACh (2 × 10 −3 M). Selective reactivation of DFP-inhibited surface AChE by 2-PAM correlates with recovery from twitch potentiation. The diaphragm muscle gave normal low frequency twitch response if surface AChE inhibition was less than 35 per cent, even when internal AChE was 78–92 per cent inhibited. Surface AChE activity as assayed on intact rat diaphragm muscle in vitro with 5 × 10 −6 M MeCh as substrate is primarily functional endplate AChE and correlates with cholinergic twitch responses.