Mechanism of active chloride secretion by shark rectal gland: role of Na-K-ATPase in chloride transport

Indirect evidence for the presence of non-specific anion channels in rabbit mandibular salivary gland acinar cells

1. Intracellular pH (pHi) was measured using the fluorescent pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein in acini isolated from the rabbit mandibular salivary gland. 2. Stimulation of the acinar cells with acetylcholine (ACh) evoked an intracellular acidosis, the size of which was dependent on the HCO3-concentration in the bathing medium. A half-maximal acidosis was observed at approximately 10 mM-HCO3-. ACh also evoked an acidosis in HCO3(-)-free solutions containing acetate; a half-maximal acidosis was observed at about 10 mM-acetate. 3. Propionate, lactate and butyrate were also able to support the ACh-evoked acidosis to varying extents. In contrast, formate, pyruvate and salicylate did not support the ACh-induced acidosis to any great extent. 4. Acetazolamide greatly reduced the size of the acidosis in HCO3(-)-buffered medium, but had no effect in acetate-buffered medium, suggesting that the inhibitory effect of acetazolamide was due to a specific inhibition of carbonic anhydrase activity. 5. The Cl- channel blockers diphenylamine-2-carboxylic acid (DPC, 1 mM) and 5-nitro-2-(3-phenylpropylamino)-benzoic acid (0.5 mM) abolished the ACh-evoked acidosis in both HCO3(-) -and acetate-buffered media. 6. The data are consistent with the presence in the acinar cell of relatively non-specific anion channels sensitive to DPC and its derivatives. Such channels, activated on stimulation with ACh, would allow HCO3- and other weak acid ions to leave the cell, leading to the observed acidosis. The existence of such channels, located in the apical membrane, could explain why HCO3- or acetate can sustain fluid secretion in the intact perfused rabbit mandibular gland in the absence of Cl-.

Effects of muscarinic, alpha-adrenergic, and substance P agonists and ionomycin on ion transport mechanisms in the rat parotid acinar cell. The dependence of ion transport on intracellular calcium

The relationship between receptor-mediated increases in the intracellular free calcium concentration [( Ca]i) and the stimulation of ion fluxes involved in fluid secretion was examined in the rat parotid acinar cell. Agonist-induced increases in [Ca]i caused the rapid net loss of up to 50-60% of the total content of intracellular chloride (Cli) and potassium (Ki), which is consistent with the activation of calcium-sensitive chloride and potassium channels. These ion movements were accompanied by a 25% reduction in the intracellular volume. The relative magnitudes of the losses of Ki and the net potassium fluxes promoted by carbachol (a muscarinic agonist), phenylephrine (an alpha-adrenergic agonist), and substance P were very similar to their characteristic effects on elevating [Ca]i. Carbachol stimulated the loss of Ki through multiple efflux pathways, including the large-conductance Ca-activated K channel. Carbachol and substance P increased the levels of intracellular sodium (Nai) to more than 2.5 times the normal level by stimulating the net uptake of sodium through multiple pathways; Na-K-2Cl cotransport accounted for greater than 50% of the influx, and approximately 20% was via Na-H exchange, which led to a net alkalinization of the cells. Ionomycin stimulated similar fluxes through these two pathways, but also promoted sodium influx through an additional pathway which was nearly equivalent in magnitude to the combined uptake through the other two pathways. The carbachol-induced increase in Nai and decrease in Ki stimulated the activity of the sodium pump, measured by the ouabain-sensitive rate of oxygen consumption, to nearly maximal levels. In the absence of extracellular calcium or in cells loaded with the calcium chelator BAPTA (bis[o-aminophenoxy]ethane-N,N,N',N'-tetraacetic acid) the magnitudes of agonist- or ionomycin-stimulated ion fluxes were greatly reduced. The parotid cells displayed a marked desensitization to substance P; within 10 min the elevation of [Ca]i and alterations in Ki, Nai, and cell volume spontaneously returned to near baseline levels. In addition to quantitating the activation of various ion flux pathways in the rat parotid acinar cell, these results demonstrate that the activation of ion transport systems responsible for fluid secretion in this tissue is closely linked to the elevation of [Ca]i.

