Characterization of Benextramine as an Irreversible α‐Adrenergic Blocker and as a Blocker of Potassium‐Activated Calcium Channels

An irreversible α‐adrenergic blocker, benextramine [N,N′‐bis(o‐methoxybenzylamine‐n‐hexyl)‐cysteamine] was used as a probe to study the possible interrelationship between α‐adrenoceptors and the K⁺‐activated Ca²⁺‐channels. Benextramine, a tetraamine disulfide, acts irreversibly both on the α₁‐adrenoceptor (t_1/2= 3 min) and the α₂‐adrenoceptors. These studies were carried out on rat brain synaptosomes, [³H]prazosin and [³H]clonidine binding. Benextramine blocked Ca²⁺ influx in rat brain synaptosomes under both depolarizing (75 mM KCl) and normal conditions (5 mM KCl). Its action at the channel is reversible with IC₅₀= 10 ± 5 μM of the net Ca²⁺ influex. This makes benextramine a most potent Ca²⁺ blocker compared to verapamil or nicardipine (IC₅₀= 200 μM and 170 μ, respectively). Pretreatment of rat brain slices with benextramine gave a synaptosomal preparation which was devoid of either α₁‐adrenergic or α₂‐adrenergic binding capacity due to the irreversible binding of benextramine, but with an undistrubed Ca²⁺ influx. Thus, these results suggest that the α‐adrenoceptors and the Ca²⁺‐channels are independent of each other, and that full occupancy of the α‐receptors does not affect the net calcium flux.