Succinate, an intermediate of glucose metabolism, and its GPR91 receptor have been implicated in different aspects of metabolic syndrome. In turn, metabolic syndrome is associated with impaired bladder function. We have previously shown that chronic administration of succinate to a rodent animal model compromises bladder morphology and function. Our aim is to further investigate the role of succinate and its receptor in the bladder in acute settings.

Conscious cystometry was performed on Sprague-Dawley (SD) rats, involving a 1-hour infusion of saline followed by a 1-hour infusion of succinate (10 mM). Bladders were collected from other SD rats and used for contractility assessment, where the detrusor strips (4 strips/bladder) were either kept intact or stripped of their urothelium. The strips were stimulated with 1 µM carbachol, allowed to stabilize and then tested with increasing concentrations of succinate (1 – 100 mM). These procedures were repeated on C57BL/6 mice and GPR91 knockout (KO) mice. Mice bladders were examined using the Masson’s trichrome stain. Paired t-tests were used to compare cystometry parameters between saline and succinate infusion. Two-way ANOVA with Bonferroni post-hoc test was used to compare the differences in detrusor strips at each succinate concentration. P<0.05 was considered significant.

In cystometry, infusion of succinate to rat bladders significantly increased micturition volumes and bladder compliance, compared to saline infusion (Table 1). There was also a trend for increased intermicturition intervals and bladder capacities with the infusion of succinate. In the C57BL/6 mice, these changes were also observed during succinate infusion, with the intermicturition intervals and bladder capacities also significantly increased. On the other hand, compared to C57BL/6 mice, GPR91 KO mice had initial lower values of the mentioned parameters, and did not experience bladder changes when succinate was infused. 
In contractility experiments, succinate induced relaxation of rat detrusor strips, which was less important in urothelium-denuded strips (Fig. 1A). Detrusor strips from C57BL/6 mice had a relaxation response to succinate that was not observed in strips from GPR91 KO mice (Fig. 1B). Bladder-to-body ratios were also higher in GPR91 KO mice compared to C57BL/6 mice (2.86 ± 0.21 vs. 1.97 ± 0.19 mg/g, respectively). Furthermore, qualitative histological analysis of the bladders showed a relatively thicker urothelium (with more cell layers) and lamina propria in GPR91 KO mice compared those of C57BL/6 mice (Fig. 1C).

Relaxation of the bladder by succinate was observed in cystometry, shown by increased bladder capacity and compliance in wild-type rats and mice. Ex vivo, succinate also relaxed bladder detrusor strips contracted with carbachol. This relaxation was less pronounced in the absence of the urothelium. The relaxation was mediated through the GPR91 receptor, as bladders of GPR91 KO mice were unaffected by succinate. Differences in cystometry parameters and bladder histology between C57BL/6 mice and GPR91 KO mice also suggest a role for succinate-GPR91 interaction in the normal development and function of the bladder.

Acute administration of succinate leads to relaxation of the bladder that is mediated by the GPR91 receptor. Increased circulating levels of succinate, associated with metabolic syndrome, may cause downregulation of GPR91-succinate signaling in the bladder, affecting bladder function and morphology. Further work is needed to fully characterize the mechanism of action involved in the relaxation caused by succinate activation of GPR91 in the bladder.