The effects of extracellular and intracellular pH on intracellular Ca2+ regulation in guinea-pig detrusor smooth muscle

TRPV4 receptor as a functional sensory molecule in bladder urothelium: Stretch-independent, tissue-specific actions and pathological implications

The newly recognized sensory role of bladder urothelium has generated intense interest in identifying its novel sensory molecules. Sensory receptor TRPV4 may serve such function. However, specific and physiologically relevant tissue actions of TRPV4, stretch-independent responses, and underlying mechanisms are unknown and its role in human conditions has not been examined. Here we showed TRPV4 expression in guinea-pig urothelium, suburothelium, and bladder smooth muscle, with urothelial predominance. Selective TRPV4 activation without stretch evoked significant ATP release-key urothelial sensory process, from live mucosa tissue, full-thickness bladder but not smooth muscle, and sustained muscle contractions. ATP release was mediated by Ca2+-dependent, pannexin/connexin-conductive pathway involving protein tyrosine kinase, but independent from vesicular transport and chloride channels. TRPV4 activation generated greater Ca2+ rise than purinergic activation in urothelial cells. There was intrinsic TRPV4 activity without exogeneous stimulus, causing ATP release. TRPV4 contributed to 50% stretch-induced ATP release. TRPV4 activation also triggered superoxide release. TRPV4 expression was increased with aging. Human bladder mucosa presented similarities to guinea pigs. Overactive bladders exhibited greater TRPV4-induced ATP release with age dependence. These data provide the first evidence in humans for the key functional role of TRPV4 in urothelium with specific mechanisms and identify TRPV4 up-regulation in aging and overactive bladders.

Purinergic and muscarinic modulation of ATP release from the urothelium and its paracrine actions

The urothelium is a newly recognized sensory structure that detects bladder fullness. Pivotal to this sensory role is the release of ATP from the urothelium. However, the routes for urothelial ATP release, its modulation by receptor-mediated pathways, and the autocrine/paracrine role of ATP are poorly understood, especially in native tissue. We examined the action of key neurotransmitters: purinergic and muscarinic agonists on ATP release and its paracrine effect. Guinea pig and human urothelial mucosa were mounted in a perfusion trough; superfusate ATP was measured using a luciferin-luciferase assay, and tissue contractions were recorded with a tension transducer. Intracellular Ca²⁺ was measured in isolated urothelial cells with fura-2. The P2Y agonist UTP but not the P2X agonist α,β-methylene-ATP generated ATP release. The muscarinic agonist carbachol and the M₂-preferential agonist oxotremorine also generated ATP release, which was antagonized by the M₂-specific agent methoctramine. Agonist-evoked ATP release was accompanied by mucosal contractions. Urothelial ATP release was differentially mediated by intracellular Ca²⁺ release, cAMP, exocytosis, or connexins. Urothelium-attached smooth muscle exhibited spontaneous contractions that were augmented by subthreshold concentrations of carbachol, which had little direct effect on smooth muscle. This activity was attenuated by desensitizing P2X receptors on smooth muscle. Urothelial ATP release was increased in aging bladders. Purinergic and muscarinic agents produced similar effects in human urothelial tissue. This is the first demonstration of specific modulation of urothelial ATP release in native tissue by purinergic and muscarinic neurotransmitters via distinct mechanisms. Released ATP produces paracrine effects on underlying tissues. This process is altered during aging and has relevance to human bladder pathologies.

Intracellular calcium stores in beta-escin skinned rat and guinea-pig bladders

Intracellular Ca2+ stores in rat and guinea-pig bladders and taenia caecum were studied in beta-escin skinned smooth muscle strips. 30 min of skinning with 40 microM and 80 microM beta-escin were the best parameters found to obtain good calcium response curves (10(-7)-10(-4) M) in rat and guinea pig, respectively. Calmodulin (1 microM) increased the calcium contractions significantly. pCa 6 was used to load intracellular stores and application of carbachol (50 microM) in all tissues then only contracted the tissues in the presence of guanosine-5'-triphosphate (GTP; 100 microM). Inositol triphosphate (IP3; 50 microM), applied after pCa 6, contracted all tissues. Carbachol added after IP3 or heparin (1 mg/ml) no longer caused a contraction in any of them. In bladders, caffeine (30 mM) but not ryanodine (5 microM) prevented the subsequent carbachol contraction. A slowly rising contraction with carbachol was elicited after caffeine (30 mM) or ryanodine (5 microM) in the taenia and after ryanodine in the bladders. Caffeine (30 mM) suppressed the calcium response curves in all tissues. Procaine (30 mM) blocked the carbachol (50 microM) contractions in bladders but not in taenia. These results suggest that calcium induced calcium release (CICR) and IP3 induced calcium release (IICR) release calcium from a common store in bladder but two different compartments in taenia.

