Inwardly rectifying, voltage-dependent and resting potassium currents in rat pancreatic acinar cells in primary culture

Calcium imaging in intact mouse acinar cells in acute pancreas tissue slices

The physiology and pathophysiology of the exocrine pancreas are in close connection to changes in intra-cellular Ca²⁺ concentration. Most of our knowledge is based on in vitro experiments on acinar cells or acini enzymatically isolated from their surroundings, which can alter their structure, physiology, and limit our understanding. Due to these limitations, the acute pancreas tissue slice technique was introduced almost two decades ago as a complementary approach to assess the morphology and physiology of both the endocrine and exocrine pancreas in a more conserved in situ setting. In this study, we extend previous work to functional multicellular calcium imaging on acinar cells in tissue slices. The viability and morphological characteristics of acinar cells within the tissue slice were assessed using the LIVE/DEAD assay, transmission electron microscopy, and immunofluorescence imaging. The main aim of our study was to characterize the responses of acinar cells to stimulation with acetylcholine and compare them with responses to cerulein in pancreatic tissue slices, with special emphasis on inter-cellular and inter-acinar heterogeneity and coupling. To this end, calcium imaging was performed employing confocal microscopy during stimulation with a wide range of acetylcholine concentrations and selected concentrations of cerulein. We show that various calcium oscillation parameters depend monotonically on the stimulus concentration and that the activity is rather well synchronized within acini, but not between acini. The acute pancreas tissue slice represents a viable and reliable experimental approach for the evaluation of both intra- and inter-cellular signaling characteristics of acinar cell calcium dynamics. It can be utilized to assess many cells simultaneously with a high spatiotemporal resolution, thus providing an efficient and high-yield platform for future studies of normal acinar cell biology, pathophysiology, and screening pharmacological substances.

CO2-dependent opening of an inwardly rectifying K+ channel

CO₂ chemosensing is a vital function for the maintenance of life that helps to control acid–base balance. Most studies have reported that CO₂ is measured via its proxy, pH. Here we report an inwardly rectifying channel, in outside-out excised patches from HeLa cells that was sensitive to modest changes in PCO₂ under conditions of constant extracellular pH. As PCO₂ increased, the open probability of the channel increased. The single-channel currents had a conductance of 6.7 pS and a reversal potential of –70 mV, which lay between the K⁺ and Cl^– equilibrium potentials. This reversal potential was shifted by +61 mV following a tenfold increase in extracellular [K⁺] but was insensitive to variations of extracellular [Cl^–]. The single-channel conductance increased with extracellular [K⁺]. We propose that this channel is a member of the Kir family. In addition to this K⁺ channel, we found that many of the excised patches also contained a conductance carried via a Cl^–-selective channel. This CO₂-sensitive Kir channel may hyperpolarize excitable cells and provides a potential mechanism for CO₂-dependent inhibition during hypercapnia.

Delayed expression of large conductance K+ channels reshaping agonist-induced currents in mouse pancreatic acinar cells

Epithelial secretory cells display cell-specific mechanisms of fluid secretion and express large conductance voltage- and Ca2+-activated K+ (Maxi-K) channels that generate the membrane negativity for effective Cl- exit to the lumen. Rat and mouse pancreatic acinar cells had been thought to be peculiar in this sense because of the previously reported lack of Maxi-K channels. However, this view is not entirely correct as evidenced in the present paper. Searching for their presence in pancreatic acinar cells in mice from 5 to 84 weeks of age with patch-clamp current measurements, we demonstrated that the expression of Maxi-K channels is regulated in an age-associated manner after birth. The expression started at approximately 12 postnatal weeks and increased steadily up to 84 weeks. In support of this, RT-PCR could not detect mSlo mRNA, the Maxi-K gene, at either 7 or 8 weeks but could at 58 and 64 postnatal weeks. These results suggest that a key steering element for fluid secretion, the Maxi-K channel, is progressively re-organized in rodent pancreas. A pancreatic secretagogue, acetylcholine, evoked Maxi-K channel current overlapping to various degrees on the previously known current response. This suggests that the rise in internal Ca2+ activates Maxi-K channels which reshape the mode of secretagogue-evoked current response and contribute to Cl- driving in fluid secretion in an age-associated fashion.

