Adreno-muscarinic synergy in the bladder trigone: Calcium-dependent and -independent mechanisms

Adreno-Muscarinic Synergy of Contractile Responses From Human Hyperplastic Prostate

PURPOSE

Adreno-muscarinic synergy, a supra-additional contractile response to simultaneous application of α-adrenoreceptor and muscarinic receptor agonists, is a feature of several lower urinary tract regions that have dual sympathetic and parasympathetic innervation. We tested the hypothesis that synergy is also a feature of prostate tissue obtained from men with benign prostatic enlargement.

METHODS

Isolated tissue strips were dissected from prostate 'chips', collected after transurethral prostate resection procedures for in vitro experiments, to measure isometric tension at 36°C.

RESULTS

Added separately to the superfusate, phenylephrine and carbachol generated contractions with mean pEC50 (-log10EC50) values of 5.36 and 5.58, respectively, although phenylephrine maximal responses were about six-fold greater. In the presence of carbachol, the mean phenylephrine pEC50 was significantly increased to 5.84 and maximal response increased by 28%; overall, a significant synergistic response was demonstrated. The synergistic response was reduced by muscarinic receptor antagonists, most potently by the M3-selective agent 4-DAMP (1,1-dimethyl-4-diphenylacetoxypiperidinium iodide), and less so by M2 and M1-selective inhibitors gallamine and pirenzepine, but with an overall profile indicating M3/M2 mediation of the synergistic response. The magnitude of the synergistic response was variable between prostate chips that provided isolated preparations suggesting regional heterogenicity, although their zonal origin could not be determined.

CONCLUSION

These experiments show that adreno-muscarinic contractile synergy is a feature of human hyperplastic prostate tissue. This has implications for the use of a combination therapy of α-blockers and anti-muscarinic agent to relieve secondary symptoms associated with benign prostatic hyperplasia, at least in men who can tolerate antimuscarinics without a risk of retention.

Adrenoceptors in the Lower Urinary Tract

Adrenoceptors importantly contribute to the physiological regulation of lower urinary tract (LUT) function and have become a target of several clinically successful treatments for major LUT diseases. In the bladder dome, β-adrenoceptor subtypes are found in multiple cell types and mediate relaxation of detrusor smooth muscle, perhaps partly indirectly by acting on afferent nerves and cells of the mucosa. β₃-adrenoceptor agonists such as mirabegron and vibegron are used to treat overactive bladder syndrome. In the bladder trigone and urethra, α₁-adrenoceptors cause contraction and thereby physiologically contribute to bladder outlet resistance. α₁-adrenoceptors in the prostate also cause contraction and pathophysiologically elevate bladder outlet resistance leading to voiding dysfunction in benign prostatic hyperplasia. α₁-adrenoceptor antagonist such as tamsulosin is widely used as a first-line option to treat LUT symptoms in men, but it remains unclear to which extent and how smooth muscle relaxation contributes to symptom relief.

Spontaneous Activity and the Urinary Bladder

The urinary bladder has two functions: to store urine, when it is relaxed and highly compliant; and void its contents, when intravesical pressure rises due to co-ordinated contraction of detrusor smooth muscle in the bladder wall. Superimposed on this description are two observations: (1) the normal, relaxed bladder develops small transient increases of intravesical pressure, mirrored by local bladder wall movements; (2) pathological, larger pressure variations (detrusor overactivity) can occur that may cause involuntary urine loss and/or detrusor overactivity. Characterisation of these spontaneous contractions is important to understand: how normal bladder compliance is maintained during filling; and the pathophysiology of detrusor overactivity. Consideration of how spontaneous contractions originate should include the structural complexity of the bladder wall. Detrusor smooth muscle layer is overlain by a mucosa, itself a complex structure of urothelium and a lamina propria containing sensory nerves, micro-vasculature, interstitial cells and diffuse muscular elements.Several theories, not mutually exclusive, have been advanced for the origin of spontaneous contractions. These include intrinsic rhythmicity of detrusor muscle; modulation by non-muscular pacemaking cells in the bladder wall; motor input to detrusor by autonomic nerves; regulation of detrusor muscle excitability and contractility by the adjacent mucosa and spontaneous contraction of elements of the lamina propria. This chapter will consider evidence for each theory in both normal and overactive bladder and how their significance may vary during ageing and development. Further understanding of these mechanisms may also identify novel drug targets to ameliorate the clinical consequences of large contractions associated with detrusor overactivity.

