Function and Distribution of β 3 -Adrenoceptors in Rat, Rabbit and Human Urinary Bladder and External Urethral Sphincter

1 Activation of β-adrenoceptors causes relaxation of the urinary bladder and contraction of the external urethral sphincter, which consists of fast-contracting skeletal muscles. A β2-adrenoceptor agonist, clenbuterol, recently has been developed as a therapeutic drug for the treatment of urinary incontinence, however β2-adrenoceptor agonists have undesirable effects on cardiac and striated muscle function. 2 In this study, we compared the effects of the β2-adrenoceptor agonist, clenbuterol and of a novel β2-adrenoceptor agonist, GS332, on urinary bladder and external urethral sphincter function in rat, rabbit and human. We also determined the distrubution of β3-adrenoceptors in human urinary bladder and external urethral sphincter, using radioligand-binding techniques. 3 Clenbuterol induced marked relaxations in rat, rabbit and human urinary bladder smooth muscles and also induced marked contractions in rat periurethral striated muscles (external urethral sphincter), while GS332 induced marked relaxations in rat and human, but not in rabbit, urinary bladder smooth muscles and induced small contractions in rat periurethral striated muscles. 4 The radioligand-binding studies showed presence of β2- and β3-adrenoceptors in human urinary bladder, external urethral sphincter and abdominal rectus muscles. The affinities of GS332 were the highest in urinary bladder and the lowest in the skeletal (abdominal rectus) muscles, while the affinities of clenbuterol were similar in urinary bladder, external urethral sphincter and the skeletal (abdominal rectus) muscles. 5 These results suggest that GS332 could, similarly clenbuterol, have a role in the treatment of urinary frequency and urinary incontinence.