NEUROGENIC MODULATION OF MICTURITION - THE RELATION BETWEEN STIMULATION INTENSITY AND THE MAXIMUM SHORTENING VELOCITY OF THE GUINEA-PIG DETRUSOR MUSCLE

The course of micturition depends on bladder contractility and urethra l resistance. The former is determined by geometrical, muscular and ne urogenic factors. The muscular aspects of bladder contractility can be characterized by the parameters P-isv, the isovolumetric detrusor pre ssure, and v(max), the maximum (unloaded) shortening velocity of the d etrusor muscle. The neurogenic control system of the urinary tract mod ulates bladder contractility, which might effectively change the value s of P-isv and v(max). These parameters also depend on the instantaneo us bladder volume. In previous work the dependence of P-isv on the int ensity of stimulation and bladder volume was measured in guinea pig bl adders in vivo and in vitro. In the present work v(max) was derived in 5 guinea pig bladders in vitro, using electrical stimulation and the stop-flow technique. This technique implies that pressure values measu red at a certain shortening velocity of the bladder circumference and in an isovolumetric contraction at the same volume are used to derive v(max) mathematically from the Hill equation. v(max) was independent o f the bladder volume in the range of 0.6 to 6.1 ml., but it was signif icantly different for the two intensities of stimulation used. Therefo re, it is concluded that the maximum shortening velocity of the guinea pig detrusor muscle depends on the intensity of stimulation. During s ubmaximal stimulation the detrusor not only generates lower pressures, it also contracts more slowly. A possible explanation for this phenom enon is that the bladder is not uniformly stimulated. The isovolumetri c pressure measured in the stop-flow test was compared with the isovol umetric pressure measured at the same bladder volume some minutes late r. It was observed that shortening had a depressant effect of approxim ately 33% on the isovolumetric pressure. This implies that the clinica lly employed stop-flow test might underestimate detrusor contraction s trength.