Basolateral K+ efflux is largely independent of maxi-K+ channels in rat submandibular glands during secretion

Ca²⁺-dependent K⁺ channels in exocrine salivary glands

In the last 15 years, remarkable progress has been realized in identifying the genes that encode the ion-transporting proteins involved in exocrine gland function, including salivary glands. Among these proteins, Ca(2+)-dependent K(+) channels take part in key functions including membrane potential regulation, fluid movement and K(+) secretion in exocrine glands. Two K(+) channels have been identified in exocrine salivary glands: (1) a Ca(2+)-activated K(+) channel of intermediate single channel conductance encoded by the KCNN4 gene, and (2) a voltage- and Ca(2+)-dependent K(+) channel of large single channel conductance encoded by the KCNMA1 gene. This review focuses on the physiological roles of Ca(2+)-dependent K(+) channels in exocrine salivary glands. We also discuss interesting recent findings on the regulation of Ca(2+)-dependent K(+) channels by protein-protein interactions that may significantly impact exocrine gland physiology.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

Regulation of membrane potential and fluid secretion by Ca2+-activated K+ channels in mouse submandibular glands

We have recently shown that the IK1 and maxi-K channels in parotid salivary gland acinar cells are encoded by the K(Ca)3.1 and K(Ca)1.1 genes, respectively, and in vivo stimulated parotid secretion is severely reduced in double-null mice. The current study tested whether submandibular acinar cell function also relies on these channels. We found that the K(+) currents in submandibular acinar cells have the biophysical and pharmacological footprints of IK1 and maxi-K channels and their molecular identities were confirmed by the loss of these currents in K(Ca)3.1- and K(Ca)1.1-null mice. Unexpectedly, the pilocarpine-stimulated in vivo fluid secretion from submandibular glands was essentially normal in double-null mice. This result and the possibility of side-effects of pilocarpine on the nervous system, led us to develop an ex vivo fluid secretion assay. Fluid secretion from the ex vivo assay was substantially (about 75%) reduced in animals with both K(+) channel genes ablated - strongly suggesting systemic complications with the in vivo assay. Additional experiments focusing on the membrane potential in isolated submandibular acinar cells revealed mechanistic details underlying fluid secretion in K(+) channel-deficient mice. The membrane potential of submandibular acinar cells from wild-type mice remained strongly hyperpolarized (-55 +/- 2 mV) relative to the Cl(-) equilibrium potential (-24 mV) during muscarinic stimulation. Similar hyperpolarizations were observed in K(Ca)3.1- and K(Ca)1.1-null mice (-51 +/- 3 and -48 +/- 3 mV, respectively), consistent with the normal fluid secretion produced ex vivo. In contrast, acinar cells from double K(Ca)3.1/K(Ca)1.1-null mice were only slightly hyperpolarized (-35 +/- 2 mV) also consistent with the ex vivo (but not in vivo) results. Finally, we found that the modest hyperpolarization of cells from the double-null mice was maintained by the electrogenic Na(+),K(+)-ATPase.

Molecular identification and physiological roles of parotid acinar cell maxi-K channels

The physiological success of fluid-secreting tissues relies on a regulated interplay between Ca(2+)-activated Cl(-) and K(+) channels. Parotid acinar cells express two types of Ca(2+)-activated K(+) channels: intermediate conductance IK1 channels and maxi-K channels. The IK1 channel is encoded by the K(Ca)3.1 gene, and the K(Ca)1.1 gene is a likely candidate for the maxi-K channel. To confirm the genetic identity of the maxi-K channel and to probe its specific roles, we studied parotid glands in mice with the K(Ca)1.1 gene ablated. Parotid acinar cells from these animals lacked maxi-K channels, confirming their genetic identity. The stimulated parotid gland fluid secretion rate was normal, but the sodium and potassium content of the secreted fluid was altered. In addition, we found that the regulatory volume decrease in acinar cells was substantially impaired in K(Ca)1.1-null animals. We examined fluid secretion from animals with both K(+) channel genes deleted. The secretion rate was severely reduced, and the ion content of the secreted fluid was significantly changed. We measured the membrane potentials of acinar cells from wild-type mice and from animals with either or both K(+) channel genes ablated. They revealed that the observed functional effects on fluid secretion reflected alterations in cell membrane voltage. Our findings show that the maxi-K channels are critical for the regulatory volume decrease in these cells and that they play an important role in the sodium uptake and potassium secretion process in the ducts of these fluid-secreting salivary glands.

