Effects of Ca2+ and calmodulin on adenylyl cyclase activity in sheep olfactory epithelium

Sheep olfactory epithelium contains an adenylyl cyclase which is stimulated by many but not all odorants. Here we report that this enzyme is activated by calmodulin in a dose-dependent manner, and that calcium ions are required for this response. Odorant stimulation of adenylyl cyclase is unaffected by the complex Ca2+/calmodulin, as suggested by the results obtained both in Ca2+/calmodulin-depleted membranes and under calmodulin antagonist treatment; this confirms the prediction that the Ca2+ binding protein and odorants stimulate the olfactory adenylyl cyclase through parallel mechanisms. The persistent activation of the regulatory component of adenylyl cyclase by GppNHp does not alter the response of the enzyme to either odorant or Ca2+/calmodulin. In sheep olfactory epithelium a cAMP-phosphodiesterase activity is also present, which is highly inhibited by IBMX and aminophylline, scarcely by RO 20-1724, and unaffected by Ca2+/calmodulin. The modulatory role exerted by calcium on cAMP system in sheep olfactory signal transduction is discussed.