Articles

Saw palmetto extracts potently and noncompetitively inhibit human alpha1-adrenoceptors in vitro.

Author

Goepel M, Hecker U, Krege S, Rubben H

Date

15/2/1999

Journal

Prostate

Abstract

BACKGROUND: We wanted to test whether phytotherapeutic agents used in the treatment of lower urinary tract symptoms have alpha1- adrenoceptor antagonistic properties in vitro. METHODS: Preparations of beta-sitosterol and extracts of stinging nettle, medicinal pumpkin, and saw palmetto were obtained from several pharmaceutical companies. They were tested for their ability to inhibit [3H]tamsulosin binding to human prostatic alpha1-adrenoceptors and [3H]prazosin binding to cloned human alpha1A- and alpha1B- adrenoceptors. Inhibition of phenylephrine-stimulated [3H]inositol phosphate formation by cloned receptors was also investigated. RESULTS: Up to the highest concentration which could be tested, preparations of beta-sitosterol, stinging nettle, and medicinal pumpkin were without consistent inhibitory effect in all assays. In contrast, all tested saw palmetto extracts inhibited radioligand binding to human alpha1-adrenoceptors and agonist-induced [3H]inositol phosphate formation. Saturation binding experiments in the presence of a single saw palmetto extract concentration indicated a noncompetitive antagonism. The relationship between active concentrations in vitro and recommended therapeutic doses for the saw palmetto extracts was slightly lower than that for several chemically defined alpha1-adrenoceptor antagonists. CONCLUSIONS: Saw palmetto extracts have alpha1-adrenoceptor-inhibitory properties. If bioavailability and other pharmacokinetic properties of these ingredients are similar to those of the chemically defined alpha1- adrenoceptor antagonists, alpha1-adrenoceptor antagonism might be involved in the therapeutic effects of these extracts in patients with lower urinary tract symptoms suggestive of benign prostatic obstruction.