Evidence for reduced Cl- and increased Na+ permeability in cystic fibrosis human primary cell cultures

1. Employing a primary cell culture system and intracellular microelectrodes, we quantitated and compared the Na+ and Cl- pathways in apical membranes of normal and cystic fibrosis (CF) human airway epithelia. 2. Like the transepithelial difference (PD) in situ, the PD of CF epithelia in culture (-27 +/- 4 mV, mean +/- S.E.M.; n = 28) exceeded the PD of normal epithelia (-10 +/- 1 mV; n = 22). The raised PD principally reflected an increase in the rate of active transport (equivalent short circuit, Ieq) for CF epithelia (61 +/- 9 microA cm-2) as compared with normal epithelia (23 +/- 3 microA cm-2). No significant differences in transepithelial resistance were detected. 3. As indicated by ion replacement studies (gluconate for Cl-), the apical membrane of normal cells exhibits an apical membrane Cl- conductance (GCl) that can be activated by isoprenaline. CF cells do not exhibit an apical membrane GCl, nor can a GCl be activated by isoprenaline. 4. CF cells exhibited a larger amiloride-sensitive Ieq and amiloride-sensitive apical membrane conductance (GNa) than normal cells. Further, the amiloride-sensitive Ieq was increased by isoprenaline in CF but not normal airway epithelia. 5. Equivalent circuit analysis yielded evidence for a more positive electromotive force (EMF) across the apical membrane and a more negative EMF across the basolateral membrane of CF cells as compared with normal cells. Baseline resistances of the apical (Ra) and basolateral (Rb) membranes did not differ for normal and CF cells. 6. Estimates of the resistance of the paracellular path to ion flow (Rs) by equivalent circuit analysis or ion substitution detected no differences in Rs between CF and normal cells. 7. We conclude that abnormalities in both cellular Cl- permeability (reduced) and Na+ permeability (increased) are characteristic of the cultured CF respiratory epithelial cell. These data suggest that a defect in the regulation of apical membrane permeabilities is a central feature of this disease.

Ca2+ not cyclic AMP mediates the fluid secretory response to isoproterenol in the rat mandibular salivary gland: whole-cell patch-clamp studies

We have performed whole-cell patch-clamp studies on dispersed secretory cells of the rat mandibular gland to determine how beta-adrenergic stimulation causes fluid secretion. When the pipette contained a high K+ solution, the resting membrane potential averaged -33 mV +/- 1.1 (SEM, n = 34) and the clamped cell showed strong outward rectification. We monitored K+ and Cl- currents for periods of 15 min by recording the currents needed to clamp the cell potential at 0 and -80 mV, respectively. Isoproterenol (1-2 mumol/l) caused increases in the clamp current at 0 mV (the K+ current) and at -80 mV (the Cl- current) in about 80% of cases, although the responses were variable in size and time-course; the responses were indistinguishable from those induced by acetylcholine or the Ca2+ ionophore, A23187. The alpha-adrenergic antagonist, phentolamine (1-2 mumol/l), had no effect on the response, but the beta-adrenergic antagonist, propranolol (10 mumol/l), blocked it completely. The isoproterenol response could not be mimicked by application to either surface of the cell membrane, of cyclic AMP (100 mumol/l), forskolin (1 or 20 mumol/l) or cholera toxin (2.5 micrograms/ml). However, increasing the Ca2+-chelating capacity of the pipette solution by raising its EGTA concentration from the customary 0.5 to 20 mmol/l, blocked the response to isoproterenol, suggesting that beta-adrenergic agonists activate Cl- and K+ channels by raising cytosolic Ca2+. Since neomycin, which blocks phospholipase C, blocked the action of isoproterenol without impairing the cell responsiveness to A23187, it appears that isoproterenol, like muscarinic agonists, increased cytosolic Ca2+ via the phosphatidylinositol cycle.