Regulation of membrane excitability by intracellular pH (pHi) changers through Ca2+-activated K+ current (BK channel) in single smooth muscle cells from rabbit basilar artery

Employing microfluorometric system and patch clamp technique in rabbit basilar arterial myocytes, regulation mechanisms of vascular excitability were investigated by applying intracellular pH (pH(i)) changers such as sodium acetate (SA) and NH(4)Cl. Applications of caffeine produced transient phasic contractions in a reversible manner. These caffeine-induced contractions were significantly enhanced by SA and suppressed by NH(4)Cl. Intracellular Ca(2+) concentration ([Ca(2+)](i)) was monitored in a single isolated myocyte and based the ratio of fluorescence using Fura-2 AM (R (340/380)). SA (20 mM) increased and NH(4)Cl (20 mM) decreased R (340/380) by 0.2 +/- 0.03 and 0.1 +/- 0.02, respectively, in a reversible manner. Caffeine (10 mM) transiently increased R (340/380) by 0.9 +/- 0.07, and the ratio increment was significantly enhanced by SA and suppressed by NH(4)Cl, implying that SA and NH(4)Cl may affect [Ca(2+)](i) (p < 0.05). Accordingly, we studied the effects of SA and NH(4)Cl on Ca(2+)-activated K(+) current (IK(Ca)) under patch clamp technique. Caffeine produced transient outward current at holding potential (V (h)) of 0 mV, caffeine induced transient outward K(+) current, and the spontaneous transient outward currents were significantly enhanced by SA and suppressed by NH(4)Cl. In addition, IK(Ca) was significantly increased by acidotic condition when pH(i) was lowered by altering the NH(4)Cl gradient across the cell membrane. Finally, the effects of SA and NH(4)Cl on the membrane excitability and basal tension were studied: Under current clamp mode, resting membrane potential (RMP) was -28 +/- 2.3 mV in a single cell level and was depolarized by 13 +/- 2.4 mV with 2 mM tetraethylammonium (TEA). SA hyperpolarized and NH(4)Cl depolarized RMP by 10 +/- 1.9 and 16 +/- 4.7 mV, respectively. SA-induced hyperpolarization and relaxation of basal tension was significantly inhibited by TEA. These results suggest that SA and NH(4)Cl might regulate vascular tone by altering membrane excitability through modulation of [Ca(2+)](i) and Ca(2+)-activated K channels in rabbit basilar artery.

Involvement of non-selective Ca2+ channels in the contraction induced by alkalinization of rat anococcygeus muscle cells

