Alpha-blocking potencies of antihypertensive agents (prazosin, terazosin, bunazosin, SGB-1534 and ketanserin) having with quinazoline or quinazolinedione as assessed by radioligand binding assay methods in rat brain.

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Abstract
The comparative effects of antihypertensive agents quinazoline or quinazolinedione residues (prazosin, bunazosin, terazosin, SGB-1534, and ketanserin), on the binding of [3H]prazosin, [3H]p-aminoclonidine and [3H]dihydroalprenolol([3H]DHA) to .alpha.1-, .alpha.2-, and .beta.-adrenoceptors in the rat brain were examined using radioligand binding assay methods. pA2 values were also obtained in the isolated rat aorta (.alpha.1-adrenoceptor) using phenylephrine as an agonist. A strong inhibition by these drugs of [3H]prazosin binding to .alpha.1-adrenoceptors was observed, while the inhibition of [3H]DHA binding to .beta.-adrenoceptors and [3H]p- aminoclonidine binding to .alpha.2-adrenoceptor was found to be very weak. The rank order of antagonistic potencies of these drugs against the .alpha.1-adrenergic receptors was determined by inhibition constants (Ki) with SGB-1534 = prazodin = bunazosin > terazosin > ketanserin. The pA2 value of these drugs, in contrast, had prazosin with higher pA2 value than that of SGB-1534. From these two different types of experiments, it was clear that these drugs antagonized .alpha.1-adrenoceptors even in the central nervous system, and the side chains bound to quinazoline and quinazolinedione residues may play an important role in the antagonistic potencies for .alpha.1-adrenoceptors in the central nervous system as in the peripheral tissues.

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