Properties and molecular basis of the mouse urinary bladder voltage-gated K+ current

Domain analysis of Kv6.3, an electrically silent channel

The subunit Kv6.3 encodes a voltage-gated potassium channel belonging to the group of electrically silent Kv subunits, i.e. subunits that do not form functional homotetrameric channels. The lack of current, caused by retention in the endoplasmic reticulum (ER), was overcome by coexpression with Kv2.1. To investigate whether a specific section of Kv6.3 was responsible for ER retention, we constructed chimeric subunits between Kv6.3 and Kv2.1, and analysed their subcellular localization and functionality. The results demonstrate that the ER retention of Kv6.3 is not caused by the N-terminal A and B box (NAB) domain nor the intracellular N- or C-termini, but rather by the S1-S6 core protein. Introduction of individual transmembrane segments of Kv6.3 in Kv2.1 was tolerated, with the exception of S6. Indeed, introduction of the S6 domain of Kv6.3 in Kv2.1 was enough to cause ER retention, which was due to the C-terminal section of S6. The S4 segment of Kv6.3 could act as a voltage sensor in the Kv2.1 context, albeit with a major hyperpolarizing shift in the voltage dependence of activation and inactivation, apparently caused by the presence of a tyrosine in Kv6.3 instead of a conserved arginine. This study suggests that the silent behaviour of Kv6.3 is largely caused by the C-terminal part of its sixth transmembrane domain that causes ER retention of the subunit.

Erectile dysfunction in mice lacking the large-conductance calcium-activated potassium (BK) channel

Penile erection is dependent on the nitric oxide (NO)/cGMP-dependent protein kinase I (PKGI) pathway. One important target of PKGI in smooth muscle is the large-conductance, calcium-activated potassium (BK) channel, which upon activation hyperpolarizes the smooth muscle cell membrane, causing relaxation. Relaxation of arterial and corpus cavernosum smooth muscle (CCSM) is necessary to increase blood flow into the corpora cavernosa that leads to penile tumescence. We investigated the functional role of BK channels in the corpus cavernosum utilizing a knock-out mouse lacking the Slo gene (Slo-/-) responsible for the pore-forming subunit of the BK channel. Whole-cell currents were recorded from isolated CCSM cells of Slo+/+ and Slo-/- mice. Iberiotoxin-sensitive voltage- and [Ca2+]-activated K+ currents, the latter activated by local transient calcium releases (calcium sparks), were present in Slo+/+ CCSM cells, but absent in Slo-/- cells. CCSM strips from Slo-/- mice demonstrated a four-fold increase in phasic contractions, in the presence of phenylephrine. Nerve-evoked relaxations of precontracted strips were reduced by 50%, both in strips from Slo-/- mice and by blocking BK channels with iberiotoxin in the Slo+/+ strips. Consistent with the in vitro results, in vivo intracavernous pressure exhibited pronounced oscillations in Slo-/- mice, but not in Slo+/+ mice. Furthermore, intracavernous pressure increases to nerve stimulation, in vivo, were reduced by 22% in Slo-/- mice. These results indicate that the BK channel has an important role in erectile function, and loss of the BK channel leads to erectile dysfunction.

Molecular diversity and function of voltage-gated (Kv) potassium channels in epithelial cells

Voltage-gated K+ channels belonging to Kv1-9 subfamilies are widely expressed in excitable cells where they play an essential role in membrane hyperpolarization during an action potential and in the propagation of action potentials along the plasma membrane. Early patch clamp studies on epithelial cells revealed the presence of K+ currents with biophysical and pharmacologic properties characteristic of Kv channels expressed in excitable cells. More recently, molecular approaches including PCR and the availability of more selective antibodies directed against Kv alpha and auxiliary subunits, have demonstrated that epithelial cells from various organ systems, express a remarkable diversity Kv channel subunits. Unlike neurons and myocytes however, epithelial cells do not typically generate action potentials or exhibit dynamic changes in membrane potential necessary for activation of Kv alpha subunits. Moreover, the fact that many Kv channels expressed in epithelial cells exhibit inactivation suggest that their activities are relatively transient, making it difficult to ascribe a functional role for these channels in transepithelial electrolyte or nutrient transport. Other proposed functions have included (i) cell migration and wound healing, (ii) cell proliferation and cancer, (iii) apoptosis and (iv) O2 sensing. Certain Kv channels, particularly Kv1 and Kv2 subfamily members, have been shown to be involved in the proliferation of prostate, colon, lung and breast carcinomas. In some instances, a significant increase in Kv channel expression has been correlated with tumorogenesis suggesting the possibility of using these proteins as markers for transformation and perhaps reducing the rate of tumor growth by selectively inhibiting their functional activity.

