The actions of isoprenaline and mirabegron in the isolated whole rat and guinea pig bladder

Inhibition of cholinergic neurotransmission by β3-adrenoceptors depends on adenosine release and A1-receptor activation in human and rat urinary bladders

The direct detrusor relaxant effect of β₃-adrenoceptor agonists as a primary mechanism to improve overactive bladder symptoms has been questioned. Among other targets, activation of β₃-adrenoceptors downmodulate nerve-evoked acetylcholine (ACh) release, but there is insufficient evidence for the presence of these receptors on bladder cholinergic nerve terminals. Our hypothesis is that adenosine formed from the catabolism of cyclic AMP in the detrusor may act as a retrograde messenger via prejunctional A₁ receptors to explain inhibition of cholinergic activity by β₃-adrenoceptors. Isoprenaline (1 µM) decreased [³H]ACh release from stimulated (10 Hz, 200 pulses) human (-47 ± 5%) and rat (-38 ± 1%) detrusor strips. Mirabegron (0.1 µM, -53 ± 8%) and CL316,243 (1 µM, -37 ± 7%) mimicked isoprenaline (1 µM) inhibition, and their effects were prevented by blocking β₃-adrenoceptors with L748,337 (30 nM) and SR59230A (100 nM), respectively, in human and rat detrusor. Mirabegron and isoprenaline increased extracellular adenosine in the detrusor. Blockage of A₁ receptors with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 100 nM) or the equilibrative nucleoside transporters (ENT) with dipyridamole (0.5 µM) prevented mirabegron and isoprenaline inhibitory effects. Dipyridamole prevented isoprenaline-induced adenosine outflow from the rat detrusor, and this effect was mimicked by the ENT1 inhibitor, S-(4-nitrobenzyl)-6-thioinosine (NBTI, 30 µM). Cystometry recordings in anesthetized rats demonstrated that SR59230A, DPCPX, dipyridamole, and NBTI reversed the decrease in the voiding frequency caused by isoprenaline (0.1-1,000 nM). Data suggest that inhibition of cholinergic neurotransmission by β₃-adrenoceptors results from adenosine release via equilibrative nucleoside transporters and prejunctional A₁-receptor stimulation in human and rat urinary bladder.

Adrenergic signaling elements in the bladder wall of the adult rat

A growing body of work is describing the absence of a significant sympathetic innervation of the detrusor implying little sympathetic regulation of bladder contractility. However, low doses of adrenergic agonists are capable of relaxing the bladder smooth muscle. If these effects underpin a physiological response then the cellular nature and operation of this system are currently unknown. The present immunohistochemistry study was done to explore the existence of alternative adrenergic signaling elements in the rat bladder wall. Using antibodies to tyrosine hydroxylase (TH) and vesicular mono-amine transporter (vmat), few adrenergic nerves were found in the detrusor although TH immunoreactive (IR) nerves were apparent in the bladder neck. TH-IR and vmat-IR nerves were however abundant surrounding blood vessels. A population of vmat-IR cells was found within the network of interstitial cells that surround the detrusor muscle bundles. These vmat-IR cells were not or only weakly TH-IR. This suggests that these interstitial cells have the capacity to store and release catecholamines that may involve noradrenaline. Cells expressing the β₁-adrenoceptor (β₁AR-IR) were also detected within the interstitial cell network. Double staining with antibodies to β₁AR and vmat suggests that the majority of vmat-IR interstitial cells show β₁AR-IR indicative of an autocrine signaling system. In conclusion, a population of interstitial cells has the machinery to store, release and respond to catecholamines. Thus, there might exist a non-neuronal β-adrenergic system operating in the bladder wall possibly linked to one component of motor activity, micro-contractions, a system that may be involved in mechanisms underpinning bladder sensation.