Molecular basis of potassium channels in pancreatic duct epithelial cells.

Potassium channels regulate excitability, epithelial ion transport, proliferation, and apoptosis. In pancreatic ducts, K(+) channels hyperpolarize the membrane potential and provide the driving force for anion secretion. This review focuses on the molecular candidates of functional K(+) channels in pancreatic duct cells, including KCNN4 (KCa 3.1), KCNMA1 (KCa 1.1), KCNQ1 (Kv 7.1), KCNH2 (Kv 11.1), KCNH5 (Kv 10.2), KCNT1 (KCa 4.1), KCNT2 (KCa 4.2), and KCNK5 (K 2P 5.1). We will give an overview of K(+) channels with respect to their electrophysiological and pharmacological characteristics and regulation, which we know from other cell types, preferably in epithelia, and, where known, their identification and functions in pancreatic ducts and in adenocarcinoma cells. We conclude by pointing out some outstanding questions and future directions in pancreatic K(+) channel research with respect to the physiology of secretion and pancreatic pathologies, including pancreatitis, cystic fibrosis, and cancer, in which the dysregulation or altered expression of K(+) channels may be of importance.

Introduction

Potassium channels (K+ channels) are very important membrane proteins present in every cell. They determine the cell membrane potential and thereby regulate the excitability of neurons and myocytes and transport of ions and water in epithelia, such as the pancreas and salivary glands. Duct epithelial cells in the pancreas secrete a HCO3–-rich pancreatic juice that neutralizes acid chyme in the duodenum. Secretin, acetylcholine, and ATP stimulate fluid secretion via signal transduction involving cAMP and Ca2+ signaling pathways. The generally accepted model for HCO3– transport involves Cl––HCO3– exchangers (SLC26A3 and SLC26A6) that operate in parallel with cAMP-activated Cl– channels (CFTR) or Ca2+-activated Cl– channels (most likely TMEM16A) on the luminal membrane and Na+-coupled transporters such Na+–K+–Cl– co-transporter (NKCC1), Na+–HCO3– co-transporter (SLC4A4), and Na+–H+ exchanger (SLC9A1) and Na+–K+-pump on the basolateral membrane (Fig. 1).- In addition, H+–K+-pumps are expressed on the luminal and basolateral membranes of pancreatic ducts. K+ channels are clearly important for setting the resting membrane potential and providing the driving force for anion exit and fluid secretion in a stimulated epithelium.-, K+ channels may also provide the transport partners for H+–K+-pumps. In addition, certain K+ channels could play an important role in pancreatic pathology, such as cystic fibrosis, pancreatitis, and pancreatic adenocarcinoma. Perhaps surprisingly, there are not so many K+ channels studies performed on pancreatic ducts.

Figure 1. Model of ion transport in a pancreatic duct cell. Intracellular HCO3– is derived from CO2 through the action of carbonic anhydrase (CA) and from HCO3– uptake via the Na+–HCO3– cotransporter. H+ is extruded at the basolateral membrane by the Na+–H+ exchanger and H+–K+ pump. HCO3– efflux across the luminal membrane is mediated by Cl––HCO3– exchangers and/or Cl– channels, and the H+–K+ pump may provide a buffering/protection zone for the alkali-secreting epithelium. K+ channels provide an exit pathway for K+ and play a vital role in maintaining the membrane potential, which is a crucial component of the driving force for anion secretion.