Intracellular pH is a modulator of cellular functions such as smooth muscle contraction. Changes in cytosolic Ca(2+) concentration ([Ca(2+)](c)) associated with contraction are brought about by Ca(2+) influx and release from the sarcoplasmic reticulum, and alterations in the intracellular pH can affect both processes. In this work, therefore, we have investigated the Ca(2+) influx pathway that contributes to the contraction induced by the alkalinizing agent NH(4)Cl in the rat anococcygeus smooth muscle. For this purpose, we measured the isometric tension in muscle preparations, and [Ca(2+)](c) was measured on isolated cells loaded with 5 micromol/l FURA2/AM by using the ratio 340/380 nm. NH(4)Cl (10 mmol/l) induced a larger increase in [Ca(2+)](c) (100%) when compared with the [Ca(2+)](c) increase induced by 0.1 micromol/l phenylephrine (57.0+/-12.3% n=4). Incubation of the muscle preparations for 1 min in Ca(2+)-free medium reduced the contractions induced by 10 mmol/l NH(4)Cl to 11.5+/-5.1% (n=5), when compared with the contractions induced in 2.5 mmol/l Ca(2+) solution (100%). After 3 min in Ca(2+) free medium, contractions stimulated with NH(4)Cl were almost abolished (0.6+/-0.4%, n=5). In the same way, incubation with 10 micromol/l 1-[beta-[3[(4-methoxyphenyl)propoxyl]-4-methoxy-phenetyl]-1H-imidazole hydrochloride (SKF96365), a non-selective Ca(2+) channels, reduced the contractions stimulated with NH(4)Cl to 47.6+/-6.7% (n=7). On the other hand, 1 micromol/l verapamil, a voltage-operated Ca(2+) channel blocker and 0.05 micromol/l calphostin C, a protein kinase-C inhibitor, did not alter the contractions induced by NH(4)Cl. On isolated cells, [Ca(2+)](c) was reduced to 72.2+/-1.7% (n=4) by 10 micromol/l SKF96365. Taken together, our results suggest that NH(4)Cl induces contraction of rat anococcygeus smooth muscle cells, as well as [Ca(2+)](c) increase due to Ca(2+) influx through non-selective Ca(2+) channels.

Ca2+ regulation in detrusor smooth muscle from developing fetal sheep bladders

Sheep fetus is a useful model to study in utero bladder outflow obstruction but little is known about cell physiology of fetal bladders. To remedy this defect we have characterised intracellular Ca(2+) regulation in fetal sheep myocytes of different developmental ages. Fetal detrusor myocytes had a similar resting [Ca(2+)](i) to adult cells and exhibited transient [Ca(2+)](i) increases in response to carbachol, ATP, high-K, caffeine and low-Na. The carbachol transients were abolished by atropine and caffeine; the ATP response was blocked by alpha,beta-methylene ATP; high-K-evoked [Ca(2+)](i) rises were antagonised by verapamil. The maximal responses to carbachol, high-K, caffeine and low-Na in fetal cells were similar to those of adult counterparts, whilst the ATP response was smaller (p < 0.05). These variables were largely similar between the three gestational groups with the exception of ATP-induced response between early fetal and adult bladders (p < 0.05). Dose-response curves to carbachol demonstrated an increase of potency between mid-gestation and early adulthood (p < 0.05). These data show that muscarinic receptors coupled to intracellular Ca(2+) release, P2X receptor-linked Ca(2+) entry, depolarisation-induced Ca(2+) rise via L-type Ca(2+) channels, Na(+)/Ca(2+) exchange and functional intracellular Ca(2+) stores are all operational in fetal bladder myocytes. Whilst most of Ca(2+) regulators are substantially developed and occur at an early fetal age, a further functional maturation for cholinergic sensitivity and purinergic efficacy continues throughout to adulthood.

Intracellular Ca2+ regulation in a human prostate stromal cell culture

AIMS

Prostate stromal cell cultures are used in vitro to study the cellular pathophysiology of benign prostatic hyperplasia (BPH), but their functional properties are poorly understood. This study characterized intracellular Ca2+ ([Ca2+]i) regulation in a cultured cell line in comparison to freshly isolated cells, as a background to understanding contractile regulation and cellular proliferation in this tissue.

METHODS

Prostate stromal cells were isolated from either PrS6 cell cultures, with an extended life span by transfection with the SV40 T-antigen, tsA58-U19, or freshly obtained transition zone prostate samples, primary cells. [Ca2+]i was measured in vitro with the indicator Fura-2 by epifluorescence microscopy.

RESULTS

Phenylephrine, high-K+, and caffeine induced Ca2+-transients in primary cells (resting [Ca2+]i 94 +/- 8 nM, n = 29; peak 193 +/- 26 nM, n = 19). In PrS6 cells resting [Ca2+]i was 96 +/- 8 nM (n = 78) and in 34 of these 78 cells, 30 microM phenylephrine increased [Ca2+]i to 296 +/- 28 nM. 5-methyl-urapidil (10-30 microM) inhibited this response in 10 of 16 cells. Spontaneous Ca2+-transients were also observed in 91% of phenylephrine-responsive cells, but in only 20% of non-responsive cells (P < 0.01). Ca2+-transients were also induced by high-K+ solution, and 20 mM caffeine. The latter abolished the response to subsequent phenylephrine application. Depletion of intracellular Ca2+ stores by caffeine or restoration from a Ca2+-free superfusate caused a substantial rise of [Ca2+]i.