Adreno-muscarinic synergy in the male human urinary outflow tract

AIM

To examine putative interaction between adrenergic and muscarinic contractile activation in the human urinary outflow tract.

METHODS

Tissue from the trigone and prostatic urethra was obtained from 12 cystectomy and 16 prostatectomy specimen. Contractions were elicited by exposure to exogenous agonists before and after inhibition of Rho kinase and protein kinase c (PKC). Immunofluorescence and Western-blot studies were performed using antibodies to muscarinic M3-receptors (M3-R) and alpha1A-adrenoreceptors (alpha1A-AR). The study is registered with ClinicalTrials.gov, number NCT01227447.

RESULTS

There was strong co-localization of M3-R and alpha1A-AR on trigonal and urethral myocytes. Western blot analysis revealed a significantly higher expression of alpha1A-AR in the superficial than in the deep trigone. Phenylephrine (PE, 1 μm) augmented contractions induced by carbachol (CA, 3 μm) to more than threefold control in the male superficial trigone, and to about sevenfold control in the proximal urethra. No such potentiation could be detected in female bladder outlet. Both PKC inhibitor GF 109203X and Rho kinase inhibitor Y-27632 reduced responses to 1 μM PE as well as to 3 μM CA significantly. However, the synergistic effect of the combined intervention remained proportionally unaffected.

CONCLUSIONS

Muscarinic and adrenergic receptor activation exerts a strong synergistic effect in the male human bladder trigone and proximal urethra.

Phorbol 12,13-dibutyrate-induced, protein kinase C-mediated contraction of rabbit bladder smooth muscle

Contraction of bladder smooth muscle is predominantly initiated by M(3) muscarinic receptor-mediated activation of the G(q/11)-phospholipase C β-protein kinase C (PKC) and the G(12/13)-RhoGEF-Rho kinase (ROCK) pathways. However, these pathways and their downstream effectors are not well understood in bladder smooth muscle. We used phorbol 12,13-dibutyrate (PDBu), and 1,2-dioctanoyl-sn-glycerol (DOG), activators of PKC, in this investigation. We were interested in dissecting the role(s) of PKC and to clarify the signaling pathways in bladder smooth muscle contraction, especially the potential cross-talk with ROCK and their downstream effectors in regulating myosin light chain phosphatase activity and force. To achieve this goal, the study was performed in the presence or absence of the PKC inhibitor bisindolylmaleimide-1 (Bis) or the ROCK inhibitor H-1152. Phosphorylation levels of Thr(38)-CPI-17 and Thr(696)/Thr(850) myosin phosphatase target subunit (MYPT1) were measured during PDBu or DOG stimulation using site specific antibodies. PDBu-induced contraction in bladder smooth muscle involved both activation of PKC and PKC-dependent activation of ROCK. CPI-17 as a major downstream effector, is phosphorylated by PKC and ROCK during PDBu and DOG stimulation. Our results suggest that Thr(696) and Thr(850)-MYPT1 phosphorylation are not involved in the regulation of a PDBu-induced contraction. The results also demonstrate that bladder smooth muscle contains a constitutively active isoform of ROCK that may play an important role in the regulation of bladder smooth muscle basal tone. Together with the results from our previous study, we developed a working model to describe the complex signaling pathways that regulate contraction of bladder smooth muscle.

Clinical and experimental aspects of Adreno-muscarinic synergy in the bladder base and prostate

Recent clinical trials have shown that combination therapy using an alpha-receptor antagonist and an antimuscarinic is more effective than either agent alone in improving quality of life and objective urodynamic variables in men with bladder outflow obstruction. There appear to be no negative effects on bladder function. The mode of action of this combination is unknown but presumed to be an antimuscarinic reduction in detrusor overactivity and the alpha-receptor antagonist reduced outflow tract resistance. We have shown with in vitro experiments that in smooth muscles influencing outflow tract resistance (prostate, trigone) there is a profound contractile synergy between adrenergic and muscarinic pathways. We propose the hypothesis that both arms of the combination therapy reduce contractile tone of the outflow tract and that their simultaneous attenuation has a disproportionately large effect on outflow tract resistance. Our data from trigone muscle suggest that adrenergic and muscarinic receptor activation increase the intracellular [Ca(2+)] but the adrenergic pathway also operates through Ca(2+)-sensitisation of the contractile apparatus, primarily through a PKC-dependent pathway.