Early electrophysiological studies using microelectrodes and patch-clamp methods indicated that pancreatic ducts expressed voltage- and Ca2+-activated K+-channels, consistent with maxi-K+ channels (BK channels), intermediate-conductance Ca2+-activated K+ channels (IK channels), and pH/HCO3– sensitive K+ channels.- Recent studies focusing on molecular candidates have shown that pancreatic ducts express the following channels that could be candidates for above functional channels: KCa1.1 channels coded by the KCNMA1 and KCNMB1 genes (α- and β-subunits of the BK channel); the KCa3.1 protein coded by the KCNN4 gene (IK channel); the KCNK5 gene (K2P5.1); and they also express: KCNQ1 (Kv7.1, KVLQT1), KCNH2 (Kv11.1, HERG), KCNH5 (Kv10.2, EAG2), KCNT1 (KCa4.1, Slack), and KCNT2 (KCa4.2, Slick), the functions of which remain unclear in duct cells.,,

It is not known whether many of these candidates are functional in pancreatic ducts or what is their localization and regulation. Therefore, their physiological and possibly pathophysiological functions have not to be confirmed. The aim of this review is to provide an overview of the above mentioned K+ channels with respect to their electrophysiological and pharmacological characteristics and functions, as we know from other cell types, preferably in epithelia, and, where known, their identification and functions in pancreatic ducts is given (Table 1). We also address some outstanding questions and future directions in pancreatic K+ channel research.

Table 1. Molecular candidates of functional K+ channels in pancreatic duct cells

Gene	Protein	Conductance (p)S)	Blockers (Ki)	Activators (Kd)	Regulation
KCNN4	KCa3.1	30–5415,28,29	charybdotoxin(2–28 nM)15,28,29,31clotrimazole(25–150 nM)15,28-31TRAM-34 (20 nM)30maurotoxin (1 nM)32	1-EBIO(15–84 μM)28,29,31,33DC-EBIO(0.8 μM)33	Ca2+ 14,15,29,31calmodulin18,29,34PKA19,42,43extracellular UTP11cell swelling47,48
KCMA1	KCa1.1	100–27050,56	tetraethylammonium(0.14 mM)50charybdotoxin(1–31 nM)57,58,62iberiotoxin(1–9 nM)58,61,62paxilline (2–9 nM)58-60	NS1608 (2 μM)60NS11021 (0.4 μM)63	membrane potential7,50,56,64Ca2+ 7,50,56,64PKA7,56extracellular UTP11
KCNQ1	Kv7.1	0.7–480,81	chromanol 293B(10–41 μM)68,82,86,87azimilide (77 μM)86XE991 (0.8 μM)88	L-364,37390	membrane potential78-81cAMP91cytosolic pH83
KCNQ1/KCNE1	Kv7.1/minK	4.5–1680,81	chromanol 293B(3–10 μM)70,82,86,87azimilide (5.6 μM)86XE991 (11 μM)88Mefloquine (0.9 μM)89	DIDS86mefenamic acid86	membrane potential78-81cAMP91,92cytosolic pH83
KCNH2	Kv11.1	10–13102-104	E-4031(7–1250 nM)104,105,108-111BeKm-1(3–12 nM)106-110ergtoxin(4.5–17 nM)107,109LY97241(2.2–19 nM)111,112	mallotoxin(0.5 μM)114PD-118057(3.1 μM)115ICA-105574(0.5 μM)108	membrane potential116PKA119,120
KCNH5	Kv10.2		LY97241(1.5 μM)113		membrane potential97PKC97
KCNT1	KCa4.1	180122	bepridil (1 μM)125quinidine (90 μM)125	bithionol(0.8 μM)125niclosamide(2.9 μM)126loxapine (4.4 μM)126niflumic acid(2.7 mM)127	membrane potential121,122Ca2+ 121Na+ 122,123Cl− 123PKC130
KCNT2	KCa4.2	140122	quinidine122isoflurane128	meclofenamic acid(80 μM)127flufenamic acid(1.1–1.4 mM)127,129niflumic acid(2.1 mM)127,129	membrane potential122Na+ 122,130Cl− 122intracellular ATP122PKC130
KCNK5	K2P5.1	50–78133,136,137	quinine (22 μM)133clofilium (25 μM)138bupivacaine (26 μM)139ropivacaine (95 μM)139	halothane, isoflurane, chloroform140	extracellular pH133,138,140,141PKC140osmolality138

