Active properties of the urinary bladder: in vitro comparative studies between adult and neonatal rats

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Abstract
Objective To determine, using in vitro comparative studies, developmental aspects associated with the active properties of the urinary bladder in neonatal and adult rats. Materials and methods Urinary bladders were removed from neonatal (1–3 days old) and adult (15 weeks old) male Sprague–Dawley rats. Anterior longitudinal muscle strips were obtained from each group and isometric tensions recorded. Nerve‐mediated contractions elicited by electrical field stimulation (0.8 ms pulse) or carbachol‐induced contractile responses in neonatal and adult bladder strips were compared. Contractile tensions were normalized using the wet weight of the bladder strip or by using the percentage contraction induced by 60 mmol/L KCl. Results Nerve‐mediated contractile responses showed that the muscarinic component predominated in the neonatal rat bladder, which contrasted with the predominant purinergic components in adult bladder. The pattern of spontaneous activity and carbachol‐induced contraction differed in the two groups. Small spontaneous contractions in the basal state occurred in adult bladder strips, while short‐lived large spontaneous contractions were present in neonatal strips. The amplitude of carbachol‐induced contractions generated in the neonatal bladder was larger than that in the adult bladder. In addition, the time to achieve peak contraction elicited by carbachol (5 µmol/L) was shorter in the neonatal bladder. Repetitive carbachol applications induced an attenuation of the contractile response (desensitization), but the neonatal bladder was more resistant to desensitization than the adult bladder. Conclusions These results show that nerve‐mediated or agonist‐induced contractile patterns, and spontaneous activity, in the neonatal bladder differ from those of the adult bladder in rats. The results suggest that in addition to neural immaturity, there are functional differences between the bladders of adult and neonatal rats.