99mTc-pertechnetate uptake in parotid acinar cells by the Na+/K+/Cl- co-transport system

99mTc-Pertechnetate (99mTcO4-) has widespread clinical use in the diagnosis and evaluation of dysfunctions in many different tissues. However, despite the broad clinical application of this radionuclide, very little is known about the mechanism by which 99mTcO4- enters a cell. We report evidence here that 99mTcO4- shares the Na+/K+/Cl- co-transport system localized to the basolateral membrane of rat parotid acinar cells. 99mTcO4- uptake by these cells was quite rapid (t1/2 approximately 30 s), was completely inhibited by the loop diuretics furosemide and bumetanide, and was markedly dependent on the presence of Na+, K+, and Cl- in the extracellular medium. Relative to uptake measured in the presence of physiological extracellular salt concentrations (Hanks' salts), 99mTcO4- uptake was inhibited 80% by sodium replacement and 50% by potassium replacement. When Cl- was replaced with the physiologically inert anion gluconate a threefold stimulation in 99mTcO4- uptake resulted. These observations provide strong evidence that 99mTcO4- can substitute for Cl- as a substrate for the Na+/K+/Cl- co-transporter and indicate that 99mTcO4- uptake by salivary glands (e.g., as seen with salivary scintiscans), and possibly by a variety of other tissues, reflects the functional activity of this co-transport mechanism.

Intracellular pH in the rat mandibular salivary gland: the role of Na-H and Cl-HCO3 antiports in secretion

Intracellular pH (pHi) in the perfused rat mandibular gland was determined from the distribution of DMO (5,5-dimethyl-2,4-oxazolidinedione). In unstimulated glands, pHi averaged 7.12 +/- 0.02. Stimulation with a "standard" (submaximal) concentration (0.3 mumol/l) of acetylcholine (ACh) caused a fall in pHi to 6.81 +/- 0.06 over 60 min, but a maximal concentration (1.0 mumol/l) caused an initial rise in pHi to 7.60 +/- 0.02, followed by a fall to 7.45 +/- 0.02 over 60 min. After replacement of perfusate Cl with gluconate, the standard ACh concentration caused a rise in pHi to 7.50 +/- 0.02 followed by a fall to 7.27 +/- 0.04 after 60 min, concomitant with a 76% fall in secretory rate and a rise in salivary HCO3 concentration from 14 +/- 0.9 to 67 +/- 1.5 mmol/l. Furosemide (1 mmol/l) had a similar effect to gluconate replacement except that secretory rate fell only by 60%. Bumetanide (1 mmol/l), which inhibited secretion by 67%, did not cause pHi to rise following ACh stimulation but prevented the fall seen with ACh alone. Acetazolamide and methazolamide (1 mmol/l) had no effect on the salivary secretory response to ACh but they caused pHi to rise, respectively, to 7.20 +/- 0.03 and 7.43 +/- 0.02. Bumetanide and methazolamide together caused pHi to rise to 7.58 +/- 0.02 and reduced the secretory response to ACh by 91%. The disulfonic stilbene, SITS, caused pHi to rise to 7.26 +/- 0.03. Ouabain and amiloride both caused resting pHi to fall closer to equilibrium and largely abolished the gland's responsiveness to ACh.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of amiloride-blockable sodium transport in adrenaline-induced lung liquid reabsorption in the fetal lamb