Physiological Roles of the Intermediate Conductance, Ca2+-activated Potassium Channel Kcnn4

Three broad classes of Ca(2+)-activated potassium channels are defined by their respective single channel conductances, i.e. the small, intermediate, and large conductance channels, often termed the SK, IK, and BK channels, respectively. SK channels are likely encoded by three genes, Kcnn1-3, whereas IK and most BK channels are most likely products of the Kcnn4 and Slo (Kcnma1) genes, respectively. IK channels are prominently expressed in cells of the hematopoietic system and in organs involved in salt and fluid transport, including the colon, lung, and salivary glands. IK channels likely underlie the K(+) permeability in red blood cells that is associated with water loss, which is a contributing factor in the pathophysiology of sickle cell disease. IK channels are also involved in the activation of T lymphocytes. The fluid-secreting acinar cells of the parotid gland express both IK and BK channels, raising questions about their particular respective roles. To test the physiological roles of channels encoded by the Kcnn4 gene, we constructed a mouse deficient in its expression. Kcnn4 null mice were of normal appearance and fertility, their parotid acinar cells expressed no IK channels, and their red blood cells lost K(+) permeability. The volume regulation of T lymphocytes and erythrocytes was severely impaired in Kcnn4 null mice but was normal in parotid acinar cells. Despite the loss of IK channels, activated fluid secretion from parotid glands was normal. These results confirm that IK channels in red blood cells, T lymphocytes, and parotid acinar cells are indeed encoded by the Kcnn4 gene. The role of these channels in water movement and the subsequent volume changes in red blood cells and T lymphocytes is also confirmed. Surprisingly, Kcnn4 channels appear to play no required role in fluid secretion and regulatory volume decrease in the parotid gland.

Delayed shrinkage triggered by the Na+-K+pump in terbutaline-stimulated rat alveolar type II cells

Terbutaline (10 microm) induced a triphasic volume change in alveolar type II (AT-II) cells: an initial shrinkage (initial phase) followed by cell swelling (second phase) and a gradual shrinkage (third phase). The present study demonstrated that the initial and the third phases are evoked by the activation of K+ and Cl- channels and the second phase is evoked by the activation of Na+ and Cl- channels. Ouabain blocked the third phase, although it did not block the initial and second phases. This suggests that the third phase is triggered by the Na+-K+ pump. Tetraethylammonium (TEA, a K+ channel blocker) decreased the volume of AT-II cells and enhanced the terbutaline-stimulated third phase, although quinidine, another K+ channel blocker, increased the volume of AT-II cells. The TEA-induced cell shrinkage was inhibited by ouabain, suggesting that TEA increases Na+-K+ pump activity. Ba2+, 2,3-diaminopyridine and a high [K+]o (30 mm) similarly decreased the volume of AT-II cells. These findings suggest that depolarization induced by TEA increases Na+-K+ pump activity, which increases [K+]i. This [K+]i increase, in turn, hyperpolarizes membrane potential. Valinomycin (a K+ ionophore), which induces hyperpolarization, decreased the volume of AT-II cells and enhanced the third phase in these cells. In conclusion, in terbutaline-stimulated AT-II cells, an increase in Na+-K+ pump activity hyperpolarizes the membrane potential and triggers the third phase by switching net ion transport from NaCl entry to KCl release.

Pharmacological investigation of the role of ion channels in salivary secretion

The role of K+ and Cl- channels in salivary secretion was investigated, with emphasis on the potential role of Ca2+ -activated K+ channels. Ligand saturation kinetic assays and autoradiography showed large-conductance (BK) K+ channels to be highly expressed in rat submandibular and parotid glands, whereas low-conductance (SK) K+ channels could not be detected. To investigate the role of K+ and Cl- channels in secretion, intact rabbit submandibular glands were vascularly perfused and secretion induced by 10 microM ACh. Secretion was inhibited by 34+/-3% following perfusion with the general K+ channel inhibitor Ba2+ (5 mM), whereas organic inhibitors of BK (200 nM paxilline) or intermediate-conductance (IK) K+ channels (5 microM clotrimazole) had no effect. Secretion was strongly influenced by Cl- channel inhibitors, as 100 microM 5-nitro-2-(3-phenylpropylamino)benzoate (NPPB) completely abolished, while 10 microM NPPB, 20 microM NS1652 and 20 microM NS3623 reduced secretion by 34+/-3%, 23+/-3% and 59+/-4%, respectively. In conclusion, although high expression levels of BK channels were demonstrated, pharmacological tools failed to demonstrate any role for BK, IK or SK channels in salivary secretion in the rabbit submandibular gland. Other types of K+ channel, however, and particularly Cl- channels, are essential for ACh-induced salivary secretion.

