Compound 48/80 increases bladder wall compliance independent of mast cells

Abstract

Changes in wall compliance are seen in many bladder pathologies, such as fibrosis and diabetes. Some of these pathologies can be caused by basic secretagogues like Compound 48/80 and its endogenous analogue, Substance P. Traditionally it is believed that basic secretagogues rapidly degranulate mast cells and release inflammatory mediators by acting on an orphan receptor, Mas‐related G protein‐coupled receptor B2 (Mrgprb2). While Compound 48/80 does increase wall compliance, the exact location and function of Mrgprb2 in the urinary bladder are still unknown. We hypothesize that changes in bladder wall compliance caused by Compound 48/80 are mediated through Mrgprb2 receptors but independent of mast cell degranulation.Whole mouse bladders from C57Bl/6 (wild‐type) and C‐kitW‐sh (mast cell‐deficient) mice were dissected and mounted in a custom designed pentaplanar reflected image macroscopy (PRIM) system for simultaneous measurement of transient pressure events, intravesical pressure, bladder volume, wall stress, and wall stretch during ex vivo bladder filling. Wall compliance was derived as a measure of the stress versus stretch relationship using the pressure‐volume curves and associated imaging data. To test our hypothesis, bladders were exposed to either vehicle or Compound 48/80 (10 µg/ml) during ex vivo filling. Compound 48/80 significantly increased the leading slope of transient pressure events in both wild‐type and mast cell‐deficient mice. Compliance was also increased, as shown by decreased stress per change in stretch. We also collected urothelium and detrusor layers from C57Bl/6 and C‐kitW‐sh mouse bladders. Layers were separated using blunt dissection, and RNA was isolated for measurement of Mrgprb2 expression using qRT‐PCR. No significant difference in the expression of Mrgprb2 mRNA was observed between the two mouse models in either the urothelium or detrusor layers. These findings suggest that Mrgprb2 receptors are also expressed outside of mast cells in the urinary bladder wall and, when activated by Compound 48/80, increase wall compliance and the leading slope of transient pressure events.