The effects of aging on the rat bladder and its innervation.

1) Measurements of the cystometrogram, of the responsiveness of bladder muscle to pelvic nerve efferent stimulation and of the sensitivity of the pelvic nerve afferents to pressure and volume during distensions have been made in the bladders of young adult (2-3 months) and aged (26-29 months) rats, anesthetized with mixtures of urethane and chloralose. 2) The pressure-volume relationship differed in young adult and aged rats. The bladders of the aged rats held up to nearly six times the volume of the young animals, and these volumes were accommodated at lower pressures in the aged animals. The pressure at which micturition contractions appeared was similar in young adult and aged animals. 3) The passive pressure associated with each of a series of distending volumes was recorded when a pelvic nerve was cut unilaterally. The distal cut end of this cut pelvic nerve was stimulated for 10 s at 20 Hz, using square wave pulses of 10 V and 1.0 ms. The active pressure-volume relationship was constructed from this data. Both the active and the passive relationships were shifted to the right in the aged animals, and it was evident that aging was associated with a reduction in the maximal pressure generated during pelvic nerve stimulation. Also the change in intravesical pressure induced by bladder contraction was less in aged animals. 4) The most sensitive mechanoreceptor afferents appear to have pressure and volume thresholds that do not change significantly during aging. While the distension-sensitive afferents in the pelvic nerve appear to have a similar sensitivity to intravesical pressure in young adult and aged rats, they were less able to monitor volume in the aged animals. The stimulus response relationship for volume was often less steep in the aged animals. 5) In this study, aging was shown to be associated with a large increase in bladder volume and a reduced sensitivity of pelvic nerve afferents to volume, and a reduced ability to raise bladder pressure during contraction of bladder smooth muscle. The changes in bladder function associated with aging are discussed.