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 characterized by ligand binding experiments and by probing receptor messenger RNA with oligonucleotide probes specific for the mammalian receptor subtypes. Competition-binding studies using I-quinuclidinyl [phenyl-4-H-3]benzilate and a series of other ligands indicated the presence of only one type of receptor in both cell types. Pharmacological characterization of its ligand-binding properties revealed similarities with the mammalian M(3) type. However, 4-[[[(3-chlorophenyl) amino]carbonyl]oxy] -N,N,N-trimethyl-2-butyn-1-aminium chloride, generally a partial agonist in cells expressing mammalian M(1) receptors, released calcium from intracellular stores in naive and stressed cells. To resolve this, we attempted to characterize the salt gland receptor by molecular means. Northern analysis of salt gland mRNA revealed weak signals only with oligonucleotide probes corresponding to the mammalian mi receptor type. However, at higher stringencies these signals faded, indicating that the salt gland receptor may resemble the mammalian mi subtype but has probably a considerable degree of sequence divergence. Such divergence may also explain the observed differences in pharmacological behavior between the avian and the mammalian glandular receptors.