The contractile response of human prostatic adenoma to a1-adrenoceptor activation is mediated by the a1A adrenoceptor subtype but possibly also by another subtype, named a1L (1). Alfuzosin is widely used for the treatment of benign prostatic hyperplasia (BPH); however no in vitro studies on the antagonistic potency of Alfuzosin in human prostatic adenoma has been reported to date.

Aim of Study:
To determine the potency of alfuzosin on a1-adrenoceptor mediating contractions of human isolated prostatic adenomas, using phenylephrine (PHE) and noradrenaline (Nad) as agonists.

Methods:
Human prostatic adenomas were obtained from patients affected by BPH undergoing transvesical adenomectomy at the Urological Surgery Department of the Institut Mutualiste Montsouris, Paris. Immediately after surgery, human prostatic adenoma specimens were placed in chilled Krebs-Henseleit solution of the following composition (mM) : NaCl 114.0, NaHCO3 25.0, CaCl2 2.5, KCl 4.7, MgSO4 1.2, KH2PO4 1.2, glucose 11.7, ascorbic acid 1. This physiological solution was oxygenated with 95% O2 / 5% CO2. Experiments were done either the same day as surgery or approximately 16 or 40 hours after surgery using tissues stored at 4°C in the refrigerator. The prostatic adenomas, divided in pieces of approximately 10x4 mm, were mounted under 1.5g of tension in 5 ml glass organ baths containing the Krebs solution gassed (95% O2/ 5% CO2). When Nad was used as an agonist, desipramine (0.1 µM) and deoxycorticosterone (3µM) were added to the Krebs solution in order to block neuronal and extraneuronal uptake, respectively. Tissues responses were measured using isometric strain gauges connected to a polygraph and to a data acquisition system. After approximately 1 hour equilibration time, the viability of all preparations was tested by exposure to 100 µM Nad. After frequent washouts and a new equilibration period of 45-60 min, tissues were incubated for 60 min with alfuzosin 0.1-0.3-1 µM (only one concentration by strip) or with the Krebs solution (controls) then a PHE or Nad concentration-response curve (CRC) was obtained by cumulative additions in half-log unit concentrations increments.Individual PHE or Nad concentration-effect data were measured in g of tension and expressed as percentage of the initial contractile response to 100µM Nad taken as 100%. Individual CRC were fitted using the Allfit program (RS1), the midpoint location (EC50) was calculated and expressed as the pD2 value. The curves obtained following alfuzosin incubation were compared with PHE or Nad curves obtained in vehicle-treated strip from the same patient (Allfit program) and antagonist concentration-ratios (CR) were calculated at the EC50 level of PHE and Nad curves in the presence of each concentration of alfuzosin. pA2 values were calculated by using the method described in (2) and a Schild plot was constructed. A statistical analysis (by RS1 software) was used to evaluate if slopes were not different from unity in order to calculate a pKB value.

Results:
The selective a1-adrenoceptor agonist, PHE (0.1-300 µM), produced concentration-dependent contractions of the human prostatic adenoma with a pD2 value of 5.31±0.04 (n=17, 7 patients). The maximal effect was obtained at 100 µM (% Nad= 35.6±6.2). Alfuzosin, studied at 0.1 µM (n=7, 5 patients), at 0.3 µM (n=8, 6 patients) and 1 µM (n=8, 4 patients), shifted the PHE CRC to the right without statistically modifying Emax. values. pA2 values were similar with the three alfuzosin concentrations and also between each patient. The Schild plot gave a pA2 value of 7.96 (7.54-8.83; 95%confidence limits) with a slope value of 0.88 which is not significantly different from unity. The pKB value calculated with the slope constrained to unity was 7.78. The non-selective a1/a2-adrenoceptor natural agonist, Nad (0.1-300 µM) produced concentration-dependent contractions of the human prostatic adenoma with a pD2 value of 5.48±0.11 (n=9, 6 patients). The maximal effect was obtained at 100 µM (% Nad=66.5 ± 4.5). Alfuzosin studied at 0.1µM (n=9, 6 patients), at 0.3 µM (n=8, 5 patients) and 1 µM (n=9, 6 patients), shifted the Nad CRC to the right without statistically modifying the Emax. values. The Schild plot gave a pA2 value of 7.40 (7.19-7.74; 95% confidence limits) with a slope value of 0.96 which is not significantly different from unity. The pKB value calculated with the slope constrained to unity was 7.37.

Conclusions:
These results performed on human prostatic adenoma strips from a total of 13 patients demonstrates that alfuzosin, at concentrations compatible with therapeutic plasma levels (3) is a potent competitive antagonist on the a1A /a1L adrenoceptor-mediated contractile responses in human prostatic adenoma. References 1. Br.J.Urol. 74: 572-578, 1994. 2. Br.J.Pharmacol.Ther. 14: 48-58, 1959. 3. Eur.J.Clin.Pharmacol. 37: 53-58, 1989.