ALPHA(1B)-ADRENOCEPTOR SUBTYPE MEDIATING THE PHENYLEPHRINE-INDUCED CONTRACTILE RESPONSE IN RABBIT CORPUS CAVERNOSUM PENIS

The alpha(1)-adrenoceptor subtype mediating contraction to phenylephri ne in rabbit corpus cavernosum penis (CCP) was investigated using sele ctive alpha(1)-adrenoceptor subtype antagonists. B4101((2-(2,6-dimetho xy-phenoxyethyl)-aminomethyl- 1,4-benzodioxane) hydrochloride), 5-meth ylurapidil and tamsulosin concentration-dependently produced a paralle l rightward shift of the concentration-response curve to phenylephrine , yielding pK(B) values of 8.05, 7.59 and 9.21, respectively. The slop es of the Schild plots were not different from unity. These antagonist s did not affect the maximum response to phenylephrine. Oxymetazoline (1 mu M), which initially caused a small contraction, produced a paral lel rightward shift of the concentration-response curve to phenylephri ne with an apparent pK(B) value of 6.99. However, oxymetazoline seemed to act as a non-surmountable antagonist to the phenylephrine-induced contraction, reducing the maximum response by 71.1%. Chloroethylclonid ine (25 and 100 mu M) produced a parallel rightward shift of the conce ntration-response curve to phenylephrine without altering the maximum response. These results show that the alpha(1)-adrenoceptor in rabbit CCP has a relatively low affinity for WB4101, 5-methylurapidil, tamsul osin and oxymetazoline and is sensitive to inactivation by chloroethyl clonidine. It is suggested that the alpha(1)-adrenoceptor subtype medi ating contraction to phenylephrine in rabbit CCP has the characteristi cs of the alpha(1B)-adrenoceptor subtype.