KCNN4 (KCa3.1, IK, SK4)

Tissue expression

KCNN4 coding for the KCa3.1 protein was cloned from the placenta and pancreas., Functional expression of the KCNN4 gene has been demonstrated in colonic crypts, salivary acini,- and pancreatic ducts., Immunoreactivity of the KCa3.1 protein has also been reported in the esophagus, stomach, small intestine, proximal colonic crypts, salivary glands, luminal membrane of lacrimal gland duct cells,- and intercalated and intralobular ducts of the pancreas., Interestingly, KCa3.1 channel immunoreactivity was shown to be localized in both the basolateral and luminal membranes in pancreatic ducts and monolayer of Capan-1, a human pancreas adenocarcinoma cell line, though its expression appeared to be stronger in the luminal membrane. Consistent with this finding, the short-circuit current (Isc) of the Capan-1 cell monolayer was enhanced by the KCa3.1 channel activator DC-EBIO in luminal or basolateral bathing solution., KCa3.1 could potentially be an important candidate for luminal K+ channels in pancreatic ducts. Importantly, equivalent-circuit analysis revealed that luminal K+ conductance contributed to a minimum of 10% of the total K+ conductance in pancreatic duct cells. Moreover, stimulation of the rat pancreas with secretin caused a marked increase in K+ concentrations in the pancreatic juice, which was equal to twice that in the plasma, indicating that K+ was secreted. K+ efflux was also shown to be mediated via mucosal KCa3.1 channels in other epithelia, such as the distal colon, and provided, in part, the driving force for agonist-induced anion secretion. Another example is salivary acini, in which both KCa1.1 and KCa3.1 were shown to be expressed on the apical membrane and contribute to optimal secretion. Furthermore, H+–K+-pumps were reported to be expressed on the luminal membranes of pancreatic ducts and their function, such as contributing to local epithelial protection, appeared to depend on the operation of K+ channels.

Channel properties

Patch-clamp studies using Xenopus oocytes and mammalian expression systems established the basic electrophysiological and pharmacological properties of KCa3.1 channels.,, Single-channel openings were observed at both positive and negative membrane potentials, and this gating showed no significant voltage dependency. The single-channel current–voltage relationship showed weak inward rectification with conductance of 30–54 pS in heterologous expression systems. Interestingly, intermediate-conductance K+ channels exhibited a conductance of 80 pS in rat pancreatic duct cells. One explanation for this discrepancy is that unidentified auxiliary proteins for KCa3.1 channels or additional KCNN4 genes may exist in rodent cells. Regarding pharmacology, KCa3.1 currents were inhibited by charybdotoxin, clotrimazole, TRAM-34, and maurotoxin with Ki values of 2–28 nM, 25–150 nM, 20 nM, and 1 nM, respectively.,- KCa3.1 currents were also activated by 1-EBIO and DC-EBIO with Kd values of 15–84 μM and 0.8 μM, respectively.,,,

Regulation

Regarding regulation, it is well established that KCa3.1 channels are activated by the Ca2+/calmodulin signaling pathway. For example, heterologously expressed KCa3.1 channels were previously shown to be activated by submicromolar free Ca2+ concentrations with EC50 values of 0.1–0.3 μM.,,, There is also strong evidence to suggest that the Ca2+ sensitivity of KCa3.1 channels is mediated by calmodulin and calmodulin kinase.,, In addition, ATP/UTP was shown to regulate KCa3.1 channels via purinergic receptors in pancreatic cell lines and rat pancreatic duct cells.,,, Both P2Y2 and P2Y4 receptors upregulated KCa3.1 activity in the Xenopus oocyte expression system. Importantly, luminal ATP/UTP, most likely delivered by secreting acini,, was reported to stimulate ductal secretion.,,-

