Bladder dysfunction is common in diabetes and can manifest as detrusor over- and underactivity. Conditions such as the overactive bladder syndrome are less efficiently treated in the presence of diabetes. As part of diabetic cystopathy, a major enlargement of the urinary bladder (often a doubling in weight) occurs in all rodent models of type 1 diabetes and many, but not all of type 2 diabetes [1, 3]. An enlargement (hypertrophy and/or hyperplasia) of tissues including prostate, skeletal muscle, heart, and kidney in non-diabetic animals is typically accompanied by fibrosis and (non-infectious) inflammation. This is also the case with bladder enlargement in euglycemic models, e.g., after bladder outlet obstruction. Nonetheless, it has been reported repeatedly that the bladder enlargement in the streptozotocin (STZ) model of type 1 diabetes is not accompanied by fibrosis, if anything the opposite [1]. However, no such information is available for other models of type 1 diabetes or any model of type 2 diabetes. Therefore, we have explored expression of markers of fibrosis (collagens I and III, and TGF-β) and of inflammation (MCP-1) in the urinary bladder of six models type 1 and 2 diabetes including examples that do and do not exhibit bladder enlargement.

Urinary bladder specimens were obtained from the following rodent models: A) STZ rats (female, type 1 model, enlarged bladder; including a treatment arm with empagliflozin n = 4-6); B) RIP-LCMV mice (both sexes; type 1 model, enlarged bladder; n = 9-11), C) ZSF rats (male, type 2 model, enlarged bladder, n = 4-6), D) insulin receptor substrate 2 (IRS2) knock-out mice (both sexes; type 2 model, no bladder enlargement, n = 8-12), E) ob/ob mice (both sexes, enlarged bladder, n = 7-15), and F) db/db mice (both sexes; including an additional ob/ob arm, enlarged bladder in ob/ob and moderately enlarged bladder in db/db, n = 13-16). Each study had been approved by the locally responsible animal committee.
mRNA isolation and expression analysis was performed by quantitative PCR using the 2(−ΔΔCT) method with normalization for dual GAPDH and β-actin expression (2). Data were excluded from the analysis if melting curves for the target mRNA or the references gene products GADPH and β-actin were inappropriate, or when a value differed by more than 2 log units from the others within the group. 
Except for the STZ rats, all data and samples came from studies primarily performed for other purposes, i.e., only the STZ rats were sacrificed for the purpose of our study. All available bladders from each study were used. Therefore, sample sizes were not based on formal power calculations for bladder endpoints, and the findings should be considered as exploratory, not as hypothesis-testing. Accordingly, no hypothesis-testing and only descriptive statistical analysis was applied. It was performed by unpaired two-tailed t-tests for the 2-armed studies and by ANOVA followed by Sidak’s multiple comparison tests for the 3-armed studies.

In contrast to many non-diabetic models and in confirmation of findings in STZ-induced diabetes, the mRNA expression of collagen I was not increased in any of the six models (Figure 1). Similarly, expression of collagen III was not increased, except in RIP-LMCV mice. Neither was expression of TGF-β. The inflammatory marker MCP-1 also was not increased in any of the six models (Figure 2).

Our data are based on studies performed for other purposes, except for that on STZ rats. However, all bladder analyses had been prespecified. This led to different sample sizes between groups, i.e., ranging from 4-6 in STZ and ZSF rats, to more than 10 in the mouse studies. While the studies with the smaller sample sizes may have had too little statistical power, the general picture was consistent across all six studies, i.e., that the enlarged diabetic bladders did not exhibit an increased expression of collagens I and III (with one exception) and TGF-β or the inflammatory marker MCP-1.

This is the first report on markers of fibrosis and inflammation in the bladder of animal models of diabetes other than the STZ model of type 1 diabetes and based on five such models including four models of type 2 diabetes. Together with previously reported data in STZ-injected rats and mice, these data show that a lack of fibrosis and inflammation (at least at the level of mRNA expression for the markers studied here) is not limited to a single model of type 1 diabetes but a universal finding in rodent models of diabetic cystopathy. Apparently, the bladder enlargement developing in diabetes has a fundamentally distinct pathophysiology from that occurring in non-diabetic animal models.

Ellenbroek JH, Arioglu-Inan E, and Michel MC. A systematic review of urinary bladder hypertrophy in experimental diabetes: Part 2. Comparison of animal models and functional consequences. Neurourol Urodyn 37: 2346-2360, 2018.
Erdogan BR, Michel MB, Matthes J, Castañeda TR, Christen U, Arioglu-Inan E, Michel MC, and Pautz A. A comparison of urinary bladder weight in male and female mice across five models of diabetes and obesity. Front Pharmacol 14: 92355, 2023.
Yesilyurt ZE, Matthes J, Hintermann E, Castañeda TR, Elvert R, Beltran-Ornelas JH, Silva-Velasco DL, Xia N, Kannt A, Christen U, Centurión D, Li H, Pautz A, Arioglu-Inan E, and Michel MC. Analysis of 16 studies in nine rodent models does not support the hypothesis that diabetic polyuria is a main reason of urinary bladder enlargement. Front Physiol 13: 923555, 2022.