BACKGROUND.
We wanted to test whether phytotherapeutic agents used in the treatment of lower urinary tract symptoms have ␣ 1 -adrenoceptor antagonistic properties in vitro. METHODS. Preparations of -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 [ 3 H]tamsulosin binding to human prostatic ␣ 1 -adrenoceptors and [ 3 H]prazosin binding to cloned human ␣ 1A -and ␣ 1B -adrenoceptors. Inhibition of phenylephrine-stimulated [ 3 H]inositol phosphate formation by cloned receptors was also investigated. RESULTS. Up to the highest concentration which could be tested, preparations of -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 ␣ 1adrenoceptors and agonist-induced [ 3 H]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 ␣ 1 -adrenoceptor antagonists. CONCLUSIONS. Saw palmetto extracts have ␣ 1 -adrenoceptor-inhibitory properties. If bioavailability and other pharmacokinetic properties of these ingredients are similar to those of the chemically defined ␣ 1 -adrenoceptor antagonists, ␣ 1 -adrenoceptor antagonism might be involved in the therapeutic effects of these extracts in patients with lower urinary tract symptoms suggestive of benign prostatic obstruction.