Enhanced K(+) secretion in dextran sulfate-induced colitis reflects upregulation of large conductance apical K(+) channels (BK; Kcnma1)

Aldosterone up-regulates basolateral Na+ -K+ -2Cl- cotransporter-1 to support enhanced large-conductance K+ channel-mediated K+ secretion in rat distal colon

Na+ -K+ -2Cl- cotransporter-1 (NKCC1) facilitates basolateral K+ and Cl- uptake, supporting their efflux across mucosal membranes of colonic epithelial cells. NKCC1 activity has also been shown to be critical for electrogenic K+ secretion induced by aldosterone, which is known to stimulate large-conductance K+ (BK) channel expression in mucosal membranes. This study was aimed to (1) identify whether aldosterone enhances NKCC1 expression specifically to support BK-mediated K+ secretion and (2) to determine whether increased NKCC1 supports electrogenic Cl- secretion in parallel to K+ secretion. Dietary Na+ depletion was used to induce secondary hyperaldosteronism in rats, or aldosterone was administered ex vivo to rat distal colonic mucosae. NKCC1-dependent electrogenic K+ or Cl- secretion was measured as a function of short circuit current (ISC ). qRT-PCR, western blot, and immunofluorescence analyses were performed using standard techniques. Aldosterone enhanced NKCC1 and BKα expression and electrogenic K+ secretion in the distal colon, which was inhibited by either serosal bumetanide (NKCC1 inhibitor) or mucosal iberiotoxin (IbTX; BK channel blocker), but not TRAM-34 (IK channel blocker). Expression of NKCC1 and BKα proteins was enhanced in crypt cells of hyper-aldosterone rats. However, neither NKCC1-dependent Cl- secretion nor CFTR (apical Cl- channel) expression was enhanced by aldosterone. We conclude that aldosterone enhances NKCC1 to support BK-mediated K+ secretion independently of Cl- secretion in the distal colon. The regulation of NKCC1 expression/K+ secretion by aldosterone may be a therapeutic target in treating gastrointestinal disorders associated with alterations in colonic K+ transport, such as colonic pseudo-obstruction, and hyperkalemia associated with renal disease.

Potassium channels in intestinal epithelial cells and their pharmacological modulation: a systematic review

Several potassium channels (KCs) have been described throughout the gastrointestinal tract. Notwithstanding, their contribution to both physiologic and pathophysiologic conditions, as inflammatory bowel disease (IBD), remains underexplored. Therefore, we aim to systematically review, for the first time, the evidence on the characteristics and modulation of KCs in intestinal epithelial cells (IECs). PubMed, Scopus, and Web of Science were searched to identify studies focusing on KCs and their modulation in IECs. The included studies were assessed using a reporting inclusiveness checklist. From the 745 identified records, 73 met the inclusion criteria; their reporting inclusiveness was moderate-high. Some studies described the physiological role of KCs, while others explored their importance in pathological settings. Globally, in IBD animal models, apical K_Ca1.1 channels, responsible for luminal secretion, were upregulated. In human colonocytes, basolateral K_Ca3.1 channels were downregulated. The pharmacological inhibition of K_2P and K_v influenced intestinal barrier function, promoting inflammation. Evidence suggests a strong association between KCs expression and secretory mechanisms in human and animal IECs. Further research is warranted to explore the usefulness of KC pharmacological modulation as a therapeutic target.

Female mice are resilient to age-related decline of substantia nigra dopamine neuron firing parameters

Degeneration of substantia nigra pars compacta dopamine neurons is a central feature in the pathology of Parkinson's disease, which is characterized by progressive loss of motor and cognitive functions. The largest risk factors for Parkinson's disease are age and sex; most cases occur after age 60 and males have nearly twice the incidence as females. Preclinical work has scarcely considered the influence of these 2 factors to disease risk and presentation. Here, we observed a progressive decline in dopamine neuron firing activity in male C57BL/6 mice by 18 months of age, while dopamine neurons from females remained largely unaffected. This was accompanied by increased mRNA expression of PINK1 in both males and females, and PARK2 primarily in males, both of which have been linked to Parkinson's. Since the declining cell properties were accompanied by only slight decreases in locomotion in both sexes, it is likely that these age-related impairments in males represent a vulnerability to further insults that could predispose the neurons to neurodegenerative processes such as in Parkinson's.