CONCLUSIONS

PrS6 prostate stromal cells express functional alpha1-adrenoceptors associated with spontaneous intracellular Ca2+-transients. They exhibit functional Ca2+ channels, intracellular Ca2+ stores, and Ca2+ entry induced by store depletion. Stromal cultures can therefore be used to characterize the cellular physiology of prostate stromal cell contraction and proliferation.

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

CHARACTERIZATION OF THE CONTROL OF INTRACELLULAR [CA 2+ ] AND THE CONTRACTILE PHENOTYPE OF CULTURED HUMAN DETRUSOR SMOOTH MUSCLE CELLS

PURPOSE

We measured the functional properties of cultured human detrusor myocytes with respect to their ability to regulate their intracellular [Ca2+] and generate force in collagen matrices.

MATERIALS AND METHODS

Human detrusor biopsies were dissociated into single cells by collagenase treatment and used immediately or cultured in D-valine medium and subsequently used after culture trypsinization. Intracellular [Ca2+] was measured in Fura-2 loaded myocytes. Cell force development was measured by incorporating cells into a collagen gel and attaching it to an isometric strain gauge.

RESULTS

Carbachol was equally effective in generating Ca transients in freshly isolated and cultured cells. Carbachol potency (pEC50) and the magnitude of Ca2+ transients were similar. Adenosine triphosphate potency was decreased in cultured cells and Ca2+ transients showed properties consistent with a purinoceptor shift from a purinergic subtype. Temporal restitution of Ca2+ transients was similar in the 2 groups, indicative of retained intracellular Ca2+ stores in cultured cells. Cultured cells (approximately 10(6)) embedded in collagen gel generated a force about 10 times greater than that generated by gel alone. The cell dependent force could be further increased by adding carbachol.

CONCLUSIONS

Cultured cells retain the ability to generate agonist induced intracellular Ca2+ transients. There was no evidence that the cell culture altered the properties of muscarinic receptors, although purinoceptor mediated properties were altered. Restitution experiments indicated that functional intracellular Ca2+ stores were retained in cultured cells. Cultured cells also retained a contractile phenotype, especially in response to carbachol. The magnitude of force was attenuated, which may be a function of the biomechanical properties of the gel used to embed the cells.

Dorzolamide induces vasodilatation in isolated pre-contracted bovine retinal arteries

The effect of the carbonic anhydrase inhibitor dorzolamide on vascular smooth muscle in pre-contracted bovine retinal arteries was examined. Ring segments of retinal arteries were placed in a small vessel myograph for measurement of contractile activity. The arteries were placed in a physiological saline solution. Vasoconstriction was induced by either 124 mM KCl (0.90 +/- 0.46 mN, n=34), 10(-4) M prostaglandin F2alpha (1.72 +/- 0.84 mN, n=10) or 10(-6) M norepinephrine (0.78 +/- 0.47 mN, n=6). Both KCl and prostaglandin F2alpha caused steady repeatable contractions but norepinephrine caused a single phasic contraction. The effect of the carbonic anhydrase inhibitor, dorzolamide on the vasoconstriction was examined. Dorzolamide, if added to the bath when the vasoconstriction had reached a maximum steady level, caused a highly significant relaxation (vasodilatation) of the arteries. This action of dorzolamide occurred irrespective of which agent was used to induce vasoconstriction. Similar results were obtained in experiments were Hepes buffer was used instead of CO2/bicarbonate buffer. The vasodilatation induced by dorzolamide was stable as long as the drug remained in the bath, and was reversible. These results show that dorzolamide causes a vasodilatation of retinal arteries, pre-contracted by three different mechanisms by direct action and presumably independent of changes in extracellular pH.