Molecular identification of Ca2+-activated K+ channels in parotid acinar cells

We used molecular biological and patch-clamp techniques to identify the Ca(2+)-activated K(+) channel genes in mouse parotid acinar cells. Two types of K(+) channels were activated by intracellular Ca(2+) with single-channel conductance values of 22 and 140 pS (in 135 mM external K(+)), consistent with the intermediate and maxi-K classes of Ca(2+)-activated K(+) channels, typified by the mIK1 (Kcnn4) and mSlo (Kcnma1) genes, respectively. The presence of mIK1 mRNA was established in acinar cells by in situ hybridization. The electrophysiological and pharmacological properties of heterologously expressed mIK1 channels matched those of the native current; thus the native, smaller conductance channel is likely derived from the mIK1 gene. We found that parotid acinar cells express a single, uncommon splice variant of the mSlo gene and that heterologously expressed channels of this Slo variant had a single-channel conductance indistinguishable from that of the native, large-conductance channel. However, the sensitivity of this expressed Slo variant to the scorpion toxin iberiotoxin was considerably different from that of the native current. RT-PCR analysis revealed the presence of two mSlo beta-subunits (Kcnmb1 and Kcnmb4) in parotid tissue. Comparison of the iberiotoxin sensitivity of the native current with that of parotid mSlo expressed with each beta-subunit in isolation and measurements of the iberiotoxin sensitivity of currents in cells from beta(1) knockout mice suggest that parotid acinar cells contain approximately equal numbers of homotetrameric channel proteins from the parotid variant of the Slo gene and heteromeric proteins composed of the parotid Slo variant in combination with the beta(4)-subunit.

SK4/IK1-like channels mediate TEA-insensitive, Ca2+-activated K+ currents in bovine parotid acinar cells

Although Ca(2+)-activated K(+) (K(Ca)) channels distinct from maxi-K(+) channels have been suggested to contribute to muscarinically stimulated K(+) currents in salivary acinar cells, the molecular nature of the channels is unclear. Using electrophysiological and RT-PCR techniques, we have now investigated the involvement of SK4/IK1-like channels in native K(Ca) currents in bovine parotid acinar (BPA) cells. Ca(2+)-dependent K(+) efflux from perfused bovine parotid tissues was not inhibited by a maxi-K(+) channel blocker, tetraethylammonium (TEA). Whole cell recordings from BPA cells showed a TEA-insensitive K(Ca) conductance, which was highly permeable to Rb(+). In inside-out macropatches, TEA-insensitive Rb(+) currents were activated by Ca(2+) with half-maximal values of 0.4 microM. 1-Ethyl-2-benzimidazolinone (1-EBIO) increased the Ca(2+) sensitivity of the currents. The calmodulin antagonists trifluoperazine, calmidazolium, and W-7 inhibited the Ca(2+)-activated Rb(+) currents. In outside-out macropatches, Ca(2+)-activated Rb(+) currents were inhibited by Ba(2+), quinine, clotrimazole, and charybdotoxin but not by d-tubocrarine or apamin. RT-PCR analysis showed transcripts of SK4/IK1 in BPA cells. These results collectively suggest that SK4/IK1-like channels mediate the native K(Ca) currents in BPA cells.

Identification and regulation of K+ and Cl- channels in human parotid acinar cells

The properties of K+ channels in these cells were studied using patch-clamp methods. Two channels, with conductances of 165+/-13 pS (n=6) and 30+/-1 pS (n=3), were identified in single-channel experiments. In cell-attached patches the reversal potentials were -67+/-8 and -74+/-2 mV for the large and small conductance channel, respectively, suggesting that both channels are K+-selective. The large conductance channel was also shown to be K+-selective in inside-out patches. The open probability (P(o)) of this channel was increased at depolarizing potentials and by increasing intracellular Ca2+ concentration ([Ca2+]i). These properties suggest that the large conductance channel is a 'maxi' Ca2+-activated K+ channel (BK(Ca)). The small conductance channel was not observed in inside-out patches. Carbachol (CCh; 10(-5) M) activated the BK(Ca) channel, but not the small conductance channel, in cell-attached patches. CCh also caused a dose-dependent increase in [Ca2+]i measured by fura-2 in microspectrofluorimetric studies, with a half-maximal response at approximately 3x10(-6) M. Neither isoproterenol (10(-5) M) nor substance P (10(-6) M) affected K+-channel activity or [Ca2+]i. In whole-cell experiments, CCh caused an increase in outward current. Charybdotoxin (10(-7) M), a BK(Ca) blocker, inhibited a large component of the CCh-induced current. A large component of the charybdotoxin-insensitive current may be carried by Ca2+-activated Cl- channels, which were also observed in human parotid acinar cells. The results indicate that BK(Ca) channels make a significant contribution to the whole-cell conductance in human parotid acinar cells.

