EFFECTS OF BETA(2)-STIMULANTS ON CONTRACTILITY AND FATIGUE OF CANINE URETHRAL SPHINCTER

The effects of Pt-stimulants [clenbuterol (CB) and terbutaline (TB)] o n the contractility of the urethral sphincter of female dogs were stud ied by measuring intraurethral pressure (IUP) during stimulation of bi lateral pudendal nerves. In nine dogs 1, 10 and 100 mu g./kg. of CB we re administered, but no changes in IUP were observed. In the other 33 dogs, sphincteric fatigue was experimentally prepared by electrically stimulating the pudendal nerves at 15V, 20 Hz for 30 to 40 minutes. In fatigued sphincters, CB (n=17) and TB (n=7) increased the contracting pressure (pressure difference between stimulation-generated peak leve l and baseline level of IUP). The inotropic effect of beta(2)-stimulan t (TB) on the fatigued urethral sphincter was abolished by a beta-bloc ker, propranolol. From the present study it was concluded that beta(2) -stimulants have little effect oil the total contractility of the nonf atigued urethral sphincter because it is composed of smooth and striat ed muscles (fast- and slow-contracting muscles). However, beta(2)-stim ulants enhanced the contractility of fatigued urethral sphincter, Thes e results suggest that beta(2)-stimulants act on fast-contracting fibe rs in the urethral sphincter because the inotropic effect of sympathom imetic amine is much greater on fatigued, fast-contracting fibers than on nonfatigued ones and its depressive effect on slow-contracting fib ers is not potentiated after fatigue.