Adrenaline was infused intravenously at rates of 0.1-1.0 microgram/min into chronically catheterized fetal lambs (125-141 days gestation) to induce slowing of secretion or reabsorption of lung liquid. There was an electrical potential difference (p.d.) of -0.3 to -9.5 mV (mean -3.4 mV) between lung liquid and plasma (lung liquid negative) during control lung liquid secretion. In response to adrenaline infusion, the p.d. increased (lung lumen more negative) and this change was greatest (1.8 +/- 0.3 mV) in experiments in which reabsorption occurred. Measurements were made of bidirectional fluxes of Na+ and Cl- across the pulmonary epithelium during control lung liquid secretion and during adrenaline infusion. Adrenaline-induced reabsorption of lung liquid was associated with an increase in Na+ flux from lung lumen to plasma. Similar but smaller changes occurred when the adrenaline response was slowing of secretion. The difference between measured flux ratios and those predicted from the forces determining passive flux provided evidence for active transport of Cl- from plasma to lung lumen, as previously demonstrated by Olver & Strang (1974). When adrenaline was infused, there was evidence of active Na+ transport in the direction lung lumen to plasma and an associated decrease in active Cl- transport in the opposite direction. These changes were greatest when the response to adrenaline was reabsorption. Amiloride, when mixed into the lung liquid to give a calculated concentration of 10(-4) M, abolished the changes in p.d. and ion flux induced by adrenaline. In experiments using amiloride concentrations between 10(-8) and 10(-4) M it was shown that 50% inhibition of the reabsorptive response to adrenaline (KI) was induced by 4 X 10(-6) M-amiloride in the lung lumen. Thus adrenaline-induced slowing of secretion or reabsorption of lung liquid is mediated by active Na+ transport from lung lumen to plasma and depends on amiloride-inhibitable Na+ channels on the luminal surface of the pulmonary epithelium.

Sodium and chloride transport by the tracheal epithelium of fetal, new-born and adult sheep

In vitro measurements were made of Na+ and Cl- isotopic fluxes across the tracheal epithelium of mature fetal lambs (130-143 days gestation), new-born lambs (up to 41 days of age) and adult sheep under conditions of continuous short circuiting. The effects of a variety of drugs were examined, but only in the case of amiloride and isoprenaline were observations made in all three groups. Experiments designed to elucidate the mechanism of basal Cl- secretion were performed in adult trachea only. Under resting conditions the net flux of Na+ from lumen to submucosa exceeds that of Cl- in the reverse direction in fetal and adult trachea. In the new-born the two fluxes are more or less equivalent in magnitude. In none of the three groups is the sum of ion fluxes significantly different from the short-circuit current (Isc). Removal of Na+ from, or addition of furosemide (10(-3) M) to, the solution bathing the submucosal surface of adult trachea has the effect of reducing Isc by an amount which approximates to the Cl- current (29%). At a concentration of 10(-4) M on the submucosal side of adult trachea, ouabain causes potential difference and Isc to fall to zero within 70 min of addition to the bathing solution. Nevertheless, there remains a significant net Na+ flux from submucosa to lumen. The addition of isoprenaline (10(-4) M) to the medium bathing the submucosal surface of both fetal and adult trachea causes an increase in the one-way flux of Cl- from submucosa to lumen with consequent increase in net Cl- flux towards the lumen. (The Na+ fluxes are unchanged.) However, in the adult the Cl- secretory response to isoprenaline is very much less and is not accompanied by an increase in electrical conductance. As judged by the change in Isc, all the post-natal fall in beta-agonist responsiveness takes place within the 3 week period following birth. Whereas, in the fetus, the effect of luminally applied amiloride on the Na+ fluxes is negligible, in the adult the one-way flux of Na+ from lumen to submucosa is reduced by 35% with a consequent 60% fall in net Na+ flux towards the submucosa.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence from O2 uptake measurements for Na+ -K+ -2 Cl- co-transport in the rabbit submandibular gland

There is evidence that the production of primary saliva by acinar cells is a consequence of Na+ -Cl- co-transport but more recently it has been proposed that in fact Na+ -K+ -2 Cl- co-transport is responsible. The latter would be energetically more efficient and the present experiments were designed to measure the stoichiometry of acinar secretion in order to distinguish between these two mechanisms. Submandibular salivary glands from anaesthetised rabbits were isolated vascularly and oxygen consumption measured from the oxygen content of arterial inflow and venous effluent blood and the total flow through the gland. Measurements were made in the steady-state at rest and during different secretion rates induced by parasympathetic nerve stimulation. The rate of sodium transport across the acinar and ductal epithelium was determined from plasma and salivary sodium concentration and salivary flow rate. Multiple regression analysis of this data showed that 22.1 mol Na+ was secreted per mol O2 consumed while 11.9 mol Na+ was reabsorbed per mol O2 consumed. Since acinar secretion is energetically about twice as efficient as ductal absorption, a mechanism for Na+ transport other than that for tight epithelia must be involved. Na+ -K+ -2 Cl- co-transport is thus more likely than Na+ -Cl- and it is suggested that Na+ -K+ -2 Cl- co-transport is the main mechanism involved in salivary acinar secretion.

