INFLUENCE OF N ‐ETHYLMALEIMIDE ON CHOLINOCEPTORS AND RESPONSES IN LONGITUDINAL MUSCLES FROM GUINEA‐PIG ILEUM

1 The binding of carbamylcholine to membranes prepared from the longitudinal muscle of guinea‐pig ileum was determined from its inhibition of the binding of [ 3 H]‐3‐quinuclidinyl benzilate. Carbamylcholine binding was resolved into high and low affinity components with apparent dissociation constants of 0.11 ± 0.02 and 11 ± 1 μ m 42% of the receptors displayed high affinity carbamylcholine binding. 2 Alkylation of longitudinal muscle membranes with N ‐ethylmaleimide increased muscarinic receptor affinity for carbamylcholine in a manner consistent with a conversion of low affinity to high affinity receptors. After exposure the muscle membrane fragments to 1 m m N ‐ethylmaleimide for 20 min at 35°C, carbamylcholine binding was resolved into two components with apparent dissociation constants of 0.11 ± 0.01 and 9 ± 2 μ m , with 74% of the receptors displaying the higher affinity. 3 Exposure of longitudinal membranes mounted in an organ chamber to 1 m m N ‐ethylmaleimide for 30 s depressed isometric contractions in response to acetylcholine by 80%, while contractions induced by K + and Ba 2+ were reduced by less than 20% and 10%, respectively. Acetylcholine dose‐response curves were shifted to the right while Ba 2+ curves were unaffected. 4 It is suggested that N ‐ethylmaleimide has a selective effect on muscarinic responses in the longitudinal muscle by disrupting processes occurring after receptor occupancy but before the induction of phospholipid turnover or calcium influx in the postsynaptic membrane.