Potentiation of carbachol-induced detrusor smooth muscle contractions by beta-adrenoceptor activation

Normalization of organ bath contraction data for tissue specimen size: does one approach fit all?

Organ bath experiments are a key technology to assess contractility of smooth muscle. Despite efforts to standardize tissue specimen sizes, they vary to a certain degree. As it appears obvious that a larger piece of tissue should develop greater force, most investigators normalize contraction data for specimen size. However, they lack agreement which parameter should be used as denominator for normalization. A pre-planned analysis of data from a recent study was used to compare denominators used for normalization, i.e., weight, length, and cross-sectional area. To increase robustness, we compared force with denominator in correlation analysis and also coefficient of variation with different denominators. This was done concomitantly with urinary bladder strips and aortic rings and with multiple contractile stimuli. Our urinary bladder data show that normalization for strip weight yielded the tightest but still only moderate correlation (e.g., r² = 0.3582 for peak carbachol responses based on 188 strips). In aorta, correlations were even weaker (e.g., r² = 0.0511 for plateau phenylephrine responses normalized for weight based on 200 rings). Normalization for strip size is less effective in reducing data variability than previously assumed; the normalization denominator of choice must be identified separately for each preparation.

β3 -Adrenoceptors in the normal and diseased urinary bladder-What are the open questions?

β₃ -Adrenoceptor agonists are used in the treatment of overactive bladder syndrome. Although the relaxant response to adrenergic stimulation in human detrusor smooth muscle cells is mediated mainly via β₃ -adrenoceptors, the plasma concentrations of the therapeutic dose of mirabegron, the only clinically approved β₃ -adrenoceptor agonist, are considerably lower than the EC₅₀ for causing direct relaxation of human detrusor, suggesting a mechanism of action other than direct relaxation of detrusor smooth muscle. However, the site and mechanism of action of β₃ -adrenoceptor agonists in the bladder have not been firmly established. Postulated mechanisms include prejunctional suppression of ACh release from the parasympathetic nerves during the storage phase and inhibition of micro-contractions through β₃ -adrenoceptors on detrusor smooth muscle cells or suburothelial interstitial cells. Implications of possible desensitization of β₃ -adrenoceptors in the bladder upon prolonged agonist exposure and possible causes of rarely observed cardiovascular effects of mirabegron are also discussed. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Therapeutic modulation of urinary bladder function: multiple targets at multiple levels

Storage dysfunction of the urinary bladder, specifically overactive bladder syndrome, is a condition that occurs frequently in the general population. Historically, pathophysiological and treatment concepts related to overactive bladder have focused on smooth muscle cells. Although these are the central effector, numerous anatomic structures are involved in their regulation, including the urothelium, afferent and efferent nerves, and the central nervous system. Each of these structures involves receptors for—and the urothelium itself also releases—many mediators. Moreover, hypoperfusion, hypertrophy, and fibrosis can affect bladder function. Established treatments such as muscarinic antagonists, β-adrenoceptor agonists, and onabotulinumtoxinA each work in part through their effects on the urothelium and afferent nerves, as do α1-adrenoceptor antagonists in the treatment of voiding dysfunction associated with benign prostatic hyperplasia; however, none of these treatments are specifically targeted to the urothelium and afferent nerves. It remains to be explored whether future treatments that specifically act at one of these structures will provide a therapeutic advantage.

β-Adrenoceptors differentially regulate vascular tone and angiogenesis of rat aorta via ERK1/2 and p38

β-Adrenoceptors (β-ARs) modulate ERK1/2 and p38 in different cells, but little is known about the contribution of these signaling pathways to the function of β-ARs in vascular tissue. Immunoblotting analysis of rat aortic rings, primary endothelial (ECs) and smooth muscle cells (SMCs) isolated from aorta showed that β-AR stimulation with isoprenaline activated p38 in aortic rings and in both cultured cell types, whereas it had a dual effect on ERK1/2 phosphorylation, decreasing it in ECs while increasing it in SMCs. These effects were reversed by propranolol, which by itself increased p-ERK1/2 in ECs. Isoprenaline β-AR mediated vasodilation of aortic rings was potentiated by the ERK1/2 inhibitor, U0126, in the presence or absence of endothelium or L-NAME, whereas inhibition of p38 had no impact. Isoprenaline moderately decreased sprouting from aorta rings in the Matrigel angiogenesis assay; conversely propranolol not only prevented isoprenaline inhibition, but stimulated angiogenesis. ERK1/2 inhibition decreased angiogenesis, while a dramatic stimulation was observed by p38 blockade. Our results suggest that ERK1/2 activation after β-ARs stimulation in the smooth muscle hinders the vasodilator effect of isoprenaline, but in the endothelium β-ARs decreases ERK1/2 and increases p38 activity reducing therefore angiogenesis.

