Effects of spin labeled acetylcholine analogs on cholinergic receptors of the leech

The spin labeled acetylcholine analog, SL-2, which is a potent, non-stimulating muscarinic antagonist on the isolated frog heart, is a potent agonist a low concentrations (3x10-5 M) and an antagonist at slightly higher concentrations (4.5x10-5 M) on the mixed cholinergic receptors of the leech. Ion the other hand, the spin labeled acetylcholine analog, SL-1, possesses no agonist activity while it retains the antagonist activity of SL-2. Furthermore, the spin labeled analog, SL-3, which causes hyperpolarization and a decrease in action potential frequency, exhibits neither agonist nor antagonist activity.