Bradykinin (BK) is a peptide mediator released in inflammation that potently excites sympathetic neurons. We have studied the mechanism of this excitation in dissociated rat sympathetic neurons and found that at low nanomolar (EC50 = 0.9 nM) concentrations, BK inhibited the M-type K+ current IK(M). Studies with the selective antagonist Hoe140 revealed that this effect was mediated via the B2 receptor subtype, and mRNA encoding this receptor was identified in these neurons by RT-PCR. IK(M) inhibition was unaffected by Pertussis toxin or microinjection of antibodies to G alpha o but was selectively inhibited by microinjection of antibodies to G alpha q/11. Thus, BK is the most potent M current inhibitor yet described in mammalian neurons, and BK inhibition of M current is mediated by a G protein pathway similar to that activated by muscarinic acetylcholine receptors.

1. Microinjection of selective antibodies into superior cervical ganglion (SCG) neurones has identified the G‐protein alpha‐subunits mediating muscarinic receptor inhibition of M‐type K+ current (IK(M)) and alpha‐adrenoceptor inhibition of Ca2+ current (ICa). 2. Antibodies specific for G alpha q/11, but not those for G alpha o, reduced M‐current inhibition by the muscarinic agonist oxotremorine‐M, whereas anti‐G alpha o antibodies, but not anti‐G alpha q/11 or anti‐G alpha i1‐3 antibodies, reduced calcium current inhibition by noradrenaline. 3. Immunoblots with specific anti‐G‐protein antibodies demonstrated the presence of both G alpha q and G alpha 11, while G alpha o1 (but virtually no G alpha o2) was present. 4. We conclude that M1 muscarinic receptor inhibition of IK(M) is transduced by G alpha q and/or G alpha 11, and that G alpha o transduces alpha‐adrenoceptor inhibition of ICa.

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The possibility that the ml muscarinic receptor subtype can inducp ~P~PW of intracellular granules and transmitters was studied by transfecting a cultured mast cell line, RBL-2H3 cells. wilh the ml receptor gene. Comparisons were made between carbachol-and antigen-induced activation ofvarious secretory responses. Likeantigen. carbachol stimulated inositol phospholipid hydrolysis and release ofarachidonic acid with concomitant dose-dependent secretion of granular contents. Carbachol also stimulated a biphasic increase in intracellular calcium, as measured by single cell fura-measurements. Although the kinetics of the carbachol-induced rise in intracellular calcium differed from that induced by antigen. they both utilized the same intracellular pool of calcium. and the second phase of the rise in intracellular calcium was dependent on estracellular calcium in both cases. Thus. the ml muscarinic receptor activates release of granules by a mechanism ostensibly similar to that of antigen.Cloned ml muscarinic receptor: Expression (RBL-2H3 ccl!): Intracellular signal: Esocytosis