Luminal Ca(2+) and the activity of sarcoplasmic reticulum Ca(2+) pumps modulate histamine-induced all-or-none Ca(2+) release in smooth muscle cells

We have studied histamine (HA)-evoked intracellular Ca(2+) release in single, freshly isolated myocytes from the guinea pig urinary bladder. Short applications of histamine (5 s) produced a thapsigargin (TG)-sensitive transient increase in intracellular calcium concentration ([Ca(2+)](i)). It was established that histamine and caffeine (Caff) released Ca(2+) from the same intracellular stores in these cells. Reducing the Ca(2+) content of internal stores by incubating cells with U-73343 or cyclopiazonic acid (CPA) inhibited the histamine-evoked Ca(2+) release in 69% and 60% of cells, respectively. Under these conditions, all cells released Ca(2+) in response to either caffeine or acetylcholine (ACh). However, decreasing internal Ca(2+) stores by removing external Ca(2+) inhibited histamine-induced Ca(2+) mobilization in only 22% of cells. A similar small fraction of cells was inhibited when sarcoplasmic reticulum (SR) Ca(2+) pumps were quickly blocked to avoid a significant reduction of luminal Ca(2+). In conclusion, lowering the luminal Ca(2+) content in combination with an impairment of the SR Ca(2+) pump activity significantly diminishes the ability of histamine to evoke an all-or-none intracellular Ca(2+) release.

The role of the L-type Ca(2+) channel in refilling functional intracellular Ca(2+) stores in guinea-pig detrusor smooth muscle

The transient rise of intracellular Ca(2+) in detrusor smooth muscle cells is due to the release of Ca(2+) from intracellular stores. However, it is not known how store refilling is maintained at a constant level to ensure constancy of the contractile response. The aim of these experiments was to characterise the role of L-type Ca(2+) channels in refilling. Experiments used isolated guinea-pig detrusor myocytes and store Ca(2+) content was estimated by measuring the magnitude of change to the intracellular [Ca(2+)] ([Ca(2+)](i)) after application of caffeine or carbachol using epifluorescence microscopy. Membrane potential was controlled when necessary by voltage clamp. After Ca(2+) stores were emptied they refilled with an exponential time course, with a time constant of 88 s. The value of the time constant was similar to that of the undershoot of [Ca(2+)](i) following store Ca(2+) release. The degree of store filling was enhanced by maintained depolarisation, or by transient depolarising pulses, and attenuated by L-type Ca(2+) channel antagonists. Inhibition of the sarcoplasmic reticular Ca(2+)-ATPase prevented refilling. Reduction of the resting [Ca(2+)](i) was accompanied by membrane depolarisation; under voltage clamp reduction of [Ca(2+)](i) decreased the number and magnitude of spontaneous transient outward currents. Ca(2+) release from intracellular stores, elicited by caffeine or carbachol, is independent of membrane potential under physiological conditions. However, store refilling occurs via Ca(2+) influx through L-type Ca(2+) channels. Ca(2+) influx is regulated by a feedback mechanism whereby a fall of [Ca(2+)](i) reduces the activity of Ca(2+)-activated K(+) channels, causing cell depolarisation and an enhancement of L-type Ca(2+) channel conductance.

Changes in extracellular pH and myocardial ischaemia alter the cardiac effects of diadenosine tetraphosphate and pentaphosphate

1. The structural conformation of diadenosine tetraphosphate (Ap(4)A) and pentaphosphate (Ap(5)A) has been reported to alter as pH is reduced. As such, it is possible that the cardiac effects of Ap(4)A and Ap(5)A vary during acidosis and myocardial ischaemia due to changes in ligand structure, receptor proteins or intracellular signalling. 2. We investigated whether the cardiac electrophysiological and coronary vasomotor effects of Ap(4)A and Ap(5)A are preserved under conditions of extracellular acidosis (pH 6.5) and alkalosis (pH 8.5) and whether Ap(4)A has any electrophysiological or antiarrhythmic effects during ischaemia. 3. Transmembrane right ventricular action potentials, refractory periods and coronary perfusion pressure were recorded from isolated, Langendorff-perfused guinea-pig hearts under constant flow conditions. The effects of 1 nM and 1 microM Ap(4)A and Ap(5)A were studied at pH 7.4, 6.5 and 8.5. The effects of 1 microM Ap(4)A were studied during global low-flow ischaemia and reperfusion. 4. At pH 7.4, Ap(4)A and Ap(5)A increased action potential duration (APD(95)) and refractory period (RP) and reduced coronary perfusion pressure. The electrophysiological effects were absent at pH 6.5 while the reductions in perfusion pressure were attenuated. At pH 8.5, Ap(4)A increased RP but the effects of Ap(4)A and Ap(5)A on perfusion pressure were attenuated. During ischaemia, Ap(4)A had no antiarrhythmic or electrophysiological effects. 5. These data demonstrate the importance of extracellular pH in influencing the effects of Ap(4)A and Ap(5)A on the heart and indicate that any potentially cardioprotective effects of these compounds during normal perfusion at physiological pH are absent during ischaemia.

