Effects of alpha1-adrenoceptor antagonists on agonist and tilt-induced changes in blood pressure: relationships to...

We evaluated the uroselectivity of a series of alpha1-adrenoceptor antagonists by comparing their potency against phenylephrine-induced increases in urethral perfusion pressure and diastolic blood pressure in the anesthetized rabbit and pithed rat. In the rabbit, Rec 15/2739 (N-[3-[4-(2-methoxyphenyl)-1-piperazinyl]propyl]-3-methyl-4-oxo-2-phenyl -4H-1-benzopyran-8-carboxamide) as well as analogs with a chlorine substituent on the methoxyphenyl ring (Rec 15/2869) or this substituent combined with the replacement of the phenyl substituent on the pyran ring by cyclohexyl (Rec 15/3011) were 2-6-fold more potent against the urethral vs. vascular response to phenylephrine. Rec 15/2841 (N-[3-[4-(2-methoxyphenyl)-1-piperazinyl]propyl]-3-methyl-4-oxo-2-cyc lohexy-4H-1-benzopyran-8-carboxamide) was only 1.5-fold more potent against the urethral response. SL 89.0591 and tamsulosin also showed selectivity for the urethral response (2-2.5-fold), while the quinazolines produced equipotent blockade of urethral and vascular responses (selectivity ratio = 0.9-1.1). The urethral selectivities of Rec 15/2739 and its derivatives were confirmed by evaluation of the response to tilt in sedated, hypovolemic rabbits. Phenylephrine challenge assays did not show any of the antagonists, with the exception of terazosin at 300 microg kg(-1), to be uroselective in the rat (selectivity ratios = 0.2-1.5); potentiation of tilt-induced hypotension in the anesthetized rat showed substantial differences from the rabbit, with Rec 15/2739, but not Rec 15/3011 and Rec 15/2841 showing orthostatic effects equivalent to that observed for prazosin. Hence, Rec 15/2739 was uroselective in the rabbit, but not in the rat, while two of its close structural analogs were highly uroselective in both species. An assay for orthostatic activity in the conscious rat yielded different results, showing prazosin and terazosin, but not Rec 15/2739, to cause a reversal of the pressor response to tilt. Hence, the apparent uroselectivity of an alpha1-adrenoceptor antagonist is both species- and assay-dependent.