Functional investigation of β-adrenoceptors in human isolated detrusor focusing on the novel selective β3-adrenoceptor agonist KUC-7322

This study aimed to characterize the β-adrenoceptor (β-AR) subtype mediating relaxation of isolated human bladder strips and to explore relaxation by the novel β3-AR-selective agonist KUC-7322 for its relaxant effect on the human isolated detrusor and for its effect on the carbachol (CCh)-induced contractile response. In two parallel studies, relaxation of isolated human bladder strips was tested for the β-AR agonists isoproterenol, clenbuterol, BRL 37344, and KUC-7322. For the isoproterenol and KUC-7322 responses, antagonism by CGP 20712A, ICI 118551, and SR59230A was determined. The potency and efficacy of the reference agonists for detrusor relaxation was in line with their known β3-AR activity. KUC-7322 relative to isoproterenol was a full agonist with a pEC(50) of 5.95 ± 0.09 and 5.92 ± 0.11 in the two studies. SR59230A exhibited antagonism of the expected potency against isoproterenol (apparent pK (B) 7.2) but not against KUC-7322. Neither isoproterenol nor KUC-7322 nor forskolin significantly attenuated CCh-induced contraction. These results suggest that KUC-7322 displays full agonistic activity in relaxing the human detrusor without inhibiting the contraction induced by cholinergic stimulation. These characteristics, if proven in vivo, may be beneficial for the treatment of overactive bladder, as increased bladder capacity with a negligible effect on voiding contractions may be anticipated.

Do β3-adrenergic receptors play a role in guinea pig detrusor smooth muscle excitability and contractility?

In many species, β3-adrenergic receptors (β3-ARs) have been reported to play a primary role in pharmacologically induced detrusor smooth muscle (DSM) relaxation. However, their role in guinea pig DSM remains controversial. The aim of this study was to investigate whether β3-ARs are expressed in guinea pig DSM and to evaluate how BRL37344 and L-755,507, two selective β3-AR agonists, modulate guinea pig DSM excitability and contractility. We used a combined experimental approach including RT-PCR, patch-clamp electrophysiology, and isometric DSM tension recordings. β3-AR mRNA message was detected in freshly isolated guinea pig DSM single cells. BRL37344 but not L-755,507 caused a slight decrease in DSM spontaneous phasic contraction amplitude and frequency in a concentration-dependent manner. In the presence of atropine (1 μM), only the spontaneous phasic contractions frequency was inhibited by BRL37344 at higher concentrations. Both BRL37344 and L-755,507 significantly decreased DSM carbachol-induced phasic and tonic contractions in a concentration-dependent manner. However, only BRL37344 inhibitory effect was partially antagonized by SR59230A (10 μM), a β3-AR antagonist. In the presence of atropine, BRL37344 and L-755,507 had no inhibitory effect on electrical field stimulation-induced contractions. Patch-clamp experiments showed that BRL37344 (100 μM) did not affect the DSM cell resting membrane potential and K(+) conductance. Although β3-ARs are expressed at the mRNA level, they play a minor to no role in guinea pig DSM spontaneous contractility without affecting cell excitability. However, BRL37344 and L-755,507 have pronounced inhibitory effects on guinea pig DSM carbachol-induced contractions. The study outlines important DSM β3-ARs species differences.

β-adrenoceptor agonist effects in experimental models of bladder dysfunction

β-adrenoceptor stimulation can enhance the storage function of the urinary bladder by acting on detrusor smooth muscle tone, mediator release from the urothelium and/or afferent nerve activity. In humans this may occur predominantly if not exclusively via the β₃-subtype. The effects of β-adrenoceptor agonists including several β₃-selective agonists have been studied in vitro and in vivo, in healthy animals of both genders and various age groups and in a wide range of animal (mostly rat) models of genetic or acquired bladder dysfunction. Such models included bladder irritation by intravesical instillation of acetic acid or prostaglandin E₂, bladder outlet obstruction, stroke, diabetes, spontaneously hypertensive rats, and NO synthase inhibition. Across all of these models β-adrenoceptor agonists had effects consistent with improved bladder storage function. β₃-adrenoceptor effects are resistant to agonist-induced desensitization in many cell types, but whether this also applies to the human bladder is unknown. The efficacy of β-adrenoceptor agonists appears to be largely unaffected by common polymorphisms of the β₃-adrenoceptor gene. Taken together these findings suggest that β₃-adrenoceptor agonists may become useful drugs for the treatment of bladder storage dysfunction, a view supported by recent phase III clinical studies for one such agent, mirabegron.

