An α-Adrenoceptor-Blocking Action of SGB-483, a New Piperazine Antihypertensive Agent in Isolated Vascular Smooth Muscles

Abstract

SGB (10^-7–10^–5 M), a new piperazinyl antihypertensive agent, like prazosin (10^–8_10^–6 M), a potent preferential α₁-adrenoceptor antagonist, inhibited the contractile response of rabbit aorta to norepinephrine and methoxamine in a competitive manner whereas in the cat coronary and rabbit basilar artery, the inhibition by SGB was not a competitive one. The potency of the inhibitory action of SGB was less than that of prazosin in all preparations used. Neither SGB nor prazosin had any inhibitory effect on the response to potassium, 5-hydroxytryptamine, prostaglandin F_2α, and excess Ca²⁺ in all preparations. In the rabbit aortic membrane preparation, the specific binding of [³H]-prazosin was inhibited, concentration-dependently, by SGB with IC₅₀ value of 1.4 ± 0.09 µ M. These studies indicated that SGB has an α_1-adrenoceptor-blocking action though its potency is weaker than prazosin.