Alison F. Brading

A myogenic basis for the overactive bladder

OBJECTIVES This article summarizes evidence supporting the conclusion that the spontaneous rises in pressure that occur in the overactive bladder, particularly in detrusor instability (DI), regardless of etiology, are myogenic. METHODS The evidence quoted has been obtained by several groups of investigators and includes electrophysiologic studies of detrusor myocytes, tension studies of strips of smooth muscle, in vivo experiments on animal models, and light and electron microscopic studies of the structure of the bladder wall. RESULTS The results of these studies demonstrate changes in the properties, structure, and innervation of the detrusor. These changes are consistent with the hypothesis that partial denervation of the detrusor may be responsible for altering the properties of the smooth muscle, leading to increased excitability and increased ability of activity to spread between cells, resulting in coordinated myogenic contractions of the whole detrusor. CONCLUSIONS It is suggested that alterations in the properties of the detrusor myocytes are a necessary prerequisite for the production of the unstable pressure rises seen in DI of any origin.

Nerve mediated relaxation of the human internal anal sphincter: the role of nitric oxide.

The aim of this study was to determine if nitric oxide (NO) is the non-adrenergic, non-cholinergic neurotransmitter, released by enteric inhibitory nerves, which mediates relaxation of the human internal anal sphincter. Isolated muscle strips were mounted for isometric tension recording in superfusion organ baths. Sodium nitroprusside, an exogenous donor of NO, relaxed the strips in a concentration dependent manner. In the presence of atropine and guanethidine, transmural field stimulation produced tetrodotoxin sensitive relaxations, which were inhibited in a dose dependent and enantiomer specific manner by antagonists of NO synthase; completely by L-nitroarginine and partially by L-N-monomethyl arginine. The effect of these antagonists was reversed by L-arginine but not D-arginine. Oxyhaemoglobin, a scavenger of nitric oxide, also abolished the relaxations but methaemoglobin had no such effect. These results strongly suggest that NO is, or is very closely associated with, the non-adrenergic, non-cholinergic neurotransmitter mediating neurogenic relaxation of the human internal anal sphincter.

Spontaneous activity of lower urinary tract smooth muscles: correlation between ion channels and tissue function

Smooth muscles from the urethra and bladder display characteristic patterns of spontaneous contractile activity in the filling phase of the micturition cycle. Tonic contractions are seen in the urethral smooth muscles, and phasic contractions occur in the detrusor. Overactivity in the detrusor is a common clinical problem. The ion channels in the smooth muscle membranes play an important role in determining the functional properties, and are obvious targets for treatment of the overactive bladder. Recent evidence suggests that interstitial cells may also play a role in determining the pattern of spontaneous activity, although their precise role is less well established in the urinary tract than in the gut. The ion channels involved in these cells are also of interest. This review discusses what is known of ion channels in these tissues, and their implications for function.

Ionic basis for the regulation of spontaneous excitation in detrusor smooth muscle cells of the guinea‐pig urinary bladder

The regulatory mechanisms of spontaneous excitation in detrusor smooth muscles of the guinea‐pig urinary bladder were investigated using intracellular microelectrode and muscle tension recording techniques. Detrusor smooth muscle cells exhibited nifedipine‐sensitive spontaneous action potentials. Their frequency was highly sensitive to membrane polarization and was reduced by lowering the temperature. Lowering the temperature also reduced the frequency of spontaneous contractions and increased their amplitude. Charybdotoxin (50 nM) and iberiotoxin (0.1 μM) increased the amplitude and duration of action potentials, and abolished after hyperpolarizations (AHPs). Both agents also increased the amplitude and duration of spontaneous contractions, and reduced their frequency. Apamin (0.1 μM) did not change the shape of action potentials but often converted individual action potentials into bursts. It also increased the amplitude and duration of spontaneous contractions, and reduced their frequency. 4‐aminopyrideine (4‐AP, 1 mM) increased the frequency of action potentials without affecting their shape, and increased the amplitude and frequency of spontaneous contractions. Cyclopiazonic acid (CPA, 10 μM) and ryanodine (50 μM) increased the amplitude of action potentials, and suppressed AHPs. Both agents also increased the amplitude and duration of spontaneous contractions, and reduced their frequency. 1,2‐(Bis (2‐aminophenoxy) ethane‐N,N,N′, N′‐tetraacetic acid tetrakis (acetoxymethyl ester) (50 μM) dramatically increased the amplitude and duration of the action potential, and abolished AHPs. Spontaneous action potentials in detrusor smooth muscles cells result from the opening of L‐type Ca2+ channels, and their frequency is regulated by voltage‐dependent mechanisms and by some metabolic process. Both the activation of large conductance Ca2+‐activated K+ (BK) channels and Ca2+‐mediated inactivation of the Ca2+ channels are involved in the repolarizing phase of action potentials. The Ca2+ influx through L‐type Ca2+ channels triggers calcium‐induced calcium release via ryanodine receptors and activates BK channels to generate AHPs. Both small conductance Ca2+‐activated K+ channels and voltage‐sensitive K+ channels may contribute to the resting membrane potential and regulate the frequency of action potentials. The regulatory mechanisms of action potentials are closely related to the regulation of spontaneous contractions.