Negative feedback regulation of nerve-mediated contractions by KCa channels in mouse urinary bladder smooth muscle

When the urinary bladder is full, activation of parasympathetic nerves causes release of neurotransmitters that induce forceful contraction of the detrusor muscle, leading to urine voiding. The roles of ion channels that regulate contractility of urinary bladder smooth muscle (UBSM) in response to activation of parasympathetic nerves are not well known. The present study was designed to characterize the role of large (BK)- and small-conductance (SK) Ca(2+)-activated K(+) (K(Ca)) channels in regulating UBSM contractility in response to physiological levels of nerve stimulation in UBSM strips from mice. Nerve-evoked contractions were induced by electric field stimulation (0.5-50 Hz) in isolated strips of UBSM. BK and SK channel inhibition substantially increased the amplitude of nerve-evoked contractions up to 2.45 +/- 0.12- and 2.99 +/- 0.25-fold, respectively. When both SK and BK channels were inhibited, the combined response was additive. Inhibition of L-type voltage-dependent Ca(2+) channels (VDCCs) in UBSM inhibited nerve-evoked contractions by 92.3 +/- 2.0%. These results suggest that SK and BK channels are part of two distinct negative feedback pathways that limit UBSM contractility in response to nerve stimulation by modulating the activity of VDCCs. Dysfunctional regulation of UBSM contractility by alterations in BK/SK channel expression or function may underlie pathologies such as overactive bladder.

Urodynamic properties and neurotransmitter dependence of urinary bladder contractility in the BK channel deletion model of overactive bladder

Overactive bladder and incontinence are major medical issues, which lack effective therapy. Previously, we showed (Meredith AL, Thornloe KS, Werner ME, Nelson MT, and Aldrich RW. J Biol Chem 279: 36746-36752, 2004) that the gene mSlo1 encodes large-conductance Ca2+-activated K+ (BK) channels of urinary bladder smooth muscle (UBSM) and that ablation of mSlo1 leads to enhanced myogenic and nerve-mediated contractility and increased urination frequency. Here, we examine the in vivo urodynamic consequences and neurotransmitter dependence in the absence of the BK channel. The sensitivity of contractility to nerve stimulation was greatly enhanced in UBSM strips from Slo-/- mice. The stimulation frequency required to obtain a 50% maximal contraction was 8.3 +/- 0.9 and 19.1 +/- 1.8 Hz in Slo-/- and Slo+/+ mice, respectively. This enhancement is at least partially due to alterations in UBSM excitability, as muscarinic-induced Slo-/- contractility is elevated in the absence of neuronal activity. Muscarinic-induced Slo-/- contractility was mimicked by blocking BK channels with iberiotoxin (IBTX) in Slo+/+ strips, whereas IBTX had no effect on Slo-/- strips. IBTX also enhanced purinergic contractions of Slo+/+ UBSM but was without effect on purinergic contractions of Slo-/- strips. In vivo bladder pressure and urine output measurements (cystometry) were performed on conscious, freely moving mice. Slo-/- mice exhibited increased bladder pressures, pronounced pressure oscillations, and urine dripping. Our results indicate that the BK channel in UBSM has a very significant role in urinary function and dysfunction and as such likely represents an important therapeutic target.

Differential regulation of Ca2+-activated K+ channels by beta-adrenoceptors in guinea pig urinary bladder smooth muscle

Stimulation of beta-adrenoceptors contributes to the relaxation of urinary bladder smooth muscle (UBSM) through activation of large-conductance Ca(2+)-activated K(+) (BK) channels. We examined the mechanisms by which beta-adrenoceptor stimulation leads to an elevation of the activity of BK channels in UBSM. Depolarization from -70 to +10 mV evokes an inward L-type dihydropyridine-sensitive voltage-dependent Ca(2+) channel (VDCC) current, followed by outward steady-state and transient BK current. In the presence of ryanodine, which blocks the transient BK currents, isoproterenol, a nonselective beta-adrenoceptor agonist, increased the VDCC current by approximately 25% and the steady-state BK current by approximately 30%. In the presence of the BK channel inhibitor iberiotoxin, isoproterenol did not cause activation of the remaining steady-state K(+) current component. Decreasing Ca(2+) influx through VDCC by nifedipine or depolarization to +80 mV suppressed the isoproterenol-induced activation of the steady-state BK current. Unlike forskolin, isoproterenol did not change significantly the open probability of single BK channels in the absence of Ca(2+) sparks and with VDCC inhibited by nifedipine. Isoproterenol elevated Ca(2+) spark (local intracellular Ca(2+) release through ryanodine receptors of the sarcoplasmic reticulum) frequency and associated transient BK currents by approximately 1.4-fold. The data support the concept that in UBSM beta-adrenoceptor stimulation activates BK channels by elevating Ca(2+) influx through VDCC and by increasing Ca(2+) sparks, but not through a Ca(2+)-independent mechanism. This study reveals key regulatory molecular and cellular mechanisms of beta-adrenergic regulation of BK channels in UBSM that could provide new targets for drugs in the treatment of bladder dysfunction.