Pancreatic two P domain K+ channels TALK-1 and TALK-2 are activated by nitric oxide and reactive oxygen species

This study firstly shows with in situ hybridization on human pancreas that TALK-1 and TALK-2, two members of the 2P domain potassium channel (K(2P)) family, are highly and specifically expressed in the exocrine pancreas and absent in Langherans islets. On the contrary, expression of TASK-2 in mouse pancreas is found both in the exocrine pancreas and in the Langherans islets. This study also shows that TALK-1 and TALK-2 channels, expressed in Xenopus oocytes, are strongly and specifically activated by nitric oxide (obtained with a mixture of sodium nitroprussate (SNP) and dithiothreitol (DTT)), superoxide anion (obtained with xanthine and xanthine oxidase) and singlet oxygen (obtained upon photoactivation of rose bengal, and with chloramine T). Other nitric oxide and reactive oxygen species (NOS and ROS) donors, as well as reducing conditions were found to be ineffective on TALK-1, TALK-2 and TASK-2 (sin-1, angeli's salt, SNP alone, tBHP, H(2)O(2), and DTT). These results suggest that, in the exocrine pancreas, specific members of the NOS and ROS families could act as endogenous modulators of TALK channels with a role in normal secretion as well as in disease states such as acute pancreatitis and apoptosis.

Effects of KCNQ1 channel blocker, 293B, on the acetylcholine-induced Cl- secretion of rat pancreatic acini

In rat pancreatic acini (RPAs), acetylcholine (ACh) typically induces a tonic depolarization of membrane potential (Vm) via increasing cytoplasmic Ca2+ concentration and subsequent activation of Cl- channels. In this study, to investigate the role of K+ channels during the ACh-induced Cl- secretion, the intracellular Cl- concentration ([Cl-]i) of RPAs was monitored using SPQ, a fluorescent dye quenchable by Cl-, and the effects of K+ channel blockers were examined. Also, the secretion of fluid and enzyme from the whole pancreas of rat was measured. The fluorescence of RPAs loaded with SPQ (FSPQ) was slightly increased by the application of ACh (ACh-Delta FSPQ), indicating net secretion of Cl-. However, the relative change of FSPQ normalized to the control fluorescence (F/F0) of RPAs was only about 20% of the effect observed in rat submandibular gland acinus. The ACh-Delta FSPQ of RPAs was not influenced by the pretreatment with 293B (20 micromol/L), a blocker of KCNQ-type K+ channels. Even the cocktail of K+ channel blockers (10 mmol/L TEA, 3 mmol/L Ba2+, 20 micromol/L 293B) exerted only minute inhibitory effects on ACh-Delta FSPQ in RPAs. In the vascularly perfused rat pancreas, the fluid and enzyme secretion induced by ACh was directly measured. 293B and HMR-1556, both specific blockers of KCNQ1 channel, did not block but even enhanced the secretion of fluid and amylase. These results suggest that the role of KCNQ1 channels may not be essential in the Ca2+-mediated Cl- secretion in rat pancreatic acini.

Morphological and functional changes of dissociated single pancreatic acinar cells: testing the suitability of the single cell as a model for exocytosis and calcium signaling

Isolated single pancreatic acinar cells have long been used as a model for studying many kinds of signaling processes due to their structural and functional polarities, but without significant validation. In this study, we examined the morphological and functional changes of dissociated single pancreatic acinar cells. Acutely isolated single cells showed a collapsed membrane potential and a much reduced secretion of zymogen granules in response to acetylcholine (ACh) stimulation, whereas clustered cells showed a much more negative membrane potential and potent exocytotic secretion. The isolated single cells became vertically flattened due to the loss of supporting adhesions with nearby cells, and the granule-attached luminal membrane was severely reduced versus that of clustered cells. However, polarized Ca(2+) signals and mitochondrial localizations were relatively well preserved in the isolated single cells, in that Ca(2+) release by ACh commenced at the indented luminal membrane. In clusters, the Ca(2+) release site was closest to the lumen where more than three cells met or at the tips of conical regions of the luminal membrane. These findings suggest that the dissociated single pancreatic acinar cells preserve an intact Ca(2+) signaling machinery but alter in shape and have impaired exocytotic functions and resting membrane potentials.

