THE EFFECT OF PRAZOSIN ON THE GUINEA‐PIG ILEUM

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Abstract
1 The α-adrenoceptor blocking activity of prazosin was compared with that of phentolamine, yohimbine and phenoxybenzamine in the guinea-pig isolated ileum. 2 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. 3 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. 4 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). 5 Phentolamine and prazosin probably block noradrenaline responses at the same α-adrenoceptor site on the postsynaptic membrane.

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