Metabolism of detrusor smooth muscle in normal and obstructed urinary bladder

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

Cyr61 and CTGF are molecular markers of bladder wall remodeling after outlet obstruction

Cysteine-rich protein (Cyr61) and connective tissue growth factor (CTGF) are key immediate early growth factors with functions in cell proliferation, differentiation, and extracellular matrix synthesis. Studies were performed to assess the gene expression profile of Cyr61 and CTGF in rat urinary bladder during growth in response to partial outlet obstruction. The mRNA levels of Cyr61 as determined by ribonuclease protection assay increased sharply after 1 day and remained elevated throughout the time period of the obstruction. This correlates well with increased bladder weight. The CTGF mRNA levels seemed to peak within the second week of the urethral obstruction and correlate well with increased type I collagen mRNA. The expression pattern of either Cyr61 or CTGF proteins corroborated that of their respective mRNAs. Immunohistochemical analyses showed that immunoreactivity of Cyr61 was confined to detrusor smooth muscle and that of CTGF was detected within both detrusor muscle and lamina propria layers. These data strongly indicate the involvement of Cyr61 and CTGF in bladder wall remodeling as a result of the outlet obstruction.

Normal detrusor is more sensitive than hypertrophied detrusor to in vitro ischemia followed by re-oxygenation

Partial outlet obstruction results in marked metabolic as well as contractile alterations. Specifically, the ratio of anaerobic to oxidative metabolism is significantly greater in hypertrophied than normal bladder smooth muscle, lactate dehydrogenase (LDH) and lactic acid production are increased, and the contractile apparatus is altered to allow for metabolically more efficient tension generation. In addition, contractile responses of hypertrophied bladder are apparently more resistant than those of normal bladder to hypoxia. In the current experiment, we studied the effects of in vitro ischemia (hypoxia + substrate deprivation) followed by an in vitro model of reperfusion (re-oxygenation + substrate replacement) on contractile responses of normal and hypertrophied urinary bladder strips. We used repetitive field stimulation (FS) during the hypoxic period as a model for hyperreflexia. The purpose of the current study was to compare the responses of normal and hypertrophied bladder smooth muscle to repetitive stimulation in the presence of in vitro ischemia followed by re-oxygenation and substrate replacement. Thirty-two rats were separated into four groups of eight each. The rats in groups 1 and 3 were subjected to partial outlet obstruction. Two weeks later, all rats were anesthetized; their bladders were isolated and cut into four strips. Each strip was mounted in an isolated bath, and after 1-hour incubation in Tyrode's solution containing glucose (in the presence of O(2)), contractile responses to FS, carbachol, and KCl were determined. After this first set of stimulations, the strips were incubated without glucose and in the presence of N(2) for 30 minutes and 1 hour (groups 1 and 2); and for 2 and 4 hours (groups 3 and 4). For groups 1 and 2, the tissues were stimulated at 5-minute intervals with FS at 32 Hz, 1-millisecond duration, 3-second trains (in vitro model of hyperreflexia). For groups 3 and 4, no stimulations were performed during the ischemic period. At the end of the ischemic period, all strips were washed and incubated for 1 hour in the presence of O(2) and with glucose. At the end of this incubation, all strips received a second set of stimulations. a) Partial outlet obstruction resulted in a significant increase in bladder weight. b) Responses to in vitro ischemia: After in vitro ischemia, contractile responses of both normal and hypertrophied tissues to FS were reduced to a significantly greater degree than were responses to carbachol and KCl. The rate of development of contractile dysfunction was significantly greater in normal bladder tissue strips than in hypertrophied bladder strips. c) Responses to repetitive stimulation: The rate of development of contractile dysfunction was significantly greater in all strips subjected to repetitive stimulation than in those not repetitively stimulated; in addition, normal bladder strips were more sensitive than hypertrophied strips to hypoxia and substrate deprivation-induced contractile dysfunction. The rate of contractile failure induced by in vitro ischemia followed by re-oxygenation and substrate replacement was significantly greater for normal bladder strips than for hypertrophied bladder strips. These results indicate that, after partial outlet obstruction, the hypertrophied tissue is more resistant than normal tissue to hypoxia and substrate deprivation.

Effect of diltiazem and pinacidil on the response of the rabbit urinary bladder to repetitive stimulation and in vitro ischemia

The effect of repetitive stimulation, in the presence and absence of diltiazem or pinacidil, on the contractile responses of isolated strips of rabbit bladder detrusor to field stimulation and carbachol, after 2 hr of incubation in a medium that serves as an in vitro model of ischemia (oxygen and substrate depleted Tyrode's solution), was determined. Our results are summarized as follows: a) The magnitude of the contractile dysfunctions after in vitro ischemia was enhanced by repetitive stimulation. b) Pre-incubation of isolated strips of detrusor with diltiazem (50 microM) inhibited the contractile responses to field stimulation (FS) and carbachol by 43 and 50%, respectively. Pinacidil (100 microM) inhibited the contractile responses to FS and carbachol by 37 and 32%, respectively. c) Neither diltiazem nor pinacidil protected the bladder strips against the effects of 2 hr of incubation in in vitro ischemia medium. However, d) both pinacidil and diltiazem reduced the level of contractile dysfunctions induced by repetitive stimulation. In conclusion, the contractile response to FS was significantly more sensitive to in vitro ischemia and repetitive stimulation than was the contractile response to carbachol. Both diltiazem and pinacidil protected the contractile responses to FS and carbachol from the degenerative effects of repetitive stimulation, but not from the effects of in vitro ischemia.