General anesthetic octanol and related compounds activate wild-type and delF508 cystic fibrosis chloride channels

1. Cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel is defective during cystic fibrosis (CF). Activators of the CFTR Cl(-) channel may be useful for therapy of CF. Here, we demonstrate that a range of general anesthetics like normal-alkanols (n-alkanols) and related compounds can stimulate the Cl(-) channel activity of wild-type CFTR and delF508-CFTR mutant. 2. The effects of n-alkanols like octanol on CFTR activity were measured by iodide ((125)I) efflux and patch-clamp techniques on three distinct cellular models: (1). CFTR-expressing Chinese hamster ovary cells, (2). human airway Calu-3 epithelial cells and (3). human airway JME/CF15 epithelial cells which express the delF508-CFTR mutant. 3. Our data show for the first time that n-alkanols activate both wild-type CFTR and delF508-CFTR mutant. Octanol stimulated (125)I efflux in a dose-dependent manner in CFTR-expressing cells (wild-type and delF508) but not in cell lines lacking CFTR. (125)I efflux and Cl(-) currents induced by octanol were blocked by glibenclamide but insensitive to 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, as expected for a CFTR Cl(-) current. 4. CFTR activation by octanol was neither due to cell-to-cell uncoupling properties of octanol nor to an intracellular cAMP increase. CFTR activation by octanol requires phosphorylation by protein kinase-A (PKA) since it was prevented by H-89, a PKA inhibitor. 5. n-Alkanols chain length was an important determinant for channel activation, with rank order of potencies: 1-heptanol<1-octanol<2-octanol<1-decanol. Our findings may be of valuable interest for developing novel therapeutic strategies for CF.

IP(3)and cyclic ADP-ribose induced Ca(2+) release from intracellular stores of pancreatic acinar cells from rat in primary culture

We have measured Ca(2+)concentration changes in intracellular Ca(2+)stores ([Ca(2+)](store)) of rat pancreatic acinar cells in primary culture in response to the Ca(2+)mobilizing substances inositol-1,4,5-trisphosphate (IP(3)) and cyclic ADP-ribose (cADPr) using the Ca(2+)-sensitive dye mag Fura-2. We found that in this cell model IP(3)releases Ca(2+)in a quantal manner. Higher Ca(2+)concentration in the stores allowed a response to lower IP(3)concentrations ([IP(3)]) indicating that the sensitivity of IP(3)receptors to IP(3)is regulated by the Ca(2+)concentration in the stores. Cyclic ADPr, that modifies 'Ca(2+)-induced-Ca(2+)-release' (CICR), was also able to release Ca(2+)from intracellular stores of pancreatic acinar cells in primary culture. In comparison to the Ca(2+)ionophore ionomycin, which induced a maximal decrease (100%) in [Ca(2+)](store), a hypermaximal [IP(3)] (10 microM) dropped [Ca(2+)](store)by 87% and cADPr had no further effect. Cyclic ADPr reduced [Ca(2+)](store)by only 56% and subsequent IP(3)addition caused further maximal decrease in [Ca(2+)](store). Furthermore, a maximal [IP(3)] caused the same decrease in [Ca(2+)](store)in all regions of the cell, whereas cADPr dropped the [Ca(2+)](store)between 20 and 80% in different cell regions. From these data we conclude that in primary cultured rat pancreatic acinar cells at least three types of Ca(2+)stores exist. One type possessing both cADPr receptors and IP(3)receptors, a second type possessing only IP(3)receptors, and a third type whose Ca(2+)can be released by ionomycin but neither by IP(3)nor by cADPr.

Molecular and functional properties of two-pore-domain potassium channels

The two-pore-domain K(+) channels, or K(2P) channels, constitute a novel class of K(+) channel subunits. They have four transmembrane segments and are active as dimers. The tissue distribution of these channels is widespread, and they are found in both excitable and nonexcitable cells. K(2P) channels produce currents with unusual characteristics. They are quasi-instantaneous and noninactivating, and they are active at all membrane potentials and insensitive to the classic K(+) channel blockers. These properties designate them as background K(+) channels. They are expected to play a major role in setting the resting membrane potential in many cell types. Another salient feature of K(2P) channels is the diversity of their regulatory mechanisms. The weak inward rectifiers TWIK-1 and TWIK-2 are stimulated by activators of protein kinase C and decreased by internal acidification, the baseline TWIK-related acid-sensitive K(+) (TASK)-1 and TASK-2 channels are sensitive to external pH changes in a narrow range near physiological pH, and the TWIK-related (TREK)-1 and TWIK-related arachidonic acid-stimulated K(+) (TRAAK) channels are the first cloned polyunsaturated fatty acids-activated and mechanogated K(+) channels. The recent demonstration that TASK-1 and TREK-1 channels are activated by inhalational general anesthetics, and that TRAAK is activated by the neuroprotective agent riluzole, indicates that this novel class of K(+) channels is an interesting target for new therapeutic developments.

