INTERACTION OF A CHEMICALLY REACTIVE PRAZOSIN ANALOG WITH ALPHA-1 ADRENOCEPTORS OF RAT-TISSUES

  • 1 April 1989
    • journal article
    • research article
    • Vol. 249  (1) , 70-77
Abstract
An alkylating analog of prazosin, SZL49 {1-(4-amino-6,7-dimethoxy-2-quinazolinyl)-4-(2-bicyclo[2.2.2]octa-2,5-diene-2-carbonyl)-piperazone}, was synthesized and its pharmacological properties examined. SZL49 competed with nanomolar potency at [3H]prazosin binding sites of rat tissues. Pretreatment of membranes with SZL49 (10 .mu.M) followed by washing led to a reduction in [3H]prazosin binding without a change in the Kd of the remaining sites. However, preincubation even at a concentration of 1 .mu.M, led to only a 50% reduction in binding. Higher concentrations of SZL49 in the preincubation mixtures increased the Kd of the remaining sites. Pretreatment of membranes with phenoxybenzamine led to greater than 80% reduction in these sites. Preincubating membranes with SZL49 together with prazosin prevented the loss of binding caused by SZL49 alone. Utilizing different buffers or altering the ratio of absoulte amounts of SZL49 and receptors in the preincubations failed to increase the blockade of [3H]prazosin binding sites. SZL49 was injected i.p. into rats and 16 hr later membranes were prepared from tissues and [3H]prazosin saturation experiments were performed. Whereas Kd values in brain and heart were no different from controls, the Kd value of the remaining kidney binding sites was increased approximately 5-fold in test animals in contrast to in vitro experiments. Maximum binding values of heart were reduced significantly by approximately 42%. Maximum binding values of kidney and brain were reduced about 21 and 36%, respectively. In functional studies with isolated rat aorta, pretreatment with SZL49, followed by a 1.5 hr washout, shifted to the right in a dose-dependent manner the dose-response curves for phenylephrine and norepinephrine. However, the maximal response obtainable with these agonists was not decreased after SZL49 treatment. This induced right-shift was not reversed by a further 2.5 hr washout treatment indicating the persistent effect of this compound. Interestingly, a very low affinity contractile response to agonists remained even after treatment of aortas with 10 .mu.M SZL49. The results suggest that SZL49 1) is an irreversible ligand at [3H]prazosin binding sites, 2) is interacting at functionally important important alpha adrenergic sites in a persistent manner and 3) may delineate a subtype of alpha-1 adrenoceptors.