Functional and molecular identification of pH-sensitive K+ channels in murine urinary bladder smooth muscle

Urinary bladder smooth muscle ion channels: expression, function, and regulation in health and disease

Urinary bladder smooth muscle (UBSM), also known as detrusor smooth muscle, forms the bladder wall and ultimately determines the two main attributes of the organ: urine storage and voiding. The two functions are facilitated by UBSM relaxation and contraction, respectively, which depend on UBSM excitability shaped by multiple ion channels. In this review, we summarize the current understanding of key ion channels establishing and regulating UBSM excitability and contractility. They include excitation-enhancing voltage-gated Ca2+ (Cav) and transient receptor potential channels, excitation-reducing K+ channels, and still poorly understood Cl- channels. Dynamic interplay among UBSM ion channels determines the overall level of Cav channel activity. The net Ca2+ influx via Cav channels increases global intracellular Ca2+ concentration, which subsequently triggers UBSM contractility. Here, for each ion channel type, we describe UBSM tissue/cell expression (mRNA and protein) profiles and their role in regulating excitability and contractility of UBSM in various animal species, including the mouse, rat, and guinea pig, and, most importantly, humans. The currently available data reveal certain interspecies differences, which complicate the translational value of published animal research results to humans. This review highlights recent developments, findings on genetic knockout models, pharmacological data, reports on UBSM ion channel dysfunction in animal bladder disease models, and the very limited human studies currently available. Among all gaps in present-day knowledge, the unknowns on expression and functional roles for ion channels determined directly in human UBSM tissues and cells under both normal and disease conditions remain key hurdles in the field.

Functional and molecular identification of a TASK-1 potassium channel regulating chloride secretion through CFTR channels in the shark rectal gland: implications for cystic fibrosis

In the shark rectal gland (SRG), apical chloride secretion through CFTR channels is electrically coupled to a basolateral K+ conductance whose type and molecular identity are unknown. We performed studies in the perfused SRG with 17 K+ channel inhibitors to begin this search. Maximal chloride secretion was markedly inhibited by low-perfusate pH, bupivicaine, anandamide, zinc, quinidine, and quinine, consistent with the properties of an acid-sensitive, four-transmembrane, two-pore-domain K+ channel (4TM-K2P). Using PCR with degenerate primers to this family, we identified a TASK-1 fragment in shark rectal gland, brain, gill, and kidney. Using 5' and 3' rapid amplification of cDNA ends PCR and genomic walking, we cloned the full-length shark gene (1,282 bp), whose open reading frame encodes a protein of 375 amino acids that was 80% identical to the human TASK-1 protein. We expressed shark and human TASK-1 cRNA in Xenopus oocytes and characterized these channels using two-electrode voltage clamping. Both channels had identical current-voltage relationships (outward rectifying) and a reversal potential of -90 mV. Both were inhibited by quinine, bupivicaine, and acidic pH. The pKa for current inhibition was 7.75 for shark TASK-1 vs. 7.37 for human TASK-1, values similar to the arterial pH for each species. We identified this protein in SRG by Western blot and confocal immunofluorescent microscopy and detected the protein in SRG and human airway cells. Shark TASK-1 is the major K+ channel coupled to chloride secretion in the SRG, is the oldest 4TM 2P family member identified, and is the first TASK-1 channel identified to play a role in setting the driving force for chloride secretion in epithelia. The detection of this potassium channel in mammalian lung tissue has implications for human biology and disease.

Ion channels of the mammalian urethra

The mammalian urethra is a muscular tube responsible for ensuring that urine remains in the urinary bladder until urination. In order to prevent involuntary urine leakage, the urethral musculature must be capable of constricting the urethral lumen to an extent that exceeds bladder intravesicular pressure during the urine-filling phase. The main challenge in anti-incontinence treatments involves selectively-controlling the excitability of the smooth muscles in the lower urinary tract. Almost all strategies to battle urinary incontinence involve targeting the bladder and as a result, this tissue has been the focus for the majority of research and development efforts. There is now increasing recognition of the value of targeting the urethral musculature in the treatment and management of urinary incontinence. Newly-identified and characterized ion channels and pathways in the smooth muscle of the urethra provides a range of potential therapeutic targets for the treatment of urinary incontinence. This review provides a summary of the current state of knowledge of the ion channels discovered in urethral smooth muscle cells that regulate their excitability.

Functional expression of KCNQ (Kv7) channels in guinea pig bladder smooth muscle and their contribution to spontaneous activity

Background and Purpose

The aim of the study was to determine whether KCNQ channels are functionally expressed in bladder smooth muscle cells (SMC) and to investigate their physiological significance in bladder contractility.

