Effects of Beta2-Stimulants on Contractility and Fatigue of Canine Urethral Sphincter

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Abstract
The effects of β2-stimulants [clenbuterol (CB) and terbutaline (TB)] on the contractility of the urethral sphincter of female dogs were studied by measuring intraurethral pressure (IUP) during stimulation of bilateral pudendal nerves. In nine dogs 1, 10 and 100 μg./kg. of CB were 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 level and baseline level of IUP). The inotropic effect of β2-stimulant (TB) on the fatigued urethral sphincter was abolished by a β-blocker, propranolol. From the present study it was concluded that β2-stimulants have little effect on the total contractility of the nonfatigued urethral sphincter because it is composed of smooth and striated muscles (fast- and slow-contracting muscles). However, β2-stimulants enhanced the contractility of fatigued urethral sphincter. These results suggest that β2-stimulants act on fast-contracting fibers in the urethral sphincter because the inotropic effect of sympathomimetic amine is much greater on fatigued, fastcontracting fibers than on nonfatigued ones and its depressive effect on slow-contracting fibers is not potentiated after fatigue.

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