MUSCARINIC RECEPTOR CHARACTERIZATION IN DIFFERENTIATING AVIAN EXOCRINE CELLS

The type of muscarinic acetylcholine receptor in exocrine cells of the avian nasal gland in the undifferentiated quiescent (naive) stage and in the partly differentiated salt-secreting (stressed) stage was char acterized by ligand binding experiments and by probing receptor messen ger RNA with oligonucleotide probes specific for the mammalian recepto r subtypes. Competition-binding studies using I-quinuclidinyl [phenyl- 4-H-3]benzilate and a series of other ligands indicated the presence o f only one type of receptor in both cell types. Pharmacological charac terization of its ligand-binding properties revealed similarities with the mammalian M(3) type. However, 4-[[[(3-chlorophenyl) amino]carbony l]oxy] -N,N,N-trimethyl-2-butyn-1-aminium chloride, generally a partia l agonist in cells expressing mammalian M(1) receptors, released calci um from intracellular stores in naive and stressed cells. To resolve t his, we attempted to characterize the salt gland receptor by molecular means. Northern analysis of salt gland mRNA revealed weak signals onl y with oligonucleotide probes corresponding to the mammalian mi recept or type. However, at higher stringencies these signals faded, indicati ng that the salt gland receptor may resemble the mammalian mi subtype but has probably a considerable degree of sequence divergence. Such di vergence may also explain the observed differences in pharmacological behavior between the avian and the mammalian glandular receptors.