Experimental Approach

KCNQ channels were examined at the genetic, protein, cellular and tissue level in guinea pig bladder smooth muscle using RT-PCR, immunofluorescence, patch-clamp electrophysiology, calcium imaging, detrusor strip myography, and a panel of KCNQ activators and inhibitors.

Key Results

KCNQ subtypes 1–5 are expressed in bladder detrusor smooth muscle. Detrusor strips typically displayed TTX-insensitive myogenic spontaneous contractions that were increased in amplitude by the KCNQ channel inhibitors XE991, linopirdine or chromanol 293B. Contractility was inhibited by the KCNQ channel activators flupirtine or meclofenamic acid (MFA). The frequency of Ca²⁺-oscillations in SMC contained within bladder tissue sheets was increased by XE991. Outward currents in dispersed bladder SMC, recorded under conditions where BK and K_ATP currents were minimal, were significantly reduced by XE991, linopirdine, or chromanol, and enhanced by flupirtine or MFA. XE991 depolarized the cell membrane and could evoke transient depolarizations in quiescent cells. Flupirtine (20 μM) hyperpolarized the cell membrane with a simultaneous cessation of any spontaneous electrical activity.

Conclusions and Implications

These novel findings reveal the role of KCNQ currents in the regulation of the resting membrane potential of detrusor SMC and their important physiological function in the control of spontaneous contractility in the guinea pig bladder.

Acid and stretch, but not capsaicin, are effective stimuli for ATP release in the porcine bladder mucosa: Are ASIC and TRPV1 receptors involved?

Stretch-evoked ATP release from the bladder mucosa is a key event in signaling bladder fullness. Our aim was to examine whether acid and capsaicin can also release ATP and to determine the receptors involved, using agonists and antagonists at TRPV1 and acid-sensing ion channels (ASICs). Strips of porcine bladder mucosa were exposed to acid, capsaicin or stretch. Strip tension was monitored. Bath fluid was collected for ATP measurement. Gene expression of ASICs and TRPV1 in porcine bladders was quantified using quantitative real-time PCR (qRT-PCR). Stretch stimulus (150% of original length) repeatedly and significantly increased ATP release to approximately 45 times basal release. Acid (pH 6.5, 6.0, 5.6) contracted mucosal strips and also increased ATP release up to 30-fold, without evidence of desensitization. Amiloride (0.3 μM) reduced the acid-evoked ATP release by approximately 70%, while capsazepine (10 μM) reduced acid-evoked ATP release at pH 6.0 and pH 5.6 (by 68% and 61%, respectively). Capsaicin (0.1-10 μM) was ineffective in causing ATP release, and also failed to contract porcine mucosal or detrusor strips. Gene expression for ASIC1, ASIC2, ASIC3 and TRPV1 was seen in the lateral wall, dome, trigone and neck of both detrusor and mucosa. In conclusion, stretch and acid induce ATP release in the porcine bladder mucosa, but capsaicin is ineffective. The pig bladder is a well-known model for the human bladder, however these data suggest that it should be used with caution, particularly for TRPV1 related studies.

Role of potassium ion channels in detrusor smooth muscle function and dysfunction

Contraction and relaxation of the detrusor smooth muscle (DSM), which makes up the wall of the urinary bladder, facilitates the storage and voiding of urine. Several families of K(+) channels, including voltage-gated K(+) (K(V)) channels, Ca(2+)-activated K(+) (K(Ca)) channels, inward-rectifying ATP-sensitive K(+) (K(ir), K(ATP)) channels, and two-pore-domain K(+) (K(2P)) channels, are expressed and functional in DSM. They control DSM excitability and contractility by maintaining the resting membrane potential and shaping the action potentials that determine the phasic nature of contractility in this tissue. Defects in DSM K(+) channel proteins or in the molecules involved in their regulatory pathways may underlie certain forms of bladder dysfunction, such as overactive bladder. K(+) channels represent an opportunity for novel pharmacological manipulation and therapeutic intervention in human DSM. Modulation of DSM K(+) channels directly or indirectly by targeting their regulatory mechanisms has the potential to control urinary bladder function. This Review summarizes our current state of knowledge of the functional role of K(+) channels in DSM in health and disease, with special emphasis on current advancements in the field.