BL-1249 [(5,6,7,8-tetrahydro-naphthalen-1-yl)-[2-(1H-tetrazol-5-yl)-phenyl]-amine]: a putative potassium channel opener with bladder-relaxant properties

BL-1249 [(5,6,7,8-tetrahydro-naphthalen-1-yl)-[2-(1H-tetrazol-5-yl)-phenyl]-amine] produced a concentration-dependent membrane hyperpolarization of cultured human bladder myocytes, assessed as either a reduction in fluorescence of the voltage-sensitive dye bis-(1,2-dibutylbarbituric acid)trimethine oxonol (EC50 = 1.26 +/- 0.6 microM) or by direct electrophysiological measurement (EC50 = 1.49 +/- 0.08 microM). BL-1249 also produced a membrane hyperpolarization of acutely dissociated rat bladder myocytes. Voltage-clamp studies in human bladder cells revealed that BL-1249 activated an instantaneous, noninactivating current that reversed near E(K). The BL-1249-evoked outward K+ current was insensitive to blockade by glyburide, tetraethylammonium, iberiotoxin, 4-aminopyridine, apamin, or Mg2+. However, the current was inhibited by extracellular Ba2+ (10 mM). In in vitro organ bath experiments, BL-1249 produced a concentration-dependent relaxation of 30 mM KCl-induced contractions in rat bladder strips (EC50 = 1.12 +/- 0.37 microM), yet had no effect on aortic strips up to the highest concentration tested (10 microM). The bladder relaxation produced by BL-1249 was partially blocked by Ba2+ (1 and 10 mM) but not by apamin, iberiotoxin, 4-aminopyridine, glyburide, or tetraethylammonium. In an anesthetized rat model, BL-1249 (1 mg/kg i.v.) decreased the number of isovolumic contractions, without significantly affecting blood pressure. Thus, BL-1249 behaves as a potassium channel activator that exhibits bladder versus vascular selectivity both in vitro and in vivo. A survey of potassium channels exhibiting sensitivity to extracellular Ba2+ at millimolar concentration revealed that the expression of the K2P2.1 (TREK-1) channel was relatively high in human bladder cells versus human aortic cells, suggesting this channel as a possible candidate target for BL-1249.

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.

Overactive bladder and incontinence in the absence of the BK large conductance Ca2+-activated K+ channel

BK large conductance voltage- and calcium-activated potassium channels respond to elevations in intracellular calcium and membrane potential depolarization, braking excitability of smooth muscle. BK channels are thought to have a particularly prominent role in urinary bladder smooth muscle function and therefore are candidate targets for overactive bladder therapy. To address the role of the BK channel in urinary bladder function, the gene mSlo1 for the pore-forming subunit of the BK channel was deleted. Slo(-/-) mice were viable but exhibited moderate ataxia. Urinary bladder smooth muscle cells of Slo(-/-) mice lacked calcium- and voltage-activated BK currents, whereas local calcium transients ("calcium sparks") and voltage-dependent potassium currents were unaffected. In the absence of BK channels, urinary bladder spontaneous and nerve-evoked contractions were greatly enhanced. Consistent with increased urinary bladder contractility caused by the absence of BK currents, Slo(-/-) mice demonstrate a marked elevation in urination frequency. These results reveal a central role for BK channels in urinary bladder function and indicate that BK channel dysfunction leads to overactive bladder and urinary incontinence.

Properties of a tonically active, sodium-permeable current in mouse urinary bladder smooth muscle

Urinary bladder smooth muscle (UBSM) elicits depolarizing action potentials, which underlie contractile events of the urinary bladder. The resting membrane potential of UBSM is approximately −40 mV and is critical for action potential generation, with hyperpolarization reducing action potential frequency. We hypothesized that a tonic, depolarizing conductance was present in UBSM, functioning to maintain the membrane potential significantly positive to the equilibrium potential for K+ ( EK; −85 mV) and thereby facilitate action potentials. Under conditions eliminating the contribution of K+ and voltage-dependent Ca2+ channels, and with a clear separation of cation- and Cl−-selective conductances, we identified a novel background conductance ( Icat) in mouse UBSM cells. Icat was mediated predominantly by the influx of Na+, although a small inward Ca2+ current was detectable with Ca2+ as the sole cation in the bathing solution. Extracellular Ca2+, Mg2+, and Gd3+ blocked Icat in a voltage-dependent manner, with Ki values at −40 mV of 115, 133, and 1.3 μM, respectively. Although UBSM Icat is extensively blocked by physiological extracellular Ca2+ and Mg2+, a tonic, depolarizing Icat was detected at −40 mV. In addition, inhibition of Icat demonstrated a hyperpolarization of the UBSM membrane potential and decreased the amplitude of phasic contractions of isolated UBSM strips. We suggest that Icat contributes tonically to the depolarization of the UBSM resting membrane potential, facilitating action potential generation and thereby a maintenance of urinary bladder tone.