TWIK-2, a new weak inward rectifying member of the tandem pore domain potassium channel family

Potassium channels are found in all mammalian cell types, and they perform many distinct functions in both excitable and non-excitable cells. These functions are subserved by several different families of potassium channels distinguishable by primary sequence features as well as by physiological characteristics. Of these families, the tandem pore domain potassium channels are a new and distinct class, primarily distinguished by the presence of two pore-forming domains within a single polypeptide chain. We have cloned a new member of this family, TWIK-2, from a human brain cDNA library. Primary sequence analysis of TWIK-2 shows that it is most closely related to TWIK-1, especially in the pore-forming domains. Northern blot analysis reveals the expression of TWIK-2 in all human tissues assayed except skeletal muscle. Human TWIK-2 expressed heterologously in Xenopus oocytes is a non-inactivating weak inward rectifier with channel properties similar to TWIK-1. Pharmacologically, TWIK-2 channels are distinct from TWIK-1 channels in their response to quinidine, quinine, and barium. TWIK-2 is inhibited by intracellular, but not extracellular, acidification. This new clone reveals the existence of a subfamily in the tandem pore domain potassium channel family with weak inward rectification properties.

Phosphorylation of components of the ER translocation site

In many eukaryotic cells, protein secretion is regulated by extracellular signalling molecules giving rise to increased intracellular Ca2+ and activation of kinases and phosphatases. To test whether components involved in the first step of secretion, the translocation of proteins across the endoplasmic reticulum (ER) membrane, are regulated by Ca2+-dependent phosphorylation and dephosphorylation, we have investigated the effect of Ca2+ on kinases associated with the rough ER. Using purified rough microsomes from dog pancreas we found that Ca2+-dependent isoforms of protein kinase C (PKC) are associated with the rough ER and phosphorylate essential components of the protein translocation machinery. Phosphorylation of microsomal proteins by PKCs increased protein translocation efficiency in vitro. We also found that proteins of the translocation machinery became phosphorylated in intact cells. This suggests a further level of regulation of protein translocation across the ER membrane.

Characterization of cell volume-sensitive chloride currents in freshly prepared and cultured pancreatic acinar cells from early postnatal rats

1. In freshly prepared and cultured exocrine pancreatic acinar cells from 5- to 7-day-old rats a chloride-selective membrane conductance could be activated by intracellular application of GTPgammaS (40-100 microM), by application of positive pressure (5 cmH2O) to the pipette interior or by challenging the cells with a hyposmolar bath solution. Hyperosmolar bath solutions inhibited the cell volume-sensitive chloride currents. 2. The anion permeability sequence of the cell volume-sensitive chloride conductance was I- > Cl- approximately Br- > F- > methanesulphonate- > glutamate-. I- had a higher permeability but lower conductance than Cl-. The permeability ratio for Pglutamate/PCl was 0.12. 3. The cell volume-sensitive chloride conductance showed outward rectification. Membrane depolarization to high positive voltages (>= +60 mV) caused a time-dependent decay in outward currents. 4. DIDS (4, 4'-diisothiocyanatostilbene-2,2'-disulphonic acid) and SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid) reversibly inhibited the cell volume-sensitive chloride current in a voltage-dependent manner. NPPB (5-nitro-2-(3-phenylpropylamino)-benzoic acid), quinidine, quinine and tamoxifen caused voltage-independent current inhibition. 5. Combined fura-2 and whole-cell current measurements showed that activation of the cell volume-sensitive chloride current does not involve cytosolic Ca2+ signals. Furthermore, there is no evidence that Ca2+-activated chloride currents play a significant role in cultured pancreatic acinar cells from 5- to 7-day-old rats. 6. Polymerase chain reaction followed by DNA sequence analysis indicated the presence of mRNA homologous to the ClC-3 chloride channel in pancreatic tissue from 5-day-old rats.