Effects of 1-[2-ethoxy-2-(3'-pyridyl)ethyl]-4-(2'-methoxyphenyl)piperazine (IP-66), prazosin, yohimbine and dihydroergotoxine mesylate on the vasoconstriction of rat mesenteric artery induced by electric stimulation and exogenous norepinephrine

The effect of (1-[2-ethoxy-2-(3'-pyridyl)ethyl]-4-(2'-methoxy-phenyl)piperazine (IP-66) on the contraction of rat mesenteric artery induced by electric stimulation (ESV) or by injection of norepinephrine (NESV) was compared with the effects of prazosin, yohimbine and dihydroergotoxine mesylate (DHEM). Interference of propranolol on the effects of the 4 alpha-blockers was also considered. Prazosin has proved to be the strongest antagonist of alpha 1-receptors towards which, however, it showed a certain bond latency. In particular conditions related to increased perfusion time and concentrations prazosin also seemed to lose selectivity towards alpha 1-receptors. It was observed that yohimbine is a weak postsynaptic antagonist whereas DHEM showed scarce selectivity. IP-66 proved to be a strong postsynaptic antagonist with negligible presynaptic effect. It was not as strong as prazosin but its bond with alpha 1-receptors appeared more rapid and steady. Results obtained with prazosin or DHEM and propranolol seemed to suggest an equilibrium between beta 2- and alpha 1-receptors shifted towards the latter.