Parallel intermediate conductance K+ and Cl- channel activity mediates electroneutral K+ exit across basolateral membranes in rat distal colon

Transepithelial K⁺ absorption requires apical K⁺ uptake and basolateral K⁺ exit. In the colon, apical H⁺-K⁺-ATPase mediates cellular K⁺ uptake, and it has been suggested that electroneutral basolateral K⁺ exit reflects K⁺-Cl^- cotransporter-1 (KCC1) operating in parallel with K⁺ and Cl^- channels. The present study was designed to identify basolateral transporter(s) responsible for K⁺ exit in rat distal colon. Active K⁺ absorption was determined by measuring ⁸⁶Rb⁺ (K⁺ surrogate) fluxes across colonic epithelia under voltage-clamp conditions. With zero Cl^- in the mucosal solution, net K⁺ absorption was reduced by 38%, indicating that K⁺ absorption was partially Cl^--dependent. Serosal addition of DIOA (KCC1 inhibitor) or Ba²⁺ (nonspecific K⁺ channel blocker) inhibited net K⁺ absorption by 21% or 61%, respectively, suggesting that both KCC1 and K⁺ channels contribute to basolateral K⁺ exit. Clotrimazole and TRAM34 (IK channel blockers) added serosally inhibited net K⁺ absorption, pointing to the involvement of IK channels in basolateral K⁺ exit. GaTx2 (CLC2 blocker) added serosally also inhibited net K⁺ absorption, suggesting that CLC2-mediated Cl^- exit accompanies IK channel-mediated K⁺ exit across the basolateral membrane. Net K⁺ absorption was not inhibited by serosal addition of either IbTX (BK channel blocker), apamin (SK channel blocker), chromanol 293B (K_V7 channel blocker), or CFTR_inh172 (CFTR blocker). Immunofluorescence studies confirmed basolateral membrane colocalization of CLC2-like proteins and Na⁺-K⁺-ATPase α-subunits. We conclude that active K⁺ absorption in rat distal colon involves electroneutral basolateral K⁺ exit, which may reflect IK and CLC2 channels operating in parallel.NEW & NOTEWORTHY This study demonstrates that during active electroneutral K⁺ absorption in rat distal colon, K⁺ exit across the basolateral membrane mainly reflects intermediate conductance K⁺ channels operating in conjunction with chloride channel 2, with a smaller, but significant, contribution from K⁺-Cl^- cotransporter-1 (KCC1) activity.

Physiology of Electrolyte Transport in the Gut: Implications for Disease

We now have an increased understanding of the genetics, cell biology, and physiology of electrolyte transport processes in the mammalian intestine, due to the availability of sophisticated methodologies ranging from genome wide association studies to CRISPR-CAS technology, stem cell-derived organoids, 3D microscopy, electron cryomicroscopy, single cell RNA sequencing, transgenic methodologies, and tools to manipulate cellular processes at a molecular level. This knowledge has simultaneously underscored the complexity of biological systems and the interdependence of multiple regulatory systems. In addition to the plethora of mammalian neurohumoral factors and their cross talk, advances in pyrosequencing and metagenomic analyses have highlighted the relevance of the microbiome to intestinal regulation. This article provides an overview of our current understanding of electrolyte transport processes in the small and large intestine, their regulation in health and how dysregulation at multiple levels can result in disease. Intestinal electrolyte transport is a balance of ion secretory and ion absorptive processes, all exquisitely dependent on the basolateral Na⁺ /K⁺ ATPase; when this balance goes awry, it can result in diarrhea or in constipation. The key transporters involved in secretion are the apical membrane Cl^- channels and the basolateral Na⁺ -K⁺ -2Cl^- cotransporter, NKCC1 and K⁺ channels. Absorption chiefly involves apical membrane Na⁺ /H⁺ exchangers and Cl^- /HCO₃ ^- exchangers in the small intestine and proximal colon and Na⁺ channels in the distal colon. Key examples of our current understanding of infectious, inflammatory, and genetic diarrheal diseases and of constipation are provided. © 2019 American Physiological Society. Compr Physiol 9:947-1023, 2019.