Oxygen consumption and active sodium and chloride transport in bovine tracheal epithelium

The O2 consumption (Jr) and the short-circuit current (Ji) were measured simultaneously in bovine tracheal epithelium in vitro. In this tissue, Ji is the sum of two active transport processes, Cl- secretion and Na+ absorption. Jr was determined from the decrease of PO2 in the incubation solution, at 37 +/- 0.05 degrees C and at a PO2 around 600 torr. Microbial contamination and leaks of dissolved O2 from the solution never exceeded 4% of the rate of PO2 decrease due to the O2 consumption of the tissue. Ji and Jr were stable over 5 h of incubation under standard conditions. Ji was 106 +/- 4 nequiv min-1 cm-2 and Jr was 39.8 +/- 1.1 nmol O2 min-1 cm-2 (mean +/- S.E., n = 46). Ji was varied with several agents known to affect ion transport across the tracheal epithelium. Na+ absorption was inhibited partly with amiloride or completely following Na+ substitution with choline. Cl- secretion was selectively suppressed by furosemide. Ji was also reduced to a very low level, using ouabain or K+ suppression to inhibit the Na+-K+-ATPase. All these manoeuvres resulted in significant reductions of both Ji and Jr. Basal Jr was not affected when Ji was modified. A plot of the relative change in suprabasal Jr versus the relative change of Ji gave a straight line (r = 0.98, n = 60). A plot using absolute values yielded a stoichiometric ratio of 13.9 ions per O2 molecule, for Na+ as well as for Cl-. The stoichiometric ratio was also calculated for each experiment. Its mean value was 14.9 ions per O2 molecule. The population of the ratios was widely dispersed, but this was explained as a predictable statistical phenomenon.

Intraepithelial current flow in rat pancreatic secretory epithelia

To assess the importance for transepithelial salt secretion of current flow across the baso-lateral membrane, we studied the effects of ouabain (1 mmol/l), Ba (3 mmol/l) and tetraethylammonium (TEA: 10 mmol/l) on secretion by the acinar (caerulein stimulated) and ductal (secretin stimulated) epithelia of the perfused rat pancreas. Within 10 min, ouabain caused a 79% inhibition of acinar secretion which was resolvable into two exponentials with half-times, respectively, of 0.24 min +/- 0.19 (S.D.) and 6.40 +/- 0.46 min. In contrast, it caused only a monoexponential inhibition of ductal secretion (61% in 10 min) with a half-time of 5.08 +/- 0.26 min. Ba caused a monoexponential inhibition of acinar secretion (70% in 10 min) with a half-time of 1.82 +/- 0.27 min, but it had no inhibitory effect on ductal secretion. The action of TEA was similar to that of Ba: it caused monoexponential inhibition of acinar secretion (26% in 10 min) with a half-time of 1.82 +/- 0.03 min, and it too had no effect on ductal secretion. For comparison, we also studied the effect of these drugs on the more rapidly secreting rat mandibular gland (stimulated with acetylcholine). All three drugs were strongly inhibitory: within 10 min, ouabain caused a 95%, Ba an 89% and TEA an 83% inhibition. The decay curves appeared to be monoexponential with half-times, respectively, of 1.49 +/- 0.12, 0.51 +/- 0.3 and 0.56 +/- 0.02 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Furosemide-sensitive Na and K fluxes in human red cells. Net uphill Na extrusion and equilibrium properties

