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1 Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
2 Pharmacology, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
3 Institute of Urology, London, United Kingdom
* To whom correspondence should be addressed. E-mail: ajk5{at}pitt.edu.
We examined the origin of spontaneous activity in neonatal and adult rat bladders and the effect of stretch and muscarinic agonists/antagonists on spontaneous activity. Rats were anesthetized and their bladders excised, cannulated and loaded with voltage- and Ca2+-sensitive dyes. Intracellular Ca2+ and membrane potential transients were mapped using photodiode arrays, in whole bladders, sheets or cross-section preparations at 37°C. In neonatal bladders and sheets, spontaneous Ca2+ and electrical signals arose at a site near the dome and spread in a coordinated manner throughout the bladder with different dome-to-neck conduction velocities (Ca2+: 3.7±0.4 mm.s-1; membrane potential, 46.2±3.1 mm.s-1) and were associated with large spontaneous contractions (10-20 cm H2O). By contrast, in adult bladders spontaneous Ca2+ and electrical activity was uncoordinated, originating at multiple sites and was associated with smaller (2-5 cm H2O) contractions. Spontaneous contractions and optical signals were insensitive to tetrodotoxin (2 µM) but were blocked by nifedipine (10 µM). Stretch or low carbachol concentrations (50 nM) applied to neonatal whole bladders enhanced the amplitude (to 20-35 cm H2O) of spontaneous activity, which was blocked by atropine. Bladder cross-sections revealed that Ca2+ and membrane potential transients produced by stretch or carbachol began near the urothelial-suburothelial interface and spread to the detrusor. Accordingly, spontaneous activity in neonatal bladders, unlike adult bladders, is highly organized, originating in the urothelium-suburothelium near the dome. Activity is enhanced by stretch or carbachol and this enhancement is blocked by atropine. We hypothesize that acetylcholine is released from the urothelium during filling to enhance spontaneous activity.
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