1999 Volume 22 Issue 4 Pages 412-417
The plasma concentration, total radioactivity and in vivo α1-adrenoceptor binding in rat tissues after intravenous (i.v.) injection of [3H]tamsulosin were measured and they were compared with those obtained after the injection of [3H]prazosin. The plasma concentration of [3H]tamsulosin was consistently higher than that of [3H]prazosin, with 1.4 times greater areas under the curve (AUC0-∞) of plasma concentration. As there was a significantly lower value of apparent volume of central compartment (Vdc) and distribution volume at steady state(Vdss) for [3H]tamsulosin than [3H]prazosin with little difference in elimination rate constant (β), the higher concentration of [3H]tamsulosin in plasma might be associated mainly with the smaller volume of distribution. The ratio of total radioactivity in tissues to the plasma unbound concentration of [3H]tamsulosin after i.v. injection of the ligand was consistently lower than that of [3H]prazosin. These observations suggest that [3H]tamsulosn is distributed in rat tissues in a more limited manner than [3H]prazosin. A significantly lower level of in vivo specific binding of [3H]tamsulosin than [3H]prazosin was observed in the spleen, heart and liver. Further, the apparent dissociation constant (Kd) and maximal number of binding sites (B<max>) for in vivo specific [3H]tamsulosin binding were considerably lower than those for [3H]prazosin binding. Therefore, these findings suggest that [3H]tamsulosin labels preferentially a subpopulation of the α1-adrenoceptor sites in rat tissues labeled by [3H]prazosin. In conclusion, the present study has shown that there is a significant difference in the pharmacokinetics and in vivo α1-adrenoceptor binding characteristics between tamsulosin and prazosin.