This paper reports experiments designed to find the concentrations of internal and external Na and K at which inward and outward furosemide-sensitive (FS) Na and K fluxes are equal, so that there is no net FS movement of Na and K. The red cell cation content was modified by using the ionophore nystatin, varying cell Na (Nai) from 0 to 34 mM (K substitution, high-K cells) and cell K (Ki) from 0 to 30 mM (Na substitution, high-Na cells). All incubation media contained NaCl (Nao = 130 or 120 nM), and KCl (Ko = 0-30 mM). In high-K cells, incubated in the absence of Ko, there was net extrusion of Na through the FS pathway. The net FS Na extrusion increased when Nai was increased. Low concentrations of Ko (0-6 mM) slightly stimulated, whereas higher concentrations of Ko inhibited, FS Na efflux. Increasing Ko stimulated the FS Na influx (K0.5 = 4 mM). Under conditions similar to those that occur in vivo (Nai = 10, Ki = 130, Nao = 130, Ko = 4 mM, Cli/Clo = 0.7), net extrusion of Na occurs through the FS pathway (180-250 mumol/liter cell X h). The concentration of Ko at which the FS Na influx and efflux and the FS K influx and efflux become equal increased when Nai increased in high-K cells and when Ki was increased in high-Na cells. The net FS Na and K fluxes both approached zero at similar internal and external Na and K concentrations. In high-K cells, under conditions when net Na and K fluxes were near zero, the ratio of FS Na to FS K unidirectional flux was found to be 2:3. In high-K cells, the empirical expression (Nai/Nao)2(Ki/Ko)3 remained at constant value (apparent equilibrium constant, Kappeq +/- SEM = 22 +/- 2) for each set of internal and external cation concentrations at which there was no net Na flux. These results indicate that in the physiological region of concentrations of internal and external Na, K, and Cl, the stoichiometry of the FS Na and K fluxes is 2 Na:3 K. In high-Na cells under conditions when net FS Na and K fluxes were near zero, the ratio of FS Na to FS K unidirectional fluxes was 3:2 (1).(ABSTRACT TRUNCATED AT 400 WORDS)

Submandibular salivary secretion in the cat and associated potassium movements: dependence on temperature and perfusate flow rate

Cat submandibular glands were perfused with Locke solution in a thermostated chamber and intermittently stimulated with 10(-5)M acetylcholine (ACh). In one series of experiments the perfusion pressure was varied within the range 90-60 mm Hg, and secretory flow rate, active K+-reuptake, passive K+-release, and resting and ACh-induced venous flow rates were measured. The ACh-induced secretory flow rate and the maximal K+-fluxes were related to the simultaneous ACh-induced venous flow rates. A proportionality was found between the maximal rate of ACh-induced K+-release and ACh-induced venous flow rates below 8 ml/min, while at higher flow rates the K+-release leveled off. The maximal rate of the post-stimulatory K+-reuptake increased proportionally to the ACh-induced perfusate flow rate throughout the range studied. The secretory flow rate was much less affected by changes in ACh-induced perfusate flow rate. In another series of experiments the gland temperature was varied within the range 12-37 degrees C, and the same parameters were measured. All parameters decreased with cooling being reduced to 50% of their 37 degrees C values at: 24 degrees C for secretion, 19 degrees C for K+-reuptake, and 14 degrees C for K+-release. It is concluded: that the rate of ACh-induced K+-release is limited by the ACh-induced perfusate flow rate (within the physiological range), the capacity of the K+-reuptake mechanism is at least one order of magnitude larger than the maximal rate of K+-reuptake in vivo, the marked temperature sensitivity of the secretory flow rate reflects the high complexity of the mechanisms involved.

Vasoactive intestinal polypeptide-induced chloride secretion by a colonic epithelial cell line. Direct participation of a basolaterally localized Na+,K+,Cl- cotransport system