Acidosis inhibits gallbladder contraction mediated by protein kinase C activation

The effects of extracellular acidosis on gallbladder contraction were investigated using gallbladder strips isolated from guinea pigs. In an acidic medium (pH 6.9), gallbladder contraction induced by histamine and prostaglandin E2 was significantly lower than that in a normal medium (pH 7.4). Acidosis affected neither gallbladder contraction induced by histamine in the absence of extracellular Ca2+ nor that induced by KCl. Acidosis significantly inhibited Ca2+-induced contraction in the presence of sodium fluoride and phorbol 12,13-dibutyrate but not that in the presence of KCl. Staurosporine (30 nM) significantly inhibited gallbladder contraction induced by histamine and prostaglandin E2, but not that by KCl. Histamine-induced contraction in the presence of staurosporine was not affected by acidosis. Acidosis significantly inhibited Ca2+-induced contraction in the presence of histamine but not that in the presence of both histamine and staurosporine. These results suggest that extracellular acidosis selectively inhibits gallbladder contraction mediated by protein kinase C activation.

Na(+)/Ca(2+) exchange and its role in intracellular Ca(2+) regulation in guinea pig detrusor smooth muscle

The role of Na(+)/Ca(2+) exchange in regulating intracellular Ca(2+) concentration ([Ca(2+)](i)) in isolated smooth muscle cells from the guinea pig urinary bladder was investigated. Incremental reduction of extracellular Na(+) concentration resulted in a graded rise of [Ca(2+)](i); 50-100 microM strophanthidin also increased [Ca(2+)](i). A small outward current accompanied the rise of [Ca(2+)](i) in low-Na(+) solutions (17.1 +/- 1.8 pA in 29.4 mM Na(+)). The quantity of Ca(2+) influx through the exchanger was estimated from the charge carried by the outward current and was approximately 30 times that which is necessary to account for the rise of [Ca(2+)](i), after correction was made for intracellular Ca(2+) buffering. Ca(2+) influx through the exchanger was able to load intracellular Ca(2+) stores. It is concluded that the level of resting [Ca(2+)](i) is not determined by the exchanger, and under resting conditions (membrane potential -50 to -60 mV), there is little net flux through the exchanger. However, a small rise of intracellular Na(+) concentration would be sufficient to generate significant net Ca(2+) influx.

The electrophysiological properties of cultured and freshly isolated detrusor smooth muscle cells

PURPOSE

We generated and characterized a convenient isolated cell model of human detrusor smooth muscle to understand mechanisms that may underlie detrusor instability and provide a suitable model to test potentially useful drugs.

MATERIALS AND METHODS

The electrophysiological properties of freshly isolated detrusor smooth muscle cells from human and guinea pig biopsies were compared with those undergoing cell culture to document in detail the changes that occur during primary culture and subsequent passages as well as the differences in the 2 species.

RESULTS

Resting electrical characteristics were changed in the cultured cells. Membrane potential was less negative (guinea pig -59 versus -42 mV.) and membrane resistance was less (138 versus 124.5 Omegacm.(2)). Regenerative action potentials were recorded in cultured and freshly isolated cells. In guinea pig cells the overall duration and initial rate of depolarization (upstroke) was slower in cultured than in freshly isolated cells, indicative of a decreased magnitude of ionic current in cultured cells. Human cells had a similar prolongation in culture but no decrease in the upstroke rate. Experiments with selective blockers indicated that depolarization is due to influx through L-type Ca2+ channels and repolarization occurred via Ca2+ dependent K+ channels in freshly isolated and cultured cells. No further changes to properties were observed in cells passaged up to 3 times from primary cultured cells.

