Cl− channels reside both in the plasma membrane and in intracellular organelles Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of e

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Molecular Structure and Physiological Function of Chloride Channels

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a fe...

Electrolyte Transport in the Mammalian Colon: Mechanisms and Implications for Disease

Calcium-activated chloride channels (CaCCs) play important roles in cellular physiology, including epithelial secretion of electrolytes and water, sensory transduction, regulation of neuronal and cardiac excitability, and regulation of vascular tone. This review discusses the physiological roles of these channels, their mechanisms of regulation and activation, and the mechanisms of anion selectivity and conduction. Despite the fact that CaCCs are so broadly expressed in cells and play such important functions, understanding these channels has been limited by the absence of specific blockers and the fact that the molecular identities of CaCCs remains in question. Recent status of the pharmacology and molecular identification of CaCCs is evaluated.

Calcium-activated chloride channels

Stimulus-secretion coupling: cytoplasmic calcium signals and the control of ion channels in exocrine acinar cells.

▪ Abstract The secretion of fluid and electrolytes by salivary gland acinar cells requires the coordinated regulation of multiple water and ion transporter and channel proteins. Notably, all the key transporter and channel proteins in this process appear to be activated, or are up-regulated, by an increase in the intracellular Ca2+ concentration ([Ca2+]i). Consequently, salivation occurs in response to agonists that generate an increase in [Ca2+]i. The mechanisms that act to modulate these increases in [Ca2+]i obviously influence the secretion of salivary fluid. Such modulation may involve effects on mechanisms of both Ca2+ release and Ca2+ entry and the resulting spatial and temporal aspects of the [Ca2+]i signal, as well as interactions with other signaling pathways in the cells. The molecular cloning of many of the transporter and regulatory molecules involved in fluid and electrolyte secretion has yielded a better understanding of this process at the cellular level. The subsequent characterization of ...

Regulation of fluid and electrolyte secretion in salivary gland acinar cells.

Our previously published whole-cell patch-clamp studies on the cells of the intralobular (granular) ducts of the mandibular glands of male mice revealed the presence of an amiloride-sensitive Na+ conductance in the plasma membrane. In this study we demonstrate the presence also of a Cl- conductance and we show that the sizes of both conductances vary with the Cl- concentration of the fluid bathing the cytosolic surface of the plasma membrane. As the cytosolic Cl- concentration rises from 5 to 150 mmol/liter, the size of the inward Na+ current declines, the decline being half-maximal when the Cl- concentration is approximately 50 mmol/liter. In contrast, as cytosolic Cl- concentration increases, the inward Cl- current remains at a constant low level until the Cl- concentration exceeds 80 mmol/liter, when it begins to increase. Studies in which Cl- in the pipette solution was replaced by other anions indicate that the Na+ current is suppressed by intracellular Br-, Cl- and NO3- but not by intracellular I-, glutamate or gluconate. Our studies also show that the Cl- conductance allows passage of Cl- and Br- equally well, I- less well, and NO3-, glutamate and gluconate poorly, if at all. The findings with NO3- are of particular interest because they show that suppression of the Na+ current by a high intracellular concentration of a particular anion does not depend on actual passage of that anion through the Cl- conductance. In mouse granular duct cells there is, thus, a reciprocal regulation of Na+ and Cl- conductances by the cytosolic Cl- concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Na+ and Cl− conductances are controlled by cytosolic Cl− concentration in the intralobular duct cells of mouse mandibular glands

We have studied a 25-pS nonselective cation channel from the apical membranes of cell line ST885, derived from neonatal mouse mandibular glands. Its Cl− permeability was not significantly different from zero. The permeabilities (relative to Na+) for inorganic cations were NH
 4
 +
 (1.87)>K+(1.12)>Li+ (1.02)>Na+(1)>Rb+(0.81)>Mg2+(0.07)>Ca2+(0.002), and for organic cations, guanidinium (1.61) 4-aminopyridine (0.66)>diethylamine (0.54)>piperazine (0.25)>Tris (0.18)>N-methylglucamine (0.12). The Tris and N-methylglucamine permeabilities differed significantly from zero. Fitting the Renkin equation indicated that the channel had an equivalent pore radius of 0.49 nm. The channel was activated by Ca2+ on the cytosolic surface (>0.1 mmol/liter) with a Hill coefficient of 1.2; it was also activated by depolarization. Open- and closed-time histograms indicated that it had at least two open and two closed states. The channel was blocked by cytosolic AMP or ATP (0.1 mmol/liter). It was also blocked by the Cl− channel blocker, diphenylamine-2-carboxylate (DPC; 0.1 mmol/liter), applied to the extracellular but not the cytosolic surface. 4-Aminopyridine, which permeated the channel when applied to the extracellular surface, blocked it when applied in low concentrations (5 mmol/liter) to the cytosolic surface. Quinine (0.1 mmol/liter) blocked from both the extracellular and cytosolic surfaces, blockade from either side being enhanced by depolarization. The channel was held open by application of SITS (0.1 mmol/liter) to the cytosolic surface. The channel shows striking similarities to the nicotinic acetylcholine receptor channel,viz., both channel types are abnormally permeable to 4-aminopyridine applied externally, and their selectivity sequences for inorganic ions are similar and for organic cations are identical.

