Comparative analysis of bladder wall compliance based on cystometry and biosensor measurements during the micturition cycle of the rat.

The stiffness characteristics of the empty and filling bladder and the modulating influence of oxybutynin were investigated using a new biosensor system. Studies were done comparing the stiffness measured using the pressure/volume relationship with direct biosensor monitoring on male and female rats during isovolumetric contractions elicited during the cystometrogram (CMG). Bladder stiffness at zero volume, measured in vitro using the biosensor, was evaluated and compared with the stiffness of the prostate, seminal vesicles, testicles, and uterus. In 5 small anesthetized male rats, in vivo isovolumetric studies were performed and bladder stiffness was measured during the storage and contraction phase of the CMG. In 6 mature female rats, change in bladder stiffness during isovolumetric contractions was investigated following intraarterial (i.a.) administration of 0.1 and 1.0 mg/kg of oxybutynin. After the in vivo CMG was completed, an in vitro CMG was done measuring bladder stiffness. The results show that bladder stiffness, measured during the storage phase of the CMG, increased in accordance with the stretched length of bladder wall. During the in vivo CMG, bladder stiffness increased consequent to a spontaneous contraction from 10.0 +/- 1.9 g/cm to 29.9 +/- 3.0 g/cm (P < 0.005). Oxybutynin produced a significant decrease in bladder stiffness during the storage phase of the CMG, as measured using the biosensor, which was concomitant with an increase in bladder compliance derived from pressure/volume data. The incremental change in stiffness, delta K, during isovolumetric contraction decreased due to i.a. oxybutynin in accordance with a decrease of maximum detrusor pressure. These results indicate that delta K is related to the active change of viscoelastic properties of bladder smooth muscle. These findings imply that direct measurement of the stiffness of the bladder wall possesses the potential to be an objective assessment of bladder biomechanical properties and of their functional response to obstruction and pharmacological intervention.