Benign prostatic hyperplasia (BPH) is one of the most common urological diseases in elderly men and is strongly associated with lower urinary tract symptoms (LUTS). Phytotherapy or the use of plant extracts for treatment of LUTS/BPH is common in Europe and is increasing in the United States. Peucedanum japonicum (PJ) is a perennial umbelliferae plant that grows naturally along southern coastal areas of Japan. The dried roots of PJ have traditionally been used as antifebriles and anodynes to treat coughs, colds, and headaches. Isosamidin is a pharmacologically active compound extracted from PJ that is classified as a coumarin. Isosamidin has pharmacological activities, including inhibition of platelet aggregation, anti-atherosclerotic effects and vasorelaxant effects [1]. Additionally, isosamidin exerts concentration-dependent inhibition of phenylephrine-stimulated contractions of isolated rabbit prostate strips or acetylcholine-stimulated contractions of isolated rabbit bladder strips in vitro [2]. Further, in rat, it significantly decreases micturition frequency in hyperactive bladders induced by intravesical infusion of acetic acid in vivo [3]. These results lead us to hypothesize that isosamidin has sufficient potency to treat patients not only with LUTS/BPH but also overactive bladder. The efficacy of isosamidin on the human lower urinary tract in vitro has not been studied. We examined whether isosamidin exerts a concentration-dependent inhibition of agonist-stimulated contraction of isolated human bladder and prostate tissue strips in vitro.

Human bladder and prostate specimens were obtained from 9 (mean age ± standard deviation: 63.4 ± 10.1 years, age range: 52–84) to 10 patients (mean age ± standard deviation: 66.8 ± 10.9 years, age range: 53–84), respectively, undergoing radical cystectomy for bladder carcinoma with no evidence of LUTS/BPH or overactive bladder. Patients with previous pelvic radiotherapy, extensive chemotherapy, or current urinary tract infection were excluded from this study. After removal of the adventitia, connective tissues, and urothelium (bladder), the bladder and prostate specimens were cut into strips measuring approximately 15 × 5 × 5 mm and 10 × 4 × 2 mm, respectively. Isolated human bladder and prostate strips were suspended in 10-ml organ baths containing Krebs' solution maintained at 37 °C and gassed with a mixture of 95% O2 and 5% CO2. Specimens that failed to contract despite the addition of 40 mM KCl were discarded. For usable strips, peak contractions of 40 mM KCl reached 5.59 ± 0.83 g in bladder strips and 1.85 ± 0.43 g in prostate strips. We changed from Krebs'/40 mM KCl solution to simple Krebs' solution, adding isosamidin to obtain concentrations of 10, 30, and 100 μM. Thirty min after administration of isosamidin (10, 30, and 100 μM) or vehicle (control), concentration-response curves were constructed for agonists (acetylcholine for bladder strips and phenylephrine for prostate strips) by cumulatively increasing agonist concentration (10-7-10-3 M) at 10 min intervals. Results are presented as mean ± standard error of the mean. The inhibitory effect of isosamidin was expressed as a percentage of the maximal contraction (100%) induced by 10-3 M acetylcholine or phenylephrine. The pD2 values (negative log concentration of an agonist that produces 50% reduction of the maximal response) and Emax (maximal effect) were determined for the agonist-induced contractions in the absence or presence of isosamidin. The Schild plots were obtained by plotting the log (dose ratio–1) against the log molar concentration of isosamidin, and the pA2 values were derived from the Schild plots. Statistical comparisons were evaluated with Student's t test or two-way ANOVA followed by Bonferroni's multiple comparisons test. Data analysis was performed using GraphPad Prism 4, statistical significance was defined as p < 0.05.

For control strips without isosamidin, peak contractions reached 1.57 ± 0.27 g for acetylcholine in bladder strips (n = 9) and 0.90 ± 0.16 g for phenylephrine in prostate strips (n = 10). Isosamidin inhibited phenylephrine-stimulated contraction of isolated human prostate tissue strips in a concentration-dependent manner, with significant differences observed in Emax and pD2 values between control and 100 μM isosamidin (Figure (a)). In contrast, isosamidin had no effect on acetylcholine-stimulated contraction of isolated human bladder tissue strips (Figure (b)). For the prostate strips, the pA2 value for isosamidin was calculated from the concentration-response curves at the concentrations described above. The pA2 value and slope of the Schild plots for isosamidin in human prostate strips is 4.71 ± 0.40 and 1.90 ± 0.18, respectively.

This study is the first to show that isosamidin exhibits an inhibitory effect on phenylephrine-stimulated contraction of isolated human prostate tissue strips in vitro. These results may support the potential clinical efficacy of isosamidin in the treatment of LUTS/BPH. This study has some limitations. First, we did not examine hyperplastic prostate specimens obtained from LUTS/BPH patients; therefore, it is still unclear whether isosamidin is effective for inhibiting phenylephrine-stimulated contractions of hyperplastic prostate tissue. Second, we could not examine the mechanisms of action of isosamidin using various agonists or antagonists. Third, to confirm whether isosamidin acts as a α1-adrenoceptor antagonist, the binding activity of isosamidin on α1-adrenoceptors should be examined. Further studies are needed to determine the mechanisms by which isosamidin inhibits phenylephrine-stimulated prostate contractions.

Isosamidin inhibits phenylephrine-stimulated prostate contractions in vitro and may have pharmacological potency in the treatment of male patients with LUTS/BPH. Further studies are required to determine the mechanisms underlying isosamidin action, and clinical studies are required to confirm the efficacy and safety of isosamidin in humans.

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