We have used a well-differentiated human colonic cell line, the T84 cell line, as a model system to study the pathways of cellular ion transport involved in vasoactive intestinal polypeptide (VIP)-induced chloride secretion. A modified Ussing chamber was used to study transepithelial Na+ and Cl- fluxes across confluent monolayer cultures of the T84 cells grown on permeable supports. In a manner analogous to isolated intestine, the addition of VIP caused an increase of net Cl- secretion which accounted for the increase in short circuit current (Isc). The effect of VIP on Isc was dose dependent with a threshold stimulation at 10(-10) M VIP, and a maximal effect at 10(-8) M. Bumetanide prevented or reversed the response to VIP. Inhibition by bumetanide occurred promptly when it was added to the serosal, but not to the mucosal bathing media. Ion replacement studies demonstrated that the response to VIP required the simultaneous presence of Na+, K+, and Cl- in the serosal media. Utilizing cellular ion uptake techniques, we describe an interdependence of bumetanide-sensitive 22Na+, 86Rb+, and 36Cl- uptake, which is indicative of a Na+,K+,Cl- cotransport system in this cell line. This transport pathway was localized to the basolateral membrane. Extrapolated initial velocities of uptake for each of the three ions was consistent with the electroneutral cotransport of 1 Na+:1 K+ (Rb+):2 Cl-. Our findings indicate that VIP-induced Cl- secretion intimately involves a bumetanide-sensitive Na+,K+,Cl- cotransport system which is functionally localized to the basolateral membrane.

Ion transport across the exocrine glands of the frog skin

Exposure of the intact frog skin to beta-adrenergic agonists stimulates chloride secretion by the exocrine glands. The secretory response is dependent on Na in the serosal bath and is inhibited by exposure to ouabain and furosemide. Thus the transport mechanism has properties similar to those described for other exocrine glands. Analysis of 3H-ouabain binding sites and determination of intracellular ions by energy dispersive x-ray microanalysis indicates that the transepithelial pathway for Cl flux may be via a distinct group of cells located at the ductal pole of the acinus of two of the gland types; termed mucous and seromucous.

Energetics of chloride secretion in canine tracheal epithelium. Comparison of the metabolic cost of chloride transport with the metabolic cost of sodium transport

Canine tracheal epithelium secretes Cl and absorbs Na; the energy for both is derived from the activity of the basolateral membrane Na-K-ATPase. These properties allowed us to examine the energetics of Cl secretion by directly comparing the metabolic cost of Cl transport with the metabolic cost of Na transport. We measured the change in short-circuit current and O2 consumption rate that was produced by a variety of maneuvers that alter Na and/or Cl transport rate. The experimental interventions included the use of the secretagogue epinephrine, the Cl transport inhibitor bumetanide, the Na transport inhibitor amiloride, the Na-K-ATPase inhibitor ouabain, and ion substitutions. The O2 consumption rates required for Na and Cl transport were compared in each individual tissue. The results indicate that the oxygen cost of Cl transport is significantly less than the oxygen cost of Na transport: two to four Cl ions are transported for the same metabolic cost that is required to transport one Na ion. These findings suggest that the basolateral membrane Na-dependent Cl entry step couples the entry of more than one Cl ion to each Na ion.

The anionic basis of fluid secretion by the rabbit mandibular salivary gland

The role played by anions in salivary secretion has been studied in experiments on the isolated, perfused mandibular gland of the rabbit, in which perfusate Cl- and/or HCO3- were replaced by other anions. Replacement of Cl- with Br- had no significant effect on salivary secretion rate, but replacement with the other anions tested caused secretory rate to fall, by 38% (I-), 50% (NO3-), 61% (isethionate, ise -), and 66% ( CH3SO4 -), respectively. Replacement of perfusate Cl- with ise - or CH3SO4 - caused the salivary HCO3- concentration to rise up to 4-fold. Replacement with Br- or I- seemed to have little effect on salivary HCO3- concentration but, in contrast to ise -, Br- and I- entered the saliva in concentrations comparable to those of Cl- during control perfusion. In glands perfused with HCO3- and ise -, the addition of methazolamide, an inhibitor of carbonic anhydrase, caused a further 60% drop in secretory rate, but the saliva remained rich in HCO3-. Replacement of perfusate HCO3- with Cl- or ise - had no effect on salivary secretion or composition. Replacement of both HCO3- and Cl- in the perfusate with ise - reduced salivary secretion to less than 2% of control levels. In control glands (i.e. perfused with both HCO3- and Cl-), administration of furosemide, an inhibitor of Na+/Cl- co-transport, reduced the secretion rate and increased salivary HCO3- in a manner indistinguishable from that seen when perfusate Cl- was replaced with ise -. In control perfused glands, administration of SITS (4-acetamido-4'- isothio cyano-2,2