The physiological role of KCa3.1 channels in pancreatic secretion could be also investigated with respect to secretin, which acts predominantly via the cAMP/cAMP-dependent protein kinase (PKA) signaling pathway, however, until this becomes available, we need to resort to studies on other cell types. A membrane-associated PKA has been proposed to activate KCa3.1 channels in human erythrocytes, the T84 human colonic crypt cell line, and rat submandibular acinar cells.,, Interestingly, the PKA consensus phosphorylation site at serine 334 in KCa3.1 channels was not involved in PKA-dependent activation. In contrast to these studies, heterologously expressed KCa3.1 channels were not affected by PKA activators and/or inhibitors,, or were inhibited by the catalytic subunit of PKA. Given these contradictory results, it is tempting to speculate that KCa3.1 channels may be activated via the phosphorylation of a closely associated protein, the expression of which is tissue-specific. One candidate for this protein is A-kinase anchoring protein (AKAP), which is able to scaffold PKA and components of cAMP signaling pathways, including G protein-coupled receptors and ion channels.

In addition to transepithelial transport, KCa3.1 channels were also shown to be stimulated by cell swelling, which triggered regulatory volume decreases., Notably, KCNN4 mRNA levels were upregulated in primary pancreatic tumors, and the growth of ductal adenocarcinoma cell lines in vitro was inhibited by blockers of KCa3.1 channels, which indicated that these were correlated with the proliferation of pancreatic cancer.

KCNMA1 (KCa1.1, Slo1, α-subunit of BK) and KCNMB (β-subunits)

The KCNMA1 coding KCa1.1 (Slo1) protein was cloned from brain and skeletal muscle. Functional expression of the KCNMA1 gene has been demonstrated in the colon, salivary acini, pancreatic acini, and pancreatic ducts., The KCa1.1 protein is located in the luminal membrane of colonic epithelia,, salivary acini and ducts,, and pancreatic ducts. It is noteworthy that there was no labeling of the basolateral membrane of guinea-pig pancreatic duct cells, although the first recordings of maxi-K+ currents were made on the basolateral membrane of rat pancreatic ducts. Venglovecz et al. proposed that luminal KCa1.1 channels, which are activated by bile acids in the lumen, regulate HCO3– secretion in pancreatic ducts. Nevertheless, experiments on KCa1.1 regulation have also indicated that some channels may be confined to the basolateral membrane (see below). Luminal KCa1.1 channels in the distal colon were shown to be responsible for resting and stimulated Ca2+-activated K+ secretion.

KCa1.1 channels have the largest single-channel conductance of all K+ selective channels: 100–270 pS in symmetrical 150 mM KCl., Maxi-K+ currents in isolated rat pancreatic duct cells had a conductance of 170–180 pS., Regarding pharmacology, the α-subunit of KCa1.1 was inhibited by tetraethylammonium, charybdotoxin, iberiotoxin, and paxilline with Ki values of 0.14 mM, 1–31 nM, 1–9 nM, and 2–9 nM, respectively.,- The α-subunit of KCa1.1 was also activated by NS1608 and NS11021 with Kd values of 2 μM and 0.4 μM, respectively., Interestingly, dehydrosoyasaponin I (DHS-I) activated the α-subunit of KCa1.1 only if co-expressed with the β1-subunit, an auxiliary protein for KCa1.1 channels.

Significant diversity has been reported in the functional characteristics of KCa1.1 channels. It is well established that KCa1.1 channels are activated by membrane depolarization alone, intracellular Ca2+ alone, or synergistically by depolarization and Ca2+.,,, The single-channel open probability of KCa1.1 channels markedly increased when the cytoplasmic face of a patch membrane was exposed to 10 μM Ca2+ and voltage was changed over a range of −60 to +80 mV. Under these conditions, the half-maximal voltage (V1/2) was +23 mV in 10 μM Ca2+; however, these were unphysiological conditions for pancreatic ducts. Importantly, maxi-K+ channels on pancreatic duct cells were activated by much lower Ca2+ concentrations. For example, maxi-K+ channels exposed to 3 μM Ca2+ reached V1/2 at −4 mV. This difference indicated that the β-subunit exists in pancreatic duct cells. Maxi-K+ channels on Xenopus oocytes that heterologously expressed both the α- and β1-subunits of KCa1.1 proteins were about 10-fold more sensitive to activation by voltage and Ca2+ concentration than channels composed of the α-subunit alone. Indeed, KCNMB1 coding the β1 subunit was detected in isolated pancreatic ducts.