Correlation between spontaneous electrical, calcium and mechanical activity in detrusor smooth muscle of the guinea-pig bladder

To investigate the cellular mechanisms underlying spontaneous excitation of smooth muscle of the guinea‐pig urinary bladder, isometric tension was measured in muscle bundles while recording the membrane potential from a cell in the bundle with a microeletrode. Changes in the intracellular calcium concentration ([Ca2+]i; calcium transients) were recorded in strips loaded with the fluorescent dye, fura‐PE3. In 40% of preparations, individual action potentials and contractions, which were abolished by nifedipine (1 μM), were generated. In the remaining preparations, bursting action potentials and contractions were generated. Contractions were again abolished by nifedipine (1 μM), while higher concentrations of nifedipine (10–30 μM) were required to prevent the electrical activity. Carbachol (0.1 μM) increased the frequency of action potentials and corresponding contractions. Apamin (0.1 μM) potentiated bursting activity and enhanced phasic contraction. Charybdotoxin (CTX, 50 nM) induced prolonged action potentials that generated enlarged contractions. In contrast, levcromakalim (0.1 μM) reduced the frequency of action potentials, action potential bursts and the size of the contractions. Forskolin (0.1 μM), 8‐bromoguanosin 3′, 5′ cyclic monophosphate (8Br‐cGMP, 0.1 mM) and Y‐26763 (10 μM) suppressed contractions without reducing the amplitude of either action potentials or Ca transients. This paper confirms that action potentials and associated calcium transients are fundamental mechanisms in generating spontaneous contractions in smooth muscles of the guinea‐pig bladder. However, in parallel with the excitation–contraction coupling, the sensitivity of the contractile proteins for Ca2+ may play an important role in regulating spontaneous excitation and can be modulated by cyclic nucleotides and Rho kinase.

The effect of sodium and calcium on the action potential of the smooth muscle of the guinea-pig taenia coli

1. Spontaneous spike activity and action potentials evoked by external field stimulation were recorded, intracellularly and with the double sucrose gap method, from the smooth muscle of guinea‐pig taenia coli.

Distribution and kinetics of CoEDTA in smooth muscle, and its use as an extracellular marker

1. [60Co]EDTA has been evaluated as an extracellular marker in guinea‐pig taenia coli and rabbit myometrium.

Mechanisms of Disease: specialized interstitial cells of the urinary tract--an assessment of current knowledge.

Scientists interested in the smooth muscles of the urinary tract, and their control, have recently been studying cells in the interstitium of tissues that express the c-kit antigen (Kit+ cells). These cells have morphologic features that are reminiscent of the well-described pacemaker cells in the gut, the interstitial cells of Cajal (ICC). The spontaneous contractile behavior of muscles in the urinary tract varies widely, and it is clear that urinary tract Kit+ interstitial cells cannot be playing an identical role to that played by the ICC in the gut. Nevertheless, there is increasing evidence that they do play a role in modulating the contractile behavior of adjacent smooth muscle, and might also be involved in mediating neural control. This review outlines the properties of ICC in the gut, and gives an account of the discovery of cells in the interstitium of the main components of the urinary tract. The physiologic properties of such cells and the functional implications of their presence are discussed, with particular reference to the bladder. In this organ, Kit+ cells are found under the lamina propria, where they might interact with the urothelium and with sensory nerves, and also between and within the smooth-muscle bundles. Confocal microscopy and calcium imaging are being used to assess the physiology of ICC and their interactions with smooth muscles. Differences in the numbers of ICC are seen in smooth muscle specimens obtained from patients with various pathologies; in particular, bladder overactivity is associated with increased numbers of these cells.

Electrical properties of detrusor smooth muscles from the pig and human urinary bladder

The electrophysiological properties of detrusor smooth muscles have been studied almost exclusively in small mammals and the relevance of the information to the human bladder has been questioned. In the present study, electrical properties of detrusor smooth muscles of the pig and human were investigated using intracellular recording techniques. Bladder smooth muscles of the pig and human exhibited nifedipine (10 μM)‐sensitive spontaneous action potentials, and their frequency was highly sensitive to membrane polarization. During bursts of action potentials, each action potential was followed by a fast after‐hyperpolarization (fast AHP). Charybdotoxin (CTX, 50 nM) increased the amplitude and duration of action potentials but failed to inhibit the fast AHPs, while apamin (0.1 μM) blocked the fast AHPs and induced action potential complexes, which were followed by slow AHPs. 4‐Aminopyridine (4‐AP, 1 mM) suppressed the slow AHP and increased action potential frequency. In the human bladder, transmural stimuli initiated inhibitory junction potential‐like hyperpolarizations, which were followed by action potential discharges. The hyperpolarizations were blocked by atropine (1 μM) and by apamin (0.1 μM) but not by CTX (50 nM). In the pig bladder, transmural stimuli evoked excitatory junction potentials (EJPs), which triggered action potentials. After desensitizing P2x receptors with α,β methylene‐ATP (10 μM), nerve‐evoked responses were similar to those of human bladder. These results indicate that detrusor smooth muscles of the pig share many features of electrical properties with those of the human. In addition to large conductance (BK) and small conductance (SK) Ca2+‐activated K+ channels, voltage‐dependent K+ (VK) channels may play an important role in the regulation of electrical activity of detrusor smooth muscles.

Measurement of intracellular chloride in guinea-pig vas deferens by ion analysis, 36chloride and micro-electrodes

1. Cl‐sensitive micro‐electrodes were used to measure the intracellular Cl activity (aCli) in smooth muscle cells of the guinea‐pig vas deferens. The values obtained were compared with those of intracellular Cl (Cli) found by both ion analysis and 36Cl efflux.