β-Adrenergic Receptor Subtypes in the Urinary Tract

Within the urinary tract, β-adrenergic receptors (AR) are found largely on smooth muscle cells but are also present, at least in the bladder, in the urothelium and on afferent nerves. Our understanding of β-AR subtype expression and function is hampered by a lack of well-validated tools, particularly with regard to β(3)-AR. Moreover, the β-AR subtypes involved in a specific function may differ between species. In the ureter, β-AR can modulate pacemaker activity and smooth muscle tone involving multiple subtypes. In the human bladder, β-AR promote urine storage. Bladder smooth muscle relaxation primarily involves β(3)-AR, and the agonists selective for this subtype are in clinical development to treat bladder dysfunction. While prostate and urethra also express β-AR, the overall physiological role in these tissues remains unclear.

β-Adrenergic receptor subtype expression in myocyte and non-myocyte cells in human female bladder

β(3)-Adrenergic receptor agonists are currently under clinical development for the treatment of overactive bladder, a condition that is prevalent in postmenopausal women. These agents purportedly relax bladder smooth muscle through a direct action at the myocyte β(3)-receptor. The aim of this study was to examine the expression of the individual beta-adrenergic receptors in full thickness sections from ageing human female bladder. We obtained a series of rabbit polyclonal antibodies generated against each of the three β-adrenergic receptors, and validated their receptor specificity in CHOK1 cells expressing each of the individual receptors. Immunostaining for β(1), β(2), and β(3) were each more prominent in the urothelium than in the detrusor, with all receptors expressed in the same cell types, indicating co-expression of all three receptors throughout the urothelium in addition to the detrusor. Staining of all receptors was also observed in suburothelial myofibroblast-like cells, intramural ganglion cells, and in Schwann cells of intramural nerves. The β(3)-receptor in the human urothelium appears to be functional, as two different selective β(3)-receptor agonists, TAK677 and BRL37344, stimulate cAMP formation in URO tsa cells. Densitometry analysis indicates a persistent expression of all receptors throughout the bladder with increasing age, with the exception of the β(2)-receptor in the urothelium of the trigone, which appears to decrease slightly in older women. These data indicate that β(3)-receptor expression is maintained with age, but may function in concert with other β-receptors. Activation of the myocyte receptor may be influenced by action on non-myocyte structures including the intramural ganglion cells and myofibroblasts.

β-Adrenoceptor-mediated differences in transverse and longitudinal strips from the rat detrusor

Contractions and relaxations of the urinary bladder occur in all directions to facilitate urine release and storage. Transverse and longitudinal contractility of detrusor smooth muscle have been studied before using various pharmacologic stimuli but not β agonists. Given the importance of β-adrenoceptors in mediating bladder relaxation, the effects of isoprenaline (IPNA) in transverse and longitudinal contractility were examined. Pretreatment with a low concentration of IPNA (0.1 or 1 μM) suppressed carbachol (CCh)-induced contractions, more in the transverse than longitudinal direction. Increasing the IPNA concentration to 10 or 100 μM resulted in greater inhibition of longitudinal contractions. Also in the longitudinal direction, IPNA-induced relaxation was greater than in the transverse direction. When precontracted with a submaximal concentration of CCh (1 μM), IPNA increased the phasic activity in the longitudinal direction only. In summary, β-adrenoceptor-mediated differences between transverse and longitudinal contractility were revealed. In testing the relaxant properties of selective β-agonists, the findings here should be considered such that other than the conventional longitudinal contractions, measurements are also made in other directions.

Rhythmic contraction generates adjustable passive stiffness in rabbit detrusor

The length-tension (L-T) relationships in airway and vascular smooth muscles have been shown to adapt with length changes over time. Our prior studies have shown that the active and passive L-T relationships in rabbit detrusor smooth muscle (DSM) can adapt and that DSM exhibits adjustable passive stiffness (APS) characterized by a passive L-T curve that is a function of strain and activation history. The present study demonstrates that passive tension due to APS can represent a substantial fraction of total tension over a broad length range. Our previous studies have shown that maximal KCl-induced contractions at short muscle lengths generate APS that is revealed by increased pseudo-steady-state passive tension at longer lengths compared with previous measurements at those lengths. The objective of the present study was to determine the mechanisms involved in APS generation. Increasing the number of KCl-induced contractions or the duration of a contraction increased the amount of APS generated. Furthermore, a fraction of APS was restored in calcium-free solution and was sensitive to the general serine and threonine protein kinase inhibitor staurosporine. Most importantly, rhythmic contraction (RC) generated APS, and because RC occurs spontaneously in human bladder, a physiological role for RC was potentially identified.