Discrimination by N-ethylmaleimide between the chronotropic and inotropic response to muscarinic receptor stimulation in rat atrium

N-ethylmaleimide (NEM) rapidly blocked the negative chronotropic effect of carbachol on rat right atrium. In contrast, NEM did not reduce the negative inotropic response to muscarinic (M) receptor stimulation. Carbachol inhibited the specific binding of [3H]-N-methylscopolamine ([3H]-NMS) to membranes of rat atria as reflected by a shallow inhibition curve. Both guanosine triphosphate (GTP) and NEM shifted the [3H]-NMS inhibition curves of carbachol to the right. Pretreatment of the atrial membranes with NEM abolished the GTP-induced rightward shift. However, when instead of the membranes the intact atria were pre-incubated with NEM, no interaction between NEM and GTP in the membranal preparation was observed. The results indicate that NEM sharply discriminated between the inotropic and chronotropic effects to M-receptor stimulation in rat atria. The inhibitory effect of NEM on the M-receptor-mediated negative chronotropic effect in rat atrium cannot be explained by an interaction of the sulfhydryl reagent with GTP-binding proteins, like Ni or No.