Visualization of the secretory process involved in Ca2+-activated fluid secretion from rat submandibular glands using the fluorescent dye, calcein

The central feature of fluid and electrolyte secretion by salivary acinar cells is transepithelial Cl- movement as a driving force for the secretion. However, little is known about the membrane localization and regulation by agonists of various anion channels. To characterize the anion transport and fluid secretion, we visualized the secretory process induced by the cholinergic agonist, carbachol (CCh), using the anionic fluorescent dye, calcein, under a confocal laser scanning microscope. The fluorescence of calcein loaded into the isolated acini was spread diffusely throughout the cytoplasm and was less intense in the secretory vesicles which occupied the apical pole. Cytoplasmic calcein was released into intercellular canaliculi just after the addition of CCh, depending upon a rise in [Ca2+]i by Ca2+ release from intracellular stores. Thereafter, the formation of watery vacuoles connected with intercellular canaliculi was visualized in the calcein-loaded acini, depending upon external Ca2+. Both the calcein release and vacuole formation were inhibited by suppressing the Ca(2+)-activated K+ efflux. The calcein release was also affected by the external anion substitution, suggesting that calcein is released through an anion channel. In the isolated, perfused glands, CCh-induced fluid secretion was sustained in two phases, whereas the loaded calcein was initially and transiently released into the saliva. By revealing the [Ca2+]i dependence and sensitivities to channel blockers, our results suggest that the initial phase of CCh-induced fluid secretion was evoked in association with the release of the organic anion, calcein, and the late phase of fluid secretion, during which calcein is less permeable, was associated with the formation of watery vacuoles. Thus, the anion channels possessing the distinct property of anion permeation may be activated in the initial phase and late phase. These results indicate that the anionic fluorescent dye, calcein, is useful for visualizing the process of Ca(2+)-dependent fluid secretion, and for clarifying the relation between fluid secretion and anion transport.

Glomerular mesangial cells: electrophysiology and regulation of contraction

Mesangial cells are smooth muscle-like pericytes that abut and surround the filtration capillaries within the glomerulus. Studies of the fine ultrastructure of the glomerulus show that the mesangial cell and the capillary basement membrane form a biomechanical unit capable of regulating filtration surface area as well as intraglomerular blood volume. Structural and functional studies suggest that mesangial cells regulate filtration rate in both a static and dynamic fashion. Mesangial excitability enables a homeostatic intraglomerular stretch reflex that integrates an increase in filtration pressure with a reduction in capillary surface area. In addition, mesangial tone is regulated by diverse vasoactive hormones. Agonists, such as angiotensin II, contract mesangial cells through a signal transduction pathway that releases intracellular stores of Ca2+, which subsequently activate nonselective cation channels and Cl- channels to depolarize the plasma membrane. The change in membrane potential activates voltage-gated Ca2+ channels, allowing Ca2+ cell entry and further activation of depolarizing conductances. Contraction and entry of cell Ca2+ are inhibited only when Ca2+-activated K+ channels (BK(Ca)) are activated and the membrane is hyperpolarized toward the K+ equilibrium potential. The mesangial BK(Ca) is a weak regulator of contraction in unstimulated cells; however, the gain of the feedback is increased by atrial natriuretic peptide, nitric oxide, and the second messenger cGMP, which activates protein kinase G and decreases both the voltage and Ca2+ activation thresholds of BK(Ca) independent of sensitivity. This enables BK(Ca) to more effectively counter membrane depolarization and voltage-gated Ca2+ influx. After hyperpolarizing the membrane, BK(Ca) rapidly inactivates because of dephosphorylation by protein phosphatase 2A. Regulation of ion channels has been linked casually to hyperfiltration during early stages of diabetes mellitus. Determining the signaling pathways controlling the electrophysiology of glomerular mesangial cells is important for understanding how glomerular filtration rate is regulated in health and disease.

Tetraethylammonium-insensitive, Ca(2+)-activated whole-cell K+ currents in rat submandibular acinar cells

Using whole-cell patch-clamp techniques, we demonstrate, for the first time, that rat submandibular acinar cells contain a tetraethylammonium (TEA)-insensitive, Ca(2+)-activated K+ conductance which is not attributable to large conductance, voltage-sensitive, Ca(2+)-dependent K+ channels (maxi-K+ channels). Taken together with our recent K+ efflux and fluid secretion studies in intact rat submandibular gland, we postulate that the K+ conductance reported here may be involved in the basolateral K+ efflux pathway activated by cytosolic Ca2+ concentration during secretion by this gland.