Interestingly, UTP was shown to inhibit KCa1.1 channels via the P2Y2 receptor, and appeared to lead to a decrease in secretion. The basolateral application of ATP/UTP inhibited K+ conductance in rat duct cells and secretion in guinea-pig ducts and human duct cell monolayers.,, These results collectively indicated that P2Y2 receptors on the basolateral membrane appeared to downregulate secretion via KCa1.1 channels in the ductal system.

Regarding the cAMP/PKA signaling pathway, cAMP-dependent phosphorylation can also activate maxi-K+ channels on pancreatic duct cells. The functional response of KCa1.1 channels to PKA phosphorylation depends on the splice-variant of the α-subunit. For example, PKA was shown to activate the ZERO splice variant, whereas PKA inhibited the STREX variant. PKA activation of the ZERO variant requires a conserved C-terminal PKA site. Indeed, the ZERO splice variant has been shown to conduct adrenaline-induced K+ secretion in the distal colon.

KCNQ1 (Kv7.1, KVLQT1) and KCNE1 (minK)

The KCNQ1 coding Kv7.1 protein was cloned from the heart. Functional expression of the KCNQ1 gene has also been demonstrated in the kidney, stomach, small intestine, colon,- pancreatic acini,,, and pancreatic ducts. Immunoreactivity of the Kv7.1 protein was reported in the parietal cells of the stomach, in the basolateral membrane of small intestinal and colonic crypt cells,,, and in acinar and duct cells of the pancreas., Kv7.1 resides in the tubulovesicular and canalicular membranes of gastric parietal cells together with H+–K+-pumps and participates in gastric acid secretion.,, Kv7.1 was localized in the luminal membrane of pancreatic duct cells, and may be involved in cell volume regulation during purinergic stimulation in epithelial transport,, and/or may potentially be associated with H+–K+-pumps expressed by pancreatic ducts.

The Kv7.1 protein can assemble with the KCNE family of regulatory β-subunits to fulfill various physiological functions. For example, minK coded by the KCNE1 gene has been shown to modify Kv7.1 activity by increasing unitary conductance, slowing activation, causing a right shift in the voltage dependence of activation, and modulating pharmacology.- It is worth noting that the acidification of cytosolic pH increased Kv7.1–minK, but decreased Kv7.1 currents, whereas alkalinization decreased Kv7.1–minK, but increased Kv7.1 currents. Indeed, the whole pancreas expresses KCNE1 and KCNE2 genes., The Kv7.1 current was shown to be strongly diminished and membrane targeting of the Kv7.1 protein was impaired in acinar cells in KCNE1 knockout mice. The expression and function of KCNE in duct cells has not yet been investigated.

