Analysis of the antagonism by prazosin of noradrenaline and phenylephrine induced contractions of the rat anococcygeus muscle.

1 Blockade of alpha-adrenoreceptors was analyzed with prazosin in the rat anococcygeus muscle contracted with noradrenaline (NA) and phenylephrine (PHE). 2 Prazosin (1.1 x 10(-8) - 8.8 x 10(-7)M) competitively blocked phenylephrine-induced contractions (pA2 = 8.93 +/- 0.04, slope = 0.85 +/- 0.07) whilst its antagonism of NA was non-competitive (pA2 = 8.45 +/- 0.04, slope = 0.46 +/- 0.03). 3 At higher concentrations, (1.76 x 10(-6) - 5.28 x 10(-6)M) prazosin competitively antagonized NA-induced contractions (pA2 = 7.58 +/- 0.11, slope = 0.93 +/- 0.09). 4 Phentolamine also reduced the effect of NA competitively (pA2 = 8.11 +/- 0.09, slope = 0.82 +/- 0.26). 5 It is concluded that low concentrations of prazosin selectively blocked an alpha 1-adrenoreceptor component of NA-mediated response whilst higher concentrations of prazosin non-selectively blocked both alpha 1- and alpha 2-adrenoreceptors.