N-demethylchlordimeform: a potent partial agonist of octopamine-sensitive adenylate cyclase
Nathanson, J.A.; Hunnicutt, E.J.
Molecular Pharmacology 20(1): 68-75
1981
ISSN/ISBN: 0026-895X PMID: 6270539 Document Number: 177589
Physiological studies suggest that the formamidine pesticides, chlordimeform (CDM) and N-demethylchlordimeform (DCDM), may affect octopaminergic neurotransmission. To elucidate the possible mechanism of this interaction, the biochemical effects of these compounds on an active octopamine-sensitive adenylate cyclase present in washed particulate preparations of the firefly light organ were studied. DCDM was a partial agonist (70% of octopamine Vmax) and was 6-fold more potent (Ka = 2.2 .times. 10-6 M) than octopamine (Ka = 1.4 .times. 10-5 M) in activating enzyme activity. At high concentrations (IC50 = 3 .times. 10-4 M), DCDM caused a 30% inhibition of octopamine stimulation. Activation by DCDM was reversible, nonadditive to that caused by octopamine and completitively inhibited by several receptor antagonists, including cyproheptadine (Ki = 2 .times. 10-6 M, clozapine (Ki = 4 .times. 10-6 M), fluphenazine (Ki = 6 .times. 10-6 M), phentolamine (Ki = 1.8 .times. 10-5 M) and propranolol (Ki = 4.7 .times. 10-5 M). These inhibitory constants correlated well with those for inhibiting octopamine stimulation. The agonist activity of DCDM was specific for tissue containing an octopamine-activated adenylate cyclase; enzyme activity in the rat caudate nucleus (activated by dopamine) and in the heart and liver (activated by isoproterenol) was little affected by DCDM. In contrast to DCDM, CDM was a weak octopamine agonist (Ka = 3 .times. 10-5 M; 9% of octopamine Vmax) in the light organ. At higher concentrations (IC50 = 3 to 10 .times. 10-4 M), CDM was an octopamine antagonist, causing nearly complete inhibition of octopamine stimulation at 1 to 3 .times. 10-3 M. This inhibitory effect of CDM was reversible, pH-dependent, and noncompetitive with octopamine. It was also nonselective, since CDM inhibited dopamine-sensitive, .beta.-adrenergic-sensitive and non-hormone-dependent adenylate cyclases in mammalian brain and liver. At low concentrations, DCDM bound specifically and reversibly to octopamine receptors with a resultant activation of adenylate cyclase. CDM had little direct effect on the octopamine receptor; its octopamimetic actions in vivo were a probable result of its conversion to DCDM. At high, nonpharmacological doses, CDM exerted noncompetitive and non-receptor-specific inhibitory effects on adenylate cyclase. These data had relevance to understanding the structural requirements necessary for interaction with octopamine-sensitive adenylate cyclase.