Kv7.1 channels have very small conductance. Noise analysis revealed estimated single-channel conductances of 0.7–4 pS., Small conductance K+ channels had 1 pS and were inhibited by chromanol 293B, a Kv7.1 blocker, in the basolateral membrane of rat pancreatic acinar cells. Chromanol 293B inhibited α-subunit of Kv7.1 with Ki values of 10–41 μM in Xenopus oocytes and mammalian expression systems.,,, Importantly, KCNE β-subunits increase the sensitivity of Kv7.1 to chromanol 293B. Ki values for Kv7.1/KCNE1, Kv7.1/KCNE2 and Kv7.1/KCNE3 were 3–10 μM, 0.4 μM, and 3–4 μM, respectively.,,,, Voltage-gated K+ currents in pancreatic acinar cells were shown to be inhibited by chromanol 293B with a Ki value of 3 μM. This result supports voltage-gated K+ channels being composed of Kv7.1 and KCNE1 β-subunit in acinar cells. Azimilide inhibited Kv7.1 and Kv7.1/KCNE1 in the same manner as chromanol 293B with Ki values of 77 μM and 5.6 μM, respectively. In contrast, XE991 inhibited Kv7.1 and Kv7.1/KCNE1 with Ki values of 0.8 μM and 11 μM, respectively. Mefloquine inhibited Kv7.1/KCNE1 with a Ki value of 0.9 μM. DIDS and mefenamic acid activated Kv7.1/KCNE1, but not Kv7.1. On the other hand, L-364,373 activated Kv7.1, but did not affect Kv7.1/KCNE1.

Regarding regulation, voltage-gated Kv7.1 channels are known to be regulated by the cAMP signaling pathway. In addition, AKAPs are required for cAMP regulation of recombinant Kv7.1 channels in mammalian cell lines. Interestingly, a K+ current was elicited by cAMP stimulation in CFTR-transfected, but not untransfected CFPAC-1 cells derived from a cystic fibrosis patient with deletion in Phe-508 in CFTR. AKAPs also mediate PKA compartmentalization with CFTR; therefore, these findings imply that functional CFTR regulates the Kv7.1 channel, presumably in the luminal membrane of pancreatic duct cells.

KCNH2 (Kv11.1, HERG) and KCNH5 (Kv10.2, EAG2)

The KCNH2 coding Kv11.1 (HERG) protein was isolated from the hippocampal cDNA library. Functional expression of the KCNH2 gene has been demonstrated in colon carcinoma cells. Immunoreactivity of the Kv11.1 protein was also reported in colon carcinoma cells and the luminal membrane of pancreatic duct cells. The KCNH5 coding Kv10.2 (EAG2) protein was identified in the thalamus and was expressed in the brain, testes, skeletal muscle, heart, placenta, lung, liver, and at low levels in the kidney and whole pancreas., Notably, Kv10.2 was shown to promote medulloblastoma tumor progression by regulating cell volume dynamics. KCNH2 and KCNH5 are clearly expressed in rodent and human pancreatic duct cells. However, the physiological or potentially pathophysiological role of Kv11.1 and Kv10.2 channels remains unclear. The related Kv10.1 (KCNH1) channel has been shown to be upregulated in several cancers including pancreatic cancer, based on studies of human pancreatic adenocarcinoma cell lines.,

Kv11.1 channels have small conductance of 10–13 pS.- Regarding pharmacology, Kv11.1 was inhibited by E-4031, BeKm-1, and ergtoxin with Ki values of 7–1250 nM, 3–12 nM, and 4.5–17 nM, respectively.- Kv11.1 channels formed with KCNE2 were about 2-fold more sensitive to E-4031. LY97241 was shown to inhibit Kv10.2 and Kv11.1 currents with Ki values of 1.5 μM and 2.2–19 nM, respectively.- Kv11.1 currents were also activated by mallotoxin, PD-118057, and ICA-105574 with Kd values of 0.5 μM, 3.1 μM, and 0.5 μM, respectively.,,

Kv11.1 currents were activated at voltages more positive than −50 mV and V1/2 was −15.1 mV, whereas Kv10.2 currents were activated at around −100 mV and V1/2 was −35.5 mV. However, a 14–3-3 protein was associated with Kv11.1 in a phosphorylation-dependent manner at specific PKA sites and shifted V1/2 in a hyperpolarizing direction by −11.1 mV. Kv11.1 may exist in a macromolecular signaling complex that includes 14–3-3 proteins and possibly AKAPs. Importantly, the Kv11.1 protein can also assemble with KCNE1 or KCNE2 regulatory β-subunits., Regarding inhibition, phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), produced a potent dose-dependent block of Kv10.2 or Kv11.1 currents., In addition, Kv11.1 currents were reduced by the cAMP-specific phosphodiesterase inhibitor Ro-20–1724 or the adenylate cyclase activator forskolin, which were shown to result in increased cAMP levels and PKA stimulation.

