Motor and inflammatory effect of hyperosmolar solutions on the rat urinary bladder in relation to capsaicin-sensitive sensory nerves.

1. Intravesical administration of hyperosmolar NaCl or urea solutions produced a concentration-dependent stimulatory action on the micturition reflex in urethane-anesthetized rats. This effect was not modified in rats pretreated with capsaicin as adults (50 mg/kg s.c. 4 days before). 2. Hyperosmolar NaCl also produced Evans blue leakage (plasma extravasation) in the rat bladder. This effect was greatly reduced by extrinsic bladder denervation and in rats desensitized to capsaicin as newborns but not as adults. 3. Cumulative addition of NaCl produced a concentration-dependent increase in tone and biphasic effects on neurogenic contractions of the rat isolated bladder. These effects were not modified by in vitro capsaicin desensitization. 4. These findings do not support the idea that true osmoreceptors are present in the rat urinary bladder. The neurogenic component of the inflammatory response to hyperosmolar NaCl could involve activation of a subpopulation of bladder sensory fibers susceptible to the neurotoxic action of capsaicin in the early postnatal period only.