Effects of saw palmetto extract on the vanilloid receptor TRPV1

Abstract

Objective: Transient receptor potential vanilloid subtype 1 (TRPV1) may play a significant role in the pathophysiology of the bladder. The present study investigated the effects of the herbal product, saw palmetto extract (SPE) on TRPV1-mediated Ca²⁺ influx and specific [³ H]resiniferatoxin ([³ H]RTX) binding to TRPV1 in HEK293 cells expressing TRPV1 (HEK293VR11 cells).

Methods: Ca²⁺ influx induced by and the direct binding activity of TRPV1 were measured using a method with Fura 2-AM, a cytoplasmic calcium indicator, and a radioligand binding assay using a [³ H]RTX, respectively.

Results: SPE did not markedly affect Ca²⁺ influx in HEK293VR11 cells; however, it significantly inhibited capsaicin-induced increases in Ca²⁺ influx in these cells. The specific binding of [³ H]RTX in HEK293VR11 cells was saturable with K_d value of 120 ± 7 pM and B_max of 1.07 ± 0.10 fmol/mg protein, and was inhibited by low concentrations of non-labeled RTX with K_i of 60.1 ± 7.6 nM. These results confirmed the pharmacological specificity of specific binding sites of [³ H]RTX to TRPV1 in HEK293VR11 cells. SPE inhibited the specific binding of [³ H]RTX in a concentration-dependent manner, with K_i of 24.2 ± 1.4 μg/mL.

Conclusions: The present study demonstrated for the first time, that SPE inhibited capsaicin-induced Ca²⁺ influx with binding to TRPV1 in HEL293VR11 cells. These results will contribute to a more detailed understanding of the pharmacological effects of SPE on urinary dysfunction.

Keywords: Ca2+ influx assay; radioreceptor assay; saw palmetto extract; transient receptor potential vanilloid subtype 1.