Alkylation of alpha-1 receptors with a chemically reactive analog of prazosin reveals low affinity sites for norepinephrine in rabbit aorta

M. T. Piascik, J. W. Kusiak, J. Pitha, B. T. Butler, H. T. Le, M. Babich

Research output: Contribution to journalArticle

13 Scopus citations

Abstract

The effect of a newly synthesized irreversible blocker of the alpha-1 receptor {1-(4-amino-6,7-dimethoxy-2-quinazolinyl)-4-(2- bicyclo[2,2,2]octa-2,5-dienylcarbonyl)-piperazine; SZL-49} has been evaluated in contractile studies in rabbit aorta and binding studies in aorta and brain. SZL-49 produced long lasting inhibition of norepinephrine-induced contractions which was apparent 21 hr after drug washout. The inhibition, which was dose and time dependent, was characterized by progressive shift to the right in the norepinephrine dose-response curve. The ED50 for norepinephrine was shifted from 10-7 M to 8 x 10-7, 3 x 10-6, 1 x 10-5 and 5 x 10-4 M after incubation (30 min) and washout of increasing concentrations of SZL-49. Surprisingly, SZL-49, irrespective of the dose or incubation time, did not decrease the maximal response of aortic rings to norepinephrine. This resulted in a norepinephrine dose-response curve after SZL-treatment that is parallel to the control. SZL-49 had no effect on the spasmogenic actions of histamine, serotonin, KCl or CaCl2. In contrast to the inhibitory pattern seen with SZL-49, incubation with 10-7 M phenoxybenzamine shifted the norepinephrine dose-response curve to the right in a nonparallel manner and significantly depressed the maximal response obtainable with norepinephrine. Incubation with 10-6 M phenoxybenzamine for 30 min virtually abolished the response to norepinephrine. Phenoxybenzamine (10-7 M) was without effect on aortic rings treated with a maximally effective dose of SZL-49. Prazosin weakly antagonized the contractile actions of norepinephrine observed after SZL-49 treatment, whereas yohimbine was without effect on these norepinephrine-induced contractions. Inn control binding studies [3H]prazosin bound to two classes of sites in both aorta and brain preparations. Affinities and densities for these sites were K1 = 67.5 pM, K2 = 309 pM; R1 = 38.2 fmol/mg, R2 = 46.47 fmol/mg in aorta and K1 = 29.6 pM, K2 = 182 pM; R1 = 6.6 fmol/mg and R2 = 30.4 fmol/mg in brain. Treatment with increasing amounts of SZL-49 (10-10 to 10-8 M) progressively reduced the number of [3H]prazosin sites without altering the affinity of the sites remaining. At 10-7 M, SZL-49 eliminated completely all specific [3H]prazosin binding. Our results indicate that the site mediating norepinephrine contraction after treatment with SZL-49 does not possess the characteristics of an alpha-1 receptor and supports the hypothesis that a low affinity site for norepinephrine and prazosin exists in vascular smooth muscle. The fact that our control binding studies reveal two high affinity [3H]prazosin sites suggests that there could be three binding sites for norepinephrine and prazosin in rabbit aorta.

Original languageEnglish (US)
Pages (from-to)1001-1011
Number of pages11
JournalJournal of Pharmacology and Experimental Therapeutics
Volume246
Issue number3
StatePublished - 1988
Externally publishedYes

ASJC Scopus subject areas

  • Pharmacology

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