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Effects of blockade of alpha 1- and alpha 2-adrenoceptors on vasoconstrictor responses to single and twin pulse stimulation in rat tail artery.

Perfused/superfused proximal segments of Sprague-Dawley rat tail artery were stimulated at supramaximal voltage with two 1 ms square wave pulses. The pulse interval was either 10 or 20 s. With either interval the response to each pulse was similar, amounting to about 20 mm Hg increase in perfusion pressure. Prazosin (0.1 nM) approximately halved the response to both pulses whereas idazoxan (30 nM) was without effect. With an interval of 10 s, cocaine (4 microM) greatly increased the response to the first but not to the second pulse; in the presence of cocaine, prazosin (1 nM) again reduced both responses whereas idazoxan (30 nM) reduced the response to the first pulse but increased the response to the second. With an interval of 20 s, cocaine increased the responses to both pulses to a similar degree; in the presence of cocaine, idazoxan reduced the responses to both pulses. The results suggest that in rat tail artery, inhibition of the neuronal uptake process is required if noradrenaline released after stimulation with a single pulse is to reach smooth muscle alpha 2-adrenoceptors, and if feedback inhibition is to persist for more than 10 s.