ABSTRACT
PURPOSE: Voiding function of the neonate is considered to be different from that of adult. Although immaturity in the central nervous system that governs urinary bladder is thought to be the cause, few well-designed physiological studies on the properties of bladder itself have been reported. MATERIALS AND METHODS: Urinary bladders were removed from male Sprague-Dawley rats, 1 to 3-day old neonate and 12 week-old adult. Anterior longitudinal muscle strips without mucosa were obtained and isometric contractions were recorded. Contractile responses induced by carbachol (CCh 0.05-10.0 microM) were obtained. To investigate length-tension relationship between adult and neonate bladder, each strip was stepwise stretched up to 300% L (taking L, 100% length in a resting state) by micropositioner. In each step it was fully permitted to reach steady state in which electrical field stimulations (0.8ms, 50Hz, 2.5s duration) were applied to assess optimal length for contraction. RESULTS: CCh produced a large phasic contraction followed by a sustained tonic contraction superimposed by rapid phasic activities (RPAs) in both group but the patterns were quite different. Neonatal bladder was characterized by its tonic contractions; lesser tone increase with dose increment and taller RPAs of shorter duration. Amplitude of CCh-induced contraction were corrected by tissue weight, neonatal bladder generated greater force than that of adult. Length-tension analysis revealed that the range of active contraction in neonatal bladder according to the stretch were similar, while passive properties were quite different from that of adult bladder. Stress in the muscle strips was derived from tension divided by cross-sectional area of the strip (g/cm2). Steep increase in total tension per area as stepwise stretch was observed in neonatal bladder, which was mainly ascribed to passive properties of the bladder. Optimal length of maximal active force generation was noted in 220% of resting length in both neonatal and adult bladder. CONCLUSIONS: Our results demonstrated that CCh-induced contractile pattern and length-tension relationship in neonatal bladder are quite different from those of adult bladder in the rats, which suggests, aside from the neural immaturity, functional difference exists in the bladder itself. During development, neonatal urinary bladder seems to undergo remarkable changes in both active and passive properties.