Characterization of a 25-pS nonselective cation channel in a cultured secretory epithelial cell line.

Models of epithelial salt secretion, involving secondary active transport of Cl- [9], locate the K+ conductance of the plasma membrane exclusively in the basolateral membrane, although there is considerable experimental evidence to show that many secretory epithelia do have a significant apical K+ conductance. We have used an equivalent circuit model to examine the effect of an apical K+ conductance on the composition and flow rate of the fluid secreted by an epithelium in which secretion is driven by the secondary active transport of Cl-. The parameters of the model were chosen to be similar to those measured in the dog tracheal mucosa when stimulated with adrenaline to secrete. We find that placing a K+ conductance in the apical membrane can actually enhance secretion provided the proportion of the total cell K+ conductance in the apical membrane is not greater than about 60%, the enabling effect on secretion being maximal when the proportion is around 10-20%. We also find that even when the entire cell K+ conductance is located in the apical membrane, the secreted fluid remains relatively Na+ rich. Analysis of the sensitivity of model behavior to the choice of values for the parameters shows that the effects of an apical K+ conductance are enhanced by increasing the ratio of the paracellular resistance to the transcellular resistance.

Effect of K+ channels in the apical plasma membrane on epithelial secretion based on secondary active Cl- transport.

Previous studies have shown that the whole-cell current-voltage (I-V) relation of unstimulated sheep parotid cells is dominated by two K+ conductances, one outwardly and the other inwardly rectifying. We now show that once these K+ conductances are blocked by replacement of pipette K+ with Na+ and by the addition of 5 mmol/liter CsCl to the bath, there remains an outwardly rectifying conductance with a reversal potential of 0 mV. Replacement of 120 mmol/liter NaCl in the pipette solution with an equimolar amount of Na-glutamate shifted the reversal potential of this residual current to -55 mV, indicating that the conductance was Cl- selective. The Cl- current was activated by increasing the free Ca2+ in the pipette solution from 10 to 100 nmol/liter. When the Ca2+ concentration in the pipette solution was 10 nmol/liter, the relaxations observed in response to membrane depolarization could be fitted with a single exponential, whose time constant increased from 81 to 183 ms as the pipette potential was increased from -30 to +60 mV. Relaxation analysis showed that the current was activated by membrane depolarization. Reversal potential measurements in experiments in which external Cl- was replaced with various anions, gave the following relative permeabilities: SCN- (1.80) > I- (1.09) > Cl- (1) > NO3- (0.92) > Br- (0.75). The relative conductances were: SCN- (2.18) > I- (1.07) > Cl- (1.00) > Br- (0.91) > NO3- (0.50). The Cl- current was blocked by NPPB (ID50 approximately 10 microM), DIDS (10 or 30 mumol/liter) and furosemide (100 mumol/liter).

A Ca(2+)-activated Cl- current in sheep parotid secretory cells.

Using whole-cell patch-clamp techniques, we demonstrate that sheep parotid secretory cells have both inwardly and outwardly rectifying currents. The outwardly rectifying current, which is blocked by 10 mmol/liter tetraethylammonium (TEA) applied extracellularly, is probably carried by the 250 pS Ca2+-and voltage-activated K+ (BK) channel which has been described in previous studies. In contrast, the inwardly rectifying current, which is also carried by K+ ions, is not sensitive to TEA. It is similar to the inwardly rectifying currents observed in many excitable tissues in that (i) its conductance is dependent on the square root of the extracellular K+, (ii) the voltage range over which it is activated is influenced by the extracellular K+ concentration and (iii) it is blocked by the addition of Cs+ ions (670 µmol/liter) to the bathing solution. Our previously published cell-attached patch studies have shown that the channel type most commonly observed in the basolateral membrane of unstimulated sheep parotid secretory cells is a K+ channel with a conductance of 30 pS and, in this study, we find that its conductance also depends on the square root of the extracellular K+ concentration. It thus seems likely that it carries the inwardly rectifying K+ current seen in the whole-cell studies.

D. I. Cook

Papers

Na+ and Cl− conductances are controlled by cytosolic Cl− concentration in the intralobular duct cells of mouse mandibular glands

Characterization of a 25-pS nonselective cation channel in a cultured secretory epithelial cell line.

Effect of K+ channels in the apical plasma membrane on epithelial secretion based on secondary active Cl- transport.

A Ca(2+)-activated Cl- current in sheep parotid secretory cells.

An inwardly rectifying potassium channel in the basolateral membrane of sheep parotid secretory cells.