KCNT1 (KCa4.1, Slo2.2, Slack) and KCNT2 (KCa4.2, Slo2.1, Slick)

KCNT1 (KCa4.1, Slo2.2, or Slack), which encodes for the Na+-activated K+ channel, was isolated from the brain cDNA library.KCNT1 and KCNT2 (KCa4.2, Slo2.1, or Slick) are expressed in the heart, kidney and testis, as well as in the brain.- The functional expression of KCa4.1 has been demonstrated in the basolateral membrane of the thick ascending limbs of Henle’s loop. Pancreatic duct cells also expressed KCNT1 and KCNT2. Interestingly, the expression pattern of KCNT1 and KCNT2 was different between Capan-1 cells expressing functional CFTR channels and CFPAC-1 cells derived from a cystic fibrosis patient with a mutation in CFTR. Capan-1 cells express KCNT1, but not KCNT2, while CFPAC-1 cells express KCNT2, but not KCNT1. This discrepancy indicates that the expression of KCNT1 and KCNT2 channels is in some way associated with the expression of functional CFTR. However, the function of these K+ channels in pancreatic duct cells remains to be investigated.

KCa4.1 and KCa4.2 channels have large conductances of 180 pS and 140 pS in symmetrical 130 mM KCl. In the basolateral membrane of the thick ascending limbs of Henle’s loop, Na+-activated K+ channels had a conductance of 140–180 pS. Regarding pharmacology, KCa4.1 was inhibited by bepridil and quinidine with Ki values of 1 μM and 90 μM, respectively. KCa4.1 was activated by bithionol, niclosamide, loxapine, and niflumic acid with Kd values of 0.8 μM, 2.9 μM, 4.4 μM, and 2.7 mM, respectively.- KCa4.2 was inhibited by 1 mM quinidine and isoflurane,, and was activated by meclofenamic acid, flufenamic acid, and niflumic acid with Kd values of 80 μM, 1.1–1.4 mM, and 2.1 mM, respectively.,

KCa4.1 was shown to be unusually inhibited by intracellular Ca2+ at 1 μM. However, KCa4.1 may co-assemble with KCa1.1 subunits to generate Ca2+-activated K+ channels. KCa4.1 and KCa4.2 channels were reported to be activated by intracellular Na+ and Kd values of 41 mM and 89 mM in the presence of 30 mM internal Cl−, respectively. These channels were also activated by intracellular Cl− or synergistically by Na+ and Cl−., Intracellular ATP inhibited KCa4.2 directly, via the presence of a consensus ATP binding motif. A similar ATP binding motif has not been demonstrated in the KCa4.1 sequences., Interestingly, the PKC activator PMA increased KCa4.1 currents, but inhibited KCa4.2 currents.

KCNK5 (K2P5.1, TASK-2)

Two-pore domain K+ channels (K2P) generate background K+ currents over the whole membrane potential range. The pH-sensitive K2P subunits (TALK-1, TALK-2 and TASK-2) were shown to be expressed in pancreatic acini. An electrophysiological study indicated that TASK-2 was expressed in HPAF, a human pancreatic ductal adenocarcinoma cell line. KCNK5 coding TASK-2 (K2P5.1) was isolated from the brain cDNA library. KCNK5 is expressed in the kidney, liver, stomach, small intestine, colon, and pancreatic acinus.- The functional expression of K2P5.1 has been demonstrated in kidney proximal convoluted tubule cells, which could be involved in volume regulation and HCO3– transport. Clofilium-sensitive K+ conductance, possibly K2P5.1, was located in the luminal membrane of the monolayer of HPAF. pH-sensitive K+ channels on the luminal membrane of pancreatic duct cells may be physiologically relevant in terms of maintaining the electrical driving force for electrogenic HCO3– secretion and providing an exit pathway for K+ secretion.

