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The mast cell activator Compound 48/80 increases bladder compliance through matrix metalloprotease release

Bladder wall stiffness and elasticity is essential to its storage and voiding functions. In addition to smooth muscle contractility, the integrity of the extracellular matrix plays a crucial role in maintaining the optimal balance between wall stiffness and compliance as the bladder distends. Lower urinary tract symptoms (e.g., bladder overactivity and fibrosis) can be caused by mast cell activation and inflammation, which increases wall stiffness and matrix remodeling. Thus, we hypothesize that mast cell activation increases muscle contractility and decreases compliance. Whole mouse bladders were dissected and mounted in a custom‐designed pentaplanar reflected image macroscopy (PRIM) system for simultaneous measurement of the following parameters during ex vivo bladder filling: intravesical pressure, bladder volume, wall deformation, mechanical stress, and mechanical stretch. Bladders were filled at a constant rate (30 µl/min) until intravesical pressure reached 25 mmHg, at which point bladders were emptied and allowed to recover for approximately 10 minutes prior to the next fill cycle. During filling, baseline bladder pressure remained relatively low over a large infused volume (~350 µL), after which pressure increased rapidly. Transient contractions began on the lateral sides of the bladder wall near the trigone, expanded upwards as the bladder filled, and corresponded to transient increases in intravesical pressure (0‐2 mmHg). Using the pressure‐volume curves and associated image data, we next calculated the bladder wall compliance (defined as the stress vs stretch relationship of the bladder wall). As with pressure, wall stress was relatively low over a large change in stretch, implying that bladder wall was highly compliant until larger infused volumes. Subsequent exposure to the mast cell activator Compound 48/80 (10 µg/ml) significantly increased the amplitude of transient pressure events but decreased wall stress per change in stretch, indicating a paradoxical increase in compliance. Incubation with the matrix metalloprotease inhibitor doxycycline (10 µM) prior to addition of compound 48/80 prevented this increase in compliance without preventing the increased amplitude of transient pressure events. These findings suggest that Compound 48/80 activates bladder mast cells to release at least two things: (1) matrix metalloproteases that rapidly degrade the extracellular matrix and increase bladder wall compliance; and (2) compounds that increase smooth muscle excitability. Thus, both decreases in stiffness and increases in smooth muscle excitability may accompany inflammatory bladder pathologies such as diabetes and interstitial cystitis.