Two-pore domain K⁺ channels regulate membrane potential of isolated human articular chondrocytes

Potassium channels that regulate resting membrane potential (RMP) of human articular chondrocytes (HACs) of the tibial joint maintained in short-term (0-3 days) non-confluent cell culture were studied using patch-clamp techniques. Quantitative PCR showed that transcripts of genes for two-pore domain K(+) channels (KCNK1, KCNK5 and KCNK6), and 'BK' Ca(2+)-activated K(+) channels (KCNMA1) were abundantly expressed. Immunocytological methods detected α-subunits for BK and K(2p)5.1 (TASK-2) K(+) channels. Electrophysiological recordings identified three distinct K(+) currents in isolated HACs: (i) a voltage- and time-dependent 'delayed rectifier', blocked by 100 nM α-dendrotoxin, (ii) a large 'noisy' voltage-dependent current that was blocked by low concentrations of tetraethylammonium (TEA; 50% blocking dose = 0.15 mM) and iberiotoxin (52% block, 100 nM) and (iii) a voltage-independent 'background' K(+) current that was blocked by acidic pH (5.5-6), was increased by alkaline pH (8.5), and was not blocked by TEA, but was blocked by the local anaesthetic bupivacaine (0.25 mM). The RMP of isolated HACs was very slightly affected by 5 mM TEA, which was sufficient to block both voltage-dependent K(+) currents, suggesting that these currents probably contributed little to maintaining RMP under 'resting' conditions (i.e. low internal [Ca(2+)]). Increases in external K(+) concentration depolarized HACs by 30 mV in response to a 10-fold increase in [K(+)], indicating a significant but not exclusive role for K(+) current in determining RMP. Increases in external [K(+)] in voltage-clamped HACs revealed a voltage-independent K(+) current whose inward current magnitude increased with external [K(+)]. Block of this current by bupivacaine (0.25-1 mM) in 5 and 25 mM external [K(+)] resulted in a large (8-25 mV) depolarization of RMP. The biophysical and pharmacological properties of the background K(+) current, together with expression of mRNA and α-subunit protein for TASK-2, strongly suggest that these two-pore domain K(+) channels contribute significantly to stabilizing the RMP of HACs.

Role of TREK-1 potassium channel in bladder overactivity after partial bladder outlet obstruction in mouse

PURPOSE

Mouse models of partial bladder outlet obstruction cause bladder hypertrophy. Expression of a number of ion channels is altered in hypertrophic detrusor muscle, resulting in bladder dysfunction. We determined whether mechanosensitive TREK-1 channels are present in the murine bladder and whether their expression is altered in partial bladder outlet obstruction, resulting in abnormal filling responses.

MATERIALS AND METHODS

Partial bladder outlet obstruction was surgically induced in CD-1 mice and the mice recovered for 14 days. Cystometry was done to evaluate bladder pressure responses during filling at 25 microl per minute in partial bladder outlet obstruction mice and sham operated controls. TREK-1 channel expression was determined at the mRNA and protein levels by quantitative reverse transcriptase-polymerase chain reaction and Western blotting, respectively, and localized in the bladder wall using immunohistochemistry.

RESULTS

Obstructed bladders showed about a 2-fold increase in weight vs sham operated bladders. TREK-1 channel protein expression on Western blots from bladder smooth muscle strip homogenates was significantly decreased in obstructed mice. Immunohistochemistry revealed a significant decrease in TREK-1 channel immunoreactivity in detrusor smooth muscle in obstructed mice. On cystometry the TREK-1 channel blocker L-methioninol induced a significant increase in premature contractions during filling in sham operated mice. L-methioninol had no significant effect in obstructed mice, which showed an overactive detrusor phenotype.

CONCLUSIONS

TREK-1 channel down-regulation in detrusor myocytes is associated with bladder overactivity in a murine model of partial bladder outlet obstruction.

Targeting TASK-1 channels as a therapeutic approach

The voltage-independent background two-pore domain K(+) channel TASK-1 sets the resting membrane potential in excitable cells and renders these cells sensitive to a variety of vasoactive factors. There is clear evidence for TASK-1 in human pulmonary artery smooth muscle cells and TASK-1 channels are likely to regulate the pulmonary vascular tone through their regulation by hypoxia, pH, inhaled anesthetics, and G protein-coupled pathways. Furthermore, TASK-1 is a strong candidate to play a role in hypoxic pulmonary vasoconstriction. On the other hand, consistent with the activation of TASK-1 channels by volatile anesthetics, TASK-1 contributes to the anesthetic-induced pulmonary vasodilation. TASK-1 channels are unique among K(+) channels because they are regulated by both, increases and decreases from physiological pH, thus contributing to their protective effect on the pulmonary arteries. Moreover, TASK-1 may also have a critical role in mediating the vasoactive response of G protein-coupled pathways in resistance arteries which can offer promising therapeutic solutions to target diseases of the pulmonary circulation.