The effects of tamsulosin, a high affinity antagonist at functional alpha 1A- and alpha 1D-adrenoceptor subtypes

Br J Pharmacol. 1997 Jan;120(2):231-8. doi: 10.1038/sj.bjp.0700907.

Abstract

1. The actions of the alpha 1-adrenoceptor antagonist tamsulosin have been examined at functional alpha 1-adrenoceptor subtypes and compared with those at the human prostate receptor. 2. At the alpha 1D-adrenoceptors of the rat aorta, tamsulosin acted as a competitive antagonist with a high affinity (pKB = 10.1). 3. At the alpha 1B-adrenoceptor of the rat spleen and rabbit corpus cavernosum penis, tamsulosin again acted as a competitive antagonist but with a significantly lower affinity (pKB = 8.9-9.2). 4. Tamsulosin acted as an unsurmountable antagonist of the alpha 1A-adrenoceptor-mediated responses of the rat and human vas deferens, reducing maximal responses to phenylephrine by 20% and 50%, respectively, at an antagonist concentration of 1 nM. Responses of depolarized (100 mM KCl) rat vas deferens preparations were unaffected by 10 nM tamsulosin but this concentration reduced maximal responses to 5-hydroxytryptamine (5-HT) in this tissue. 5. When longer antagonist incubation periods (> or = 60 min) were used, tamsulosin behaved as a competitive antagonist on the human prostate with a significantly higher affinity (pKB = 10.0) than obtained at the alpha 1B-adrenoceptor. 6. The data demonstrate that tamsulosin is a high affinity antagonist at functional alpha 1-adrenoceptors with a selectivity alpha 1D > or = alpha 1A > alpha 1B. In some tissues the compound exhibits an additional unsurmountable antagonist action, the clinical significance of which is unknown.

MeSH terms

  • Adrenergic alpha-1 Receptor Antagonists*
  • Adrenergic alpha-Antagonists / pharmacology*
  • Adult
  • Aged
  • Animals
  • Dose-Response Relationship, Drug
  • Humans
  • In Vitro Techniques
  • Male
  • Middle Aged
  • Prostate / drug effects
  • Rabbits
  • Rats
  • Rats, Wistar
  • Sulfonamides / pharmacology*
  • Tamsulosin
  • Vas Deferens / drug effects
  • Vas Deferens / physiology

Substances

  • Adrenergic alpha-1 Receptor Antagonists
  • Adrenergic alpha-Antagonists
  • Sulfonamides
  • Tamsulosin