2005 Volume 41 Issue 5 Pages 247-256
The effects of different alpha-1 adrenoceptor blockers on the urethra and the cardiovascular system were evaluated using an in vivo isovolumetric intra-urethral pressure model in New Zealand white rabbits. The urethra of anesthetized male rabbits was cannulated through the bladder and secured at the vesico-urethral junction. The distal side of urethra under the pubic bone was also closed to allow measurement of the intra-urethral pressure. Both the intra-urethral pressure and the femoral arterial pressure were monitored. The effects of five different alpha-1 adrenoceptor blockers on the increases in both the intra-urethral pressure and blood pressure induced by phenylephrine were then examined. The inhibition rate of the alpha-1 adrenoceptor blockers prazosin, bunazosin, terazosin, alfuzosin and tamsulosin on the increase in intra-urethral pressure caused as a result of contraction by phenylephrine was 87.5 ± 4.5% (mean ± S.E.), 88.0 ± 7.2%, 86.2 ± 6.2%, 81.4 ± 4.8% and 92.5 ± 5.0% respectively. The potency ranking of these alpha-1 adrenoceptor blockers was tamsulosin > bunazosin > prazosin > terazosin > alfuzosin. Their inhibition rate of the arterial pressure increase induced by phenylephrine was 81.9 ± 5.0%, 86.2 ± 5.9%, 76.0 ± 6.0%, 63.6 ± 5.7% and 58.0 ± 5.2% respectively, with a potency ranking of bunazosin > prazosin > terazosin > alfuzosin > tamsulosin. We therefore conclude that the alpha-1 adrenoceptor blockers bunazosin and prazosin have a more potent action on both the urethra and the vascular system. However, tamsulosin and alfuzosin displayed a marked blockade of the increased urethral pressure induced by phenylephrine, with much less of a blockade of arterial pressure. In the present study, tamsulosin has been shown to be the most sensitive and powerful of the alpha-1 adrenoceptor blockers on urethral smooth muscle.