K2P5.1 channels have an intermediate conductance of 50–78 pS.,, Regarding pharmacology, K2P5.1 was inhibited by quinine, clofilium, bupivacaine, and ropivacaine with Ki values of 22 μM, 25 μM, 26 μM, and 95 μM, respectively.,, K2P5.1 was activated by halothane, isoflurane, and chloroform, which are volatile anesthetics.

K2P5.1 is very sensitive to extracellular pH in the physiological range, with a pKa value of 7.5–8.3.,,, Phorbol 12,13-dibutyrate and PMA, activators of PKC, were shown to potentiate K2P5.1 currents in Xenopus oocytes. Extracellular ATP activated TASK-like channels (K2P3.1 and/or K2P5.1), possibly via the P2Y11 receptor in thoracic aorta myocytes. P2Y11 receptors were reported to be expressed on the basolateral membrane of canine pancreatic duct epithelia, which increased cAMP and Isc. The K2P5.1 channel is also osmosensitive and participates in cell volume regulation. Therefore, pH-sensitive K+ channels may be important on both the luminal (alkaline) and basolateral (acid) membranes of pancreatic ducts.

Potassium Channels in Pancreatic Cancer

Ion channels have been associated with the malignant phenotype of cancer cells, as well as contributing to virtually all basic cellular processes, including crucial roles in maintaining tissue homeostasis such as proliferation, differentiation, and apoptosis. Several potassium channels have been suggested as the hallmarks of cancer, including pancreatic duct adenocarcinoma. For example, KCa3.1 channels have been correlated with the proliferation of pancreatic cancer. In addition, the expression of G protein-activated inward rectifier potassium channel 1 (Kir3.1) was markedly higher in pancreatic adenocarcinomas than in a normal pancreas, whereas Kv1.3 expression was decreased in pancreatic adenocarcinomas. Downregulation in the expression of Kv1.3 has been associated with metastatic tumors. Kv1.5 was also shown to be highly expressed in pancreatic adenocarcinomas. Furthermore, a specific monoclonal antibody that inhibits the function of Kv10.1 (EAG1) reduced tumor growth of BxPC3, a human pancreas adenocarcinoma cell line, which implicates this channel in cancer progression. Altered pH homeostasis is known to be one of the key hallmarks of cancer., Thus, pH-sensitive K2P channels may also play a role in pancreatic adenocarcinoma. The human duct adenocarcinoma cell line, HPAF cells, were reported to express K2P5.1 channels. However, its contribution to cancer progression is still unknown. Although further studies on K+ channels in pancreatic cancer must be performed, some candidates, such as Kv10.1, already have the potential to be diagnostic tools and therapeutic targets.

Concluding remarks

This review described the current status on the molecular basis for a number of K+ channels found in pancreatic ducts. Electrophysiological studies on ducts and duct cells using microelectrode, patch-clamp, and Ussing chamber methods showed how some of these K+ channels contribute to physiological processes in ductal secretion by providing the driving forces for anion transport and as partial accompanying partners in secretion. Future studies are needed to verify the localization of K+ channels to a polarized ductal epithelium and affirm their physiological function in secretion or associated cell processes such as cell volume regulation, as well as their participation in cell proliferation and apoptosis. The pancreas and especially the ductal epithelium are involved in a number of diseases including cystic fibrosis and pancreatitis. Some target therapies should include K+ channel openers to maintain or upregulate pancreatic secretion. Our knowledge regarding the role of K+ channels in duct cell homeostasis remains relatively sparse. Because some K+ channels are being regarded as the hallmark of cancer progression and emerging studies on pancreatic adenocarcinoma foreshadow similar trends, more knowledge is required in this area before specific K+ channel openers or inhibitors can be used in the treatment of pancreatic diseases.