0022-5347/94/1515-1405$03.00/0 THE JOURNAL OF UROLOGY Copyright © 1994 by AMERICAN UROLOGiCAL ASSOCIATION, INC. Vol. 151, CONTRACTILITY PARA1VIETERS OF THE GUINEA PIG BLADDER IN SITU: SIMILARITY TO HUMAN BLADDER CONTRACTILITY Jo GROEN,* R VAN MASTRIGT, E. VAN ASSELT, Go Ao VAN KOEVERINGE AND R. BOSCH From the Department of Urology, Erasmus University Rotterdam, Rotterdam, The Netherlands ABSTRACT The parameters P isv (active isovolumetric detrusor pressure) and V max (maximum shortening velocity), which characterize the contractility of the detrusor muscle, were determined in guinea pigso To this end it was necessary to develop a method of measuring flow rates in these small animalso The values found were used to calculate the contractility parameter W max' Thirteen animals were usedo The results found for P isv and V max were 4300 ± 3.7 cmo H 20 and 2002 ± 307 mmo per second, respectivelyo The latter corresponded to about 0038 muscle lengths per second, which is similar to values reported for bladder strips from other species. Previous work showed that in vitro P isv decreased with increasing bladder volume over a wide range of volumeso In vivo P isv seemed to be independent of bladder volumeo This suggests that neurogenic stimulation intensifies as volume increaseso V max also was independent of volumeo W max appeared to be suitable for detecting differences in the contractility of the bladders of different animalso Values were not significantly different in isovolumetric and nonisovolumetric contractionso Normalized to the size of the bladder, the W max values indicated that the power generated by the guinea pig bladder is similar to the power generated by the human bladder. KEY WORDS: bladder, urination, guinea pigs, muscle contraction, urodynamics In a micturition contraction the detrusor pressure rises above the (passive) pressure determined by the viscoelastic properties of the bladder wall to the amount of P det, the active detrusor pressure. The relation between P det and the shortening velocity v of the bladder circumference can be modelled by the equation' (P det/Pisv + a/PisvHv + b) = (1 + a/P isv )' b (1) where P isv is the active detrusor pressure in an isovolumetric contraction and a and b are constants (fig. 1)0 It has been shown experimentally that a/P isv is approximately 0.25.' By definition, b = (a/Pisv ) X Vmax, where V max is the potential maximum shortening velocity (at zero pressure), and consequently b = v max/4. The above means that the contractility of a particular bladder can be completely characterized by the constants P isv and vmax . Not all micturitions result in pressure/flow rate plots from which good estimations of these parameters can be derived. 2 Also, the use of two parameters hampers comparison of the contractility of different bladders. Several methods have there- fore been developed to quantify bladder contractility in pa- tients. 3 - 8 The parameter VI! max is considered to give a good estimation of the contraction strength of a given detrusor in the general population0 5 This parameter is based on the meas- urement of pressure and flow rate in a nonmanipulated mictu- rition and gives an approximated value for the maximum power generated by the detrusor per bladder unit surface area. To study the physiological basis and relevance of the con- tractility parameters P isv , Vrnax and the combined parameter W max, pressure/flow rate studies were done in guinea pigs. The values derived for these parameters were compared with values from clinical studies. Because the volumes voided by the guinea pig are small, a special method of flow rate measurement had to be develope do MATERIALS AND METHODS Thirteen male guinea pigs with a mean weight of 930 gmo (range: 710 to 1335 gm.) were used. Each animal was anesthe- Accepted for publication November 16, 1993. * Requests for reprints: Department of Urology, Room EE 1630, Erasmus University Rotterdam, P.O. Box 1738, 3000 DR Rotterdam, The Netherlands. tized with urethane (1.2 gm./kg. intraperitoneally). A small incision was made in the abdominal wall to localize the bladder, into which a 24 G angiocatheter was inserted. Subsequently, the abdomen was loosely closed with silk sutureso Body tem- perature was maintained by means of a heating pad. A Harvard infusion pump was used to fill the bladder via the catheter with room temperature saline at a rate of 0053 mL per minute until a micturition contraction occurred. Pressure was measured via the same catheter using a disposable pressure transducer. The pressure signal was sampled with a PDP 11 type computer at a rate of 10 Hz. Each bladder was filled 4 to 10 times, with intervals of 10 to 15 minutes0 9 In six animals, the urethra was alternately allowed to leak and occluded in the bulbar part with a 3.5 metric silk suture. In these animals two kinds of micturition contractions were distinguished: voiding contractions (when the urethra was allowed to leak) and iso- volumetric contractions (when it was occluded). In two animals isovolumetric contractions alone were measured, and in five animals only voiding contractions were measured. Via a cone- shaped funnel, the voided volume was collected in a glass tube (diameter 407 mmo) equipped with a central glass rod (diameter 1.5 mmo) to prevent the formation of bubbles. The tube was connected to a disposable pressure transducero The volume signal thus measured was also sampled with the computer. The total voided volume was determined using a measuring glass. Residual urine after voiding and after an isovolumetric con- traction was removed with a syringe attached to the catheter. In the voiding contractions, the residual volume was expressed as a percentage of the bladder volume at the onset of micturi- tiono The largest bladder volume in each animal at which a micturition contraction was measured was taken as the cysto- metric capacity. (Occasionally, after an isovolumetric contrac- tion the bladder was filled further without emptying until the next contraction occurredo The corresponding volume was not considered to represent capacity) 0 Compliance was calculated from the bladder volume at the start of micturition and the difference between the pressure at the start of filling and at the start of the micturition contraction. Following the experiments the bladders were blotted and 1405