Hypothesis / aims of study
Recent in vivo animal studies have demonstrated that induction of chronic inflammation of the prostate leads to symptoms of bladder overactivity [1]. A pharmacological intervention study has indicated that it is possible to counteract the induced bladder symptoms by treatment with a soluble guanylate cyclase inhibitor [2]. However, to ensure that these findings have clinical relevance it is of importance to pinpoint the mechanism for how local inflammation of the prostate leads to alterations in bladder function. Recent data indicate that alterations in local bladder contractile properties arise upon induction of chronic prostatitis. It has been proposed that cross-organ sensitization is the most plausible mechanistic explanation for this, i.e, that local inflammation in the prostate alters bladder function and leads to innate alterations in the bladder via shared nerves. However, there is also a possibility that the bladder alterations could be due to local detrimental effects in the prostate disturbing other parts of the lower urinary tract simply by proximity. Thus, this scientific question needs to be addressed.
The aim of the current study was to examine if (a) innate contractile properties and (b) functional receptor expression in the prostate and urethra is altered in a state of chronic prostatitis.
Study design, materials and methods
To achieve the objectives, the innate contractile properties of the prostate and urethra in health and in a state of prostate inflammation were examined in a rat model of chronic prostatitis. For this purpose, 32 adult male Sprague-Dawley rats (270-450 g; Charles River Laboratories, Calco, Italy) were used. The rats were randomly divided into two groups, receiving either an intraprostatic (dorsal prostate) injection with vehicle (10 µl sterile saline), serving as control, or zymosan (0.1 mg in 10 µl sterile saline), in order to induce inflammation. On the 14th day post intraprostatic injection, the dorsal prostate and urethra were excised. Subsequently, the animals were euthanized. From each animal, two strips of dorsal prostate and the full-length urethra were mounted in an organ bath setup. After a short period of recovery, contractile responses to electrical field stimulation (EFS), the muscarinic agonist methacholine and the adrenergic agonist phenylephrine were examined. Subsequently, the tissues were examined immunohistochemically for grading of muscarinic M3 and adrenergic α1 receptor expression.
Statistical calculations were performed using GraphPad Prism version 9.5.0 (GraphPad Software Inc., San Diego, USA). Two-way ANOVA followed by Bonferroni`s post-hoc test for multiple comparisons was used for statistical comparisons of organ bath data. Immunohistochemical findings were statistically compared using the Mann-Whitney U test. Statistical significance was regarded for p-values < 0.05.
Results
Induction of chronic inflammation of the prostate with zymosan led to a slight but not statistically significant increase of contractile responses to EFS and methacholine in urethral tissues (Fig 1a, b). Urethral contractile responses to phenylephrine were identical in tissues from animals with chronic prostatitis, as compared to healthy controls (Fig 1c). Comparably, upon induction of chronic prostatitis, immunohistochemical evaluation of urethral tissue revealed no changes in expression of α1 adrenoceptors, but a significant increase in the expression of muscarinic receptors of the M3 subtype (p = 0.013).
Induction of chronic inflammation of the prostate with zymosan did not lead to any alterations of contractile responses to neither EFS, methacholine nor phenylephrine in prostate tissue strips (Fig 2). Likewise, no changes in expression of α1 adrenoceptors or muscarinic M3 receptors were revealed by immunohistochemical evaluation.
Interpretation of results
In light of previous studies that demonstrated that induction of chronic prostatitis leads to bladder overactivity and innate alterations of bladder contractile responses [1,2], the current study examined if induction of chronic inflammation of the prostate with zymosan also leads to alterations in contractile responses in prostatic and urethral tissue. Further, the expression of the main functional receptor subtypes, i.e. α1 adrenoceptors and muscarinic M3 receptors, was investigated. The current data show that despite induction of chronic inflammation, the contractile responses of the prostate, as well as expression of functional receptor subtypes, remain unchanged. Considering the dramatic effects that induction of chronic prostatitis has been shown to have in the urinary bladder, this finding was somewhat surprising. There does seem to be some impact on contraction of the urethra, including a statistically significant increase of the expression of muscarinic M3 receptors. However, the current data are not sufficiently convincing as no significant differences in contractile responses could be demonstrated. Set in relation to previous findings, the current data strengthen the proposed idea of cross-organ sensitization being involved in the bladder alterations seen during chronic prostatitis.
Concluding message
To the best of our knowledge, the current study is the first to examine how contractile properties and receptor expression in the isolated prostate and urethra is affected by induction of chronic prostatitis. The findings demonstrated that induction of prostate inflammation does not lead to changes in neither prostatic contractile responses nor expression of receptors involved in contraction of prostate smooth muscle. Expression of muscarinic receptors of the M3 subtype is upregulated in urethra, without having a significant impact on urethral contractile responses. Considering previous reports of bladder overactivity and alterations in local bladder contractile properties upon induction of inflammation of the prostate, the current data support the idea of cross-organ sensitization, rather than alterations arising due to local detrimental effects in the prostate. Future studies on chronic prostatitis should focus on bladder alterations, and how to alleviate these.