CONCLUSIONS

Cell culture qualitatively preserves the electrophysiological properties of detrusor smooth muscle cells, although there is some decrease in channel density.

The actions of metabolic inhibition on human detrusor smooth muscle contractility from stable and unstable bladders

OBJECTIVES

To determine the important cellular site(s) of action of a brief exposure to NaCN (chosen to reduce mitochondrial respiration and hence mimic cellular hypoxia) on the mechanical properties and regulation of intracellular [Ca2+] in human detrusor smooth muscle. Using muscle samples obtained from patients with stable and unstable bladders, to determine whether the unstable bladder is associated with changes in the functional properties of detrusor muscle under these circumstances. Materials and methods Experiments were conducted in vitro on muscle strips or isolated cells. Isometric tension was recorded in muscle strips during electrical stimulation or exposure to agonists. Intracellular [Ca2+] and [H+] were measured by epifluorescence microscopy, and cell autofluorescence measured as an index of mitochondrial function.

RESULTS

There were no differences in the responses to electrical stimulation and varying concentrations of carbachol in muscle strips from stable and unstable bladders. NaCN (2 mmol/L) reduced the contraction induced by carbachol (10 micromol/L) by a mean (SD) of 43 (16)% and 56 (15)% in the two groups; the reduction in the unstable was significantly less than in the stable group. NaCN similarly reduced the response to 10 mmol/L caffeine, but had no effect on the KCl-induced contraction. NaCN significantly increased the resting sarcoplasmic [Ca2+] and attenuated the calcium transients evoked by carbachol and caffeine, but again had no effect on the KCl-induced transient. The reduction of the carbachol calcium transient was also less in cells from unstable bladders than in those from stable bladders. There was no effect of NaCN on intracellular pH, except for a brief, transient alkalosis.

CONCLUSIONS

NaCN reduces both the contraction and Ca-transient to carbachol by reducing Ca2+ accumulation by intracellular stores, because the carbachol- and caffeine-evoked responses were similar. Any effect on transmembrane Ca2+ flux was minimal because there was no effect on KCl-induced responses. The greater resilience of tissue from unstable bladders to acute cellular hypoxia may reflect some adaptation acquired in vivo.

Cloning, tissue distribution, genomic organization, and functional characterization of NBC3, a new member of the sodium bicarbonate cotransporter family

Previous functional studies have demonstrated that muscle intracellular pH regulation is mediated by sodium-coupled bicarbonate transport, Na+/H+ exchange, and Cl-/bicarbonate exchange. We report the cloning, sequence analysis, tissue distribution, genomic organization, and functional analysis of a new member of the sodium bicarbonate cotransporter (NBC) family, NBC3, from human skeletal muscle. mNBC3 encodes a 1214-residue polypeptide with 12 putative membrane-spanning domains. The approximately 7.8-kilobase transcript is expressed uniquely in skeletal muscle and heart. The NBC3 gene (SLC4A7) spans approximately 80 kb and is composed of 25 coding exons and 24 introns that are flanked by typical splice donor and acceptor sequences. Expression of mNBC3 cRNA in Xenopus laevis oocytes demonstrated that the protein encodes a novel stilbene-insensitive 5-(N-ethyl-N-isopropyl)-amiloride-inhibitable sodium bicarbonate cotransporter.

Electrophysiological properties of the bladder

The electrophysiological properties of detrusor smooth muscle are described, in particular with regard to their influence on the contractile properties of the tissue. The Ca2+ and K+ channel activities are most important in generating action potentials, but the role of several other ionic currents is described, including Cl-, Ca2+-activated, stretch-activated and ligand-gated channels. The variable appearance and functions of different ionic currents in disease states is discussed, as well as the question of whether electrical activity can transmit between adjacent smooth muscle cells. In addition, the precise role that electrophysiological phenomena play in the regulation of the contractile state of the smooth muscle cells, as well as the generation of bladder electromyograms, is discussed.