Characterization of the functional subunit combination of nicotinic acetylcholine receptors in bovine adrenal chromaffin cells

The combination of nicotinic acetylcholine receptors (nAChRs) subunits conn ecting with the secretion of catecholamines in bovine adrenal chromaffin ce lls was pharmacologically investigated using selective agonists and antagon ists for their nAChRs. The EC50 values (muM) for the agonists that stimulat e the catecholamine secretion and the rank order were as follows: nicotine (3.3) greater than or equal to 1,1-dimethyl-4-phenylpiperazinium (3.5) > (E )-N-methyl-4-(3-pyridinyl)-3-butene-1-amine (14) > cytisine (23) greater th an or equal to acetylcholine (25). However, because both the rank order and the EC50 values differed considerably from those in the various subunits' combinations expressed in Xenopus oocytes or mammalian cells (e.g. alpha2 b eta2, alpha3 beta4, alpha4 beta4, etc.), we could not compare them. On the other hand, the IC50 values (muM) for the antagonists that inhibit the secr etion and the rank order were mecamylamine (0.08) > alpha -conotoxin-MII (a lpha -CTX-MII) (0.71) > dihydro-beta -erythroidine (DH betaE) (48) > alpha -bungarotoxin (alpha -BTX) (no effect). Mecamylamine is a highly selective antagonist for alpha3 beta4 nAChRs, and alpha -CTX-MII and alpha -BTX are s pecific antagonists for alpha3 beta2 and alpha7 nAChRs, respectively. DH be taE is a selective antagonist for the alpha4 beta2. It has already been sho wn that the mRNAs for alpha3, alpha5, alpha7 and beta4 subunits are express ed in the chromaffin cells. Therefore, the subunit combination of nAChRs as sociated with the catecholamine secretion from bovine adrenal chromaffin ce lls is suggested to be at least alpha3 beta4 or alpha3 beta4 alpha5. Furthe r, the results indicate that the utilization of the nicotinic agonists as s elective ligands for the subunit combination of nAChRs may be not suitable for the characterization of nAChRs. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.