Colonic Potassium Absorption and Secretion in Health and Disease

The colon has large capacities for K⁺ absorption and K⁺ secretion, but its role in maintaining K⁺ homeostasis is often overlooked. For many years, passive diffusion and/or solvent drag were thought to be the primary mechanisms for K⁺ absorption in human and animal colon. However, it is now clear that apical H⁺ ,K⁺ -ATPase, in coordination with basolateral K⁺ -Cl^- cotransport and/or K⁺ and Cl^- channels operating in parallel, mediate electroneutral K⁺ absorption in animal colon. We now know that K⁺ absorption in rat colon reflects ouabain-sensitive and ouabain-insensitive apical H⁺ ,K⁺ -ATPase activities. Ouabain-insensitive and ouabain-sensitive H⁺ ,K⁺ -ATPases are localized in surface and crypt cells, respectively. Colonic H⁺ ,K⁺ -ATPase consists of α- (HKC_α ) and β- (HKC_β ) subunits which, when coexpressed, exhibit ouabain-insensitive H⁺ ,K⁺ -ATPase activity in HEK293 cells, while HKC_α coexpressed with the gastric β-subunit exhibits ouabain-sensitive H⁺ ,K⁺ -ATPase activity in Xenopus oocytes. Aldosterone enhances apical H⁺ ,K⁺ -ATPase activity, HKC_α specific mRNA and protein expression, and K⁺ absorption. Active K⁺ secretion, on the other hand, is mediated by apical K⁺ channels operating in a coordinated way with the basolateral Na⁺ -K⁺ -2Cl^- cotransporter. Both Ca²⁺ -activated intermediate conductance K⁺ (IK) and large conductance K⁺ (BK) channels are located in the apical membrane of colonic epithelia. IK channel-mediated K⁺ efflux provides the driving force for Cl^- secretion, while BK channels mediate active (e.g., cAMP-activated) K⁺ secretion. BK channel expression and activity are increased in patients with end-stage renal disease and ulcerative colitis. This review summarizes the role of apical H⁺ ,K⁺ -ATPase in K⁺ absorption, and apical BK channel function in K⁺ secretion in health and disease. © 2018 American Physiological Society. Compr Physiol 8:1513-1536, 2018.

Dextran sulfate sodium-induced chronic colitis attenuates Ca2+-activated Cl- secretion in murine colon by downregulating TMEM16A

Attenuated Ca2+-activated Cl- secretion has previously been observed in the model of dextran sulfate sodium (DSS)-induced colitis. Prior studies have implicated dysfunctional muscarinic signaling from basolateral membranes as the potential perpetrator leading to decreased Ca2+-activated Cl- secretion. However, in our chronic model of DSS-colitis, cholinergic receptor muscarinic 3 ( Chrm3) transcript (1.028 ± 0.12 vs. 1.029 ± 0.27, P > 0.05) and CHRM3 protein expression (1.021 ± 0.24 vs. 0.928 ± 0.09, P > 0.05) were unchanged. Therefore, we hypothesized that decreased carbachol (CCH)-stimulated Cl- secretion in DSS-induced colitis could be attributed to a loss of Ca2+-activated Cl- channels (CaCC) in apical membranes of colonic epithelium. To establish this chemically-induced colitis, Balb/C mice were exposed to 4% DSS for five alternating weeks to stimulate a more moderate, chronic colitis. Upon completion of the protocol, whole thickness sections of colon were mounted in an Ussing chamber under voltage-clamp conditions. DSS-induced colitis demonstrated a complete inhibition of basolateral administration of CCH-stimulated Cl- secretion that actually displayed a reversal in polarity (15.40 ± 2.22 μA/cm2 vs. -2.47 ± 0.25 μA/cm2). Western blotting of potential CaCCs, quantified by densitometric analysis, demonstrated no change in bestrophin-2 and cystic fibrosis transmembrane regulator, whereas anoctamin-1 [ANO1, transmembrane protein 16A (TMEM16A)] was significantly downregulated (1.001 ± 0.13 vs. 0.510 ± 0.12, P < 0.05). Our findings indicate that decreased expression of TMEM16A in DSS-induced colitis contributes to the decreased Ca2+-activated Cl- secretion in murine colon.

Potassium Channelopathies and Gastrointestinal Ulceration

Potassium channels and transporters maintain potassium homeostasis and play significant roles in several different biological actions via potassium ion regulation. In previous decades, the key revelations that potassium channels and transporters are involved in the production of gastric acid and the regulation of secretion in the stomach have been recognized. Drugs used to treat peptic ulceration are often potassium transporter inhibitors. It has also been reported that potassium channels are involved in ulcerative colitis. Direct toxicity to the intestines from nonsteroidal anti-inflammatory drugs has been associated with altered potassium channel activities. Several reports have indicated that the long-term use of the antianginal drug Nicorandil, an adenosine triphosphate-sensitive potassium channel opener, increases the chances of ulceration and perforation from the oral to anal regions throughout the gastrointestinal (GI) tract. Several of these drug features provide further insights into the role of potassium channels in the occurrence of ulceration in the GI tract. The purpose of this review is to investigate whether potassium channelopathies are involved in the mechanisms responsible for ulceration that occurs throughout the GI tract.

Role of epithelial ion transports in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic inflammatory disorder with a complex pathogenesis. Diarrhea is a highly prevalent and often debilitating symptom of IBD patients that results, at least in part, from an intestinal hydroelectrolytic imbalance. Evidence suggests that reduced electrolyte absorption is more relevant than increased secretion to this disequilibrium. This systematic review analyses and integrates the current evidence on the roles of epithelial Na(+)-K(+)-ATPase (NKA), Na(+)/H(+) exchangers (NHEs), epithelial Na(+) channels (ENaC), and K(+) channels (KC) in IBD-associated diarrhea. NKA is the key driving force of the transepithelial ionic transport and its activity is decreased in IBD. In addition, the downregulation of apical NHE and ENaC and the upregulation of apical large-conductance KC all contribute to the IBD-associated diarrhea by lowering sodium absorption and/or increasing potassium secretion.

Maintaining K+ balance on the low-Na+, high-K+ diet

A low-Na⁺, high-K⁺ diet (LNaHK) is considered a healthier alternative to the "Western" high-Na⁺ diet. Because the mechanism for K⁺ secretion involves Na⁺ reabsorptive exchange for secreted K⁺ in the distal nephron, it is not understood how K⁺ is eliminated with such low Na⁺ intake. Animals on a LNaHK diet produce an alkaline load, high urinary flows, and markedly elevated plasma ANG II and aldosterone levels to maintain their K⁺ balance. Recent studies have revealed a potential mechanism involving the actions of alkalosis, urinary flow, elevated ANG II, and aldosterone on two types of K⁺ channels, renal outer medullary K⁺ and large-conductance K⁺ channels, located in principal and intercalated cells. Here, we review these recent advances.

Targeting Ion Channels: An Important Therapeutic Implication in Gastrointestinal Dysmotility in Patients With Spinal Cord Injury

Gastrointestinal (GI) dysmotility is a severe, and common complication in patients with spinal cord injury (SCI). Current therapeutic methods using acetylcholine analogs or laxative agents have unwanted side effects, besides often fail to have desired effect. Various ion channels such as ATP-sensitive potassium (K_ATP) channel, calcium ions (Ca²⁺)-activated potassium ions (K⁺) channels, voltage-sensitive Ca²⁺ channels and chloride ion (Cl⁻) channels are abundantly expressed in GI tissues, and play an important role in regulating GI motility. The release of neurotransmitters from the enteric nerve terminal, innervating GI interstitial cells of Cajal (ICC), and smooth muscle cells (SMC), causes inactivation of K⁺ and Cl⁻ channels, increasing Ca²⁺ influx into cytoplasm, resulting in membrane depolarization and smooth muscle contraction. Thus, agents directly regulating ion channels activity either in ICC or in SMC may affect GI peristalsis and would be potential therapeutic target for the treatment of GI dysmotility with SCI.

Na-H Exchanger Isoform-2 (NHE2) Mediates Butyrate-dependent Na+ Absorption in Dextran Sulfate Sodium (DSS)-induced Colitis

Diarrhea associated with ulcerative colitis (UC) occurs primarily as a result of reduced Na(+) absorption. Although colonic Na(+) absorption is mediated by both epithelial Na(+) channels (ENaC) and Na-H exchangers (NHE), inhibition of NHE-mediated Na(+) absorption is the primary cause of diarrhea in UC. As there are conflicting observations reported on NHE expression in human UC, the present study was initiated to identify whether NHE isoforms (NHE2 and NHE3) expression is altered and how Na(+) absorption is regulated in DSS-induced inflammation in rat colon, a model that has been used to study UC. Western blot analyses indicate that neither NHE2 nor NHE3 expression is altered in apical membranes of inflamed colon. Na(+) fluxes measured in vitro under voltage clamp conditions in controls demonstrate that both HCO3 (-)-dependent and butyrate-dependent Na(+) absorption are inhibited by S3226 (NHE3-inhibitor), but not by HOE694 (NHE2-inhibitor) in normal animals. In contrast, in DSS-induced inflammation, butyrate-, but not HCO3 (-)-dependent Na(+) absorption is present and is inhibited by HOE694, but not by S3226. These observations indicate that in normal colon NHE3 mediates both HCO3 (-)-dependent and butyrate-dependent Na(+) absorption, whereas DSS-induced inflammation activates NHE2, which mediates butyrate-dependent (but not HCO3 (-)-dependent) Na(+) absorption. In in vivo loop studies HCO3 (-)-Ringer and butyrate-Ringer exhibit similar rates of water absorption in normal rats, whereas in DSS-induced inflammation luminal butyrate-Ringer reversed water secretion observed with HCO3 (-)-Ringer to fluid absorption. Lumen butyrate-Ringer incubation activated NHE3-mediated Na(+) absorption in DSS-induced colitis. These observations suggest that the butyrate activation of NHE2 would be a potential target to control UC-associated diarrhea.

A whole-genome RNAi screen uncovers a novel role for human potassium channels in cell killing by the parasite Entamoeba histolytica

The parasite Entamoeba histolytica kills human cells resulting in ulceration, inflammation and invasion of the colonic epithelium. We used the cytotoxic properties of ameba to select a genome-wide RNAi library to reveal novel host factors that control susceptibility to amebic killing. We identified 281 candidate susceptibility genes and bioinformatics analyses revealed that ion transporters were significantly enriched among susceptibility genes. Potassium (K⁺) channels were the most common transporter identified. Their importance was further supported by colon biopsy of humans with amebiasis that demonstrated suppressed K⁺ channel expression. Inhibition of human K⁺ channels by genetic silencing, pharmacologic inhibitors and with excess K⁺ protected diverse cell types from E. histolytica-induced death. Contact with E. histolytica parasites triggered K⁺ channel activation and K⁺ efflux by intestinal epithelial cells, which preceded cell killing. Specific inhibition of Ca²⁺-dependent K⁺ channels was highly effective in preventing amebic cytotoxicity in intestinal epithelial cells and macrophages. Blockade of K⁺ efflux also inhibited caspase-1 activation, IL-1β secretion and pyroptotic death in THP-1 macrophages. We concluded that K⁺ channels are host mediators of amebic cytotoxicity in multiple cells types and of inflammasome activation in macrophages.

Anoctamin 1 in secretory epithelia

Fluid and electrolyte releasing from secretory epithelia are elaborately regulated by orchestrated activity of ion channels. The activity of chloride channel at the apical membrane decides on the direction and the rate of secretory fluid and electrolyte. Chloride-dependent secretion is conventionally associated with intracellular increases in two second messengers, cAMP and Ca(2+), responding to luminal purinergic and basolateral adrenergic or cholinergic stimulation. While it is broadly regarded that cAMP-dependent Cl(-) secretion is regulated by cystic fibrosis transmembrane conductance regulator (CFTR), Ca(2+)-activated Cl(-) channel (CaCC) had been veiled for quite some time. Now, Anoctamin 1 (ANO1 or TMEM16A) confers Ca(2+)-activated Cl(-) currents. Ano 1 and its paralogs have been actively investigated for multiple functions underlying Ca(2+)-activated Cl(-) efflux and fluid secretion in a variety of secretory epithelial cells. In this review, we will discuss recent advances in the secretory function and signaling of ANO1 in the secretory epithelia, such as airways, intestines, and salivary glands.