To compare the pharmacologic properties of tamsulosin and alfuzosin in isolated prostatic and iris dilator smooth muscle from pigmented rabbits.
UROsphere Laboratories, Université Paul Sabatier, Toulouse, France.
Prostatic and iris dilator smooth muscle strips were placed in organ baths. A concentration-response curve to phenylephrine was compared before and after incubation with tamsulosin or alfuzosin.
Both drugs were approximately 30 times less potent in iris dilator than prostatic smooth muscle. In the iris, tamsulosin acted as a competitive antagonist starting at the 0.03 μM concentration (pA2 = 7.96). This is in the same range as the maximum plasma concentration after a 0.4 mg dose of tamsulosin in humans (0.025 μM). The antagonistic effect of alfuzosin in the iris was weaker (calculated mean pA2 value of 5.63 ± 0.19). Concentrations with an equipotent antagonistic effect on rabbit iris dilator muscle (3.0 and 10.0 μM) were approximately 100 to 300 times higher than the maximum plasma concentrations after a 10.0 mg dose of alfuzosin in humans (0.032 μM).
Tamsulosin was more effective than alfuzosin at blocking adrenergic contraction of the iris dilator muscle in pigmented rabbits. Both drugs were less potent in the iris than in the prostate, which suggests that an additional iris receptor could be involved. If valid in humans, our results suggest that attainable plasma concentrations of tamsulosin are able to antagonize iris dilator smooth muscle contraction, whereas those of alfuzosin are not. This could explain the higher frequency of intraoperative floppy-iris syndrome in patients treated with tamsulosin than with alfuzosin.