Comparative Studies on the Fatigue of Urinary Bladder following Repeated Contractions in Neonatal and Adult Rats

Oh, Seung June; Kim, Sung Joon; Kim, Ki Whan; Lee, Ahnkie; Kim, Kwang Myung; Choi, Hwang

Abstract

PURPOSE: The function of urinary bladder in neonate and adult are regarded as different but little is investigated on the physiological function of the neonatal urinary bladder. In this study, we compared the contractile responses of bladder muscle strips between neonate and adult rats to repeated electrical field stimulations(EFSs).
MATERIALS AND METHODS
Postnatal 1 to 3-day-old Sprague-Dawley(SD) rats (7.3+/-1.5gm, mean+/-SE, n=15) and 12-week-old adult SD rat(386+/-21gm, n=15) were sacrificed and longitudinal strips of the anterior wall of the bladder were prepared. Each preparation was mounted in a vertical chamber and connected to a force transducer (FT03, Grass, U.S.A.). Isometric contractions were induced by three different methods; 1) intrinsic nerve activation by applying EFSs(5, 20, or 60Hz frequency, square pulse of 0.8ms width, 70V/cm, 2,5s duration) every 50 seconds, 2) the activation of muscarinic receptors on smooth muscle cell membrane with 5.0microM carbachol(CCh), 3) direct smooth muscle stimulation by applying EFSs(60Hz frequency, 50ms width, 70V/cm, 2.5s duration) in the presence of tetrodotoxin every 50 seconds.
RESULTS
Contractile responses to intrinsic nerve stimulations reached maximal amplitude at 60Hz stimulation in both adult and neonate rat bladder. Recovery from the fatigue in neonate and adult bladder muscle was evaluated by the amplitude changes in contractions evoked by EFS with 3 min. or 1 min. rest periods. Poor recovery was induced in neonates even by 3 min. rest period. In neonate group, the amplitude of contractions declined gradually (fatigue response) with 20 and 60Hz stimulations, while the fatigue responses were negligible in adult group. The responses to the exogenous CCh before and immediately after the repeated stimulations were not significantly different in both groups. Repeated direct muscle stimulations did not induce significant decreases of contractile responses in both groups.
CONCLUSIONS
These results demonstrate that neonatal bladder becomes fatigued faster than adult bladder and the fatigue seems to occur at pre-receptor level of the smooth muscle cells rather than at the muscle itself.