We have characterized odorant-stimulated adenylate cyclase activity in isolated chemosensory cilia prepared from frog and rat olfactory epithelium. Cilia from both species exhibit high levels of adenylate cyclase activity. Basal activity is stimulated approximately 2-fold by GTP and approximately 5-fold by guanosine 5'-(3-O-thio)triphosphate and forskolin. Odorants augment enzyme activity 30-65% above the basal level in a tissue-specific and GTP-dependent manner. Calcium reduces GTP-stimulated activity with a 50% effective concentration at 10 microM. Odorants vary in their influence upon olfactory adenylate cyclase activity. Most fruity, floral, minty, and herbaceous odorants stimulate the enzyme. 3,7-Dimethyl-2,6-octadienenitrile (citralva), menthone, D-carvone, L-carvone, and 2-isobutyl-3-methoxypyrazine display similar potencies in activating the adenylate cyclase upto concentrations of 100 microM. Putrid odorants, such as isovaleric acid, triethylamine, pyridine, thiazole, and methoxypyrazine, and odorous chemical solvents, do not stimulate enzyme activity. In homologous series of pyrazine, thiazole, and pyridine odorants, compounds with the longest hydrocarbon side chains are best able to enhance enzyme activity. The failure of certain odorants to affect adenylate cyclase activity suggests that additional transduction mechanisms besides the formation of cAMP are involved in olfaction.
