THE EFFECT OF PRAZOSIN ON THE GUINEA‐PIG ILEUM

Abstract

The α‐adrenoceptor blocking activity of prazosin was compared with that of phentolamine, yohimbine and phenoxybenzamine in the guinea‐pig isolated ileum. Phentolamine (2.56 × 10−8 to 2.56 × 10−6 m) and yohimbine (5.12 × 10−8 to 5.12 × 10−6 m) antagonized the sympathetic inhibition of twitches induced by transmural stimulation, whereas prazosin (5.28 × 10−8 to 5.28 × 10−6 m) and phenoxybenzamine (4.2 × 10−9 to 4.2 × 10−7 m) had no effect. Phentolamine, yohimbine, prazosin and phenoxybenzamine antagonized the relaxant response of the ileum to periarterial nerve stimulation and to exogenous noradrenaline in the absence of transmural stimulation. The pA2 value for phentolamine against noradrenaline in the transmurally‐stimulated gut (presynaptic α‐adrenoceptor site; 8.17 ± 0.04) was significantly different from that in the non‐stimulated gut (postsynaptic α‐adrenoceptor site; 6.88 ± 0.12). Phentolamine and prazosin probably block noradrenaline responses at the same α‐adrenoceptor site on the postsynaptic membrane. 1980 British Pharmacological Society