Effect of guanethidine on α-adrenoceptor blocking potency of phentolamine in vitro

The effect of guanethidine on the α-adrenoceptor blocking property of phentolamine was studied using the rabbit aortic strip and rat vas deferens. The agonists used were noradrenaline (in presence of DOCA and cocaine which block uptake mechanisms) and xylometazone (which is not taken by uptake mechanisms). Guanethidine augmented the responses to noradrenaline and xylometazoline and increased the pA 2 value of phentolamine against both agonist. Increased affinity of α-receptors may be partly responsible for the guanethidine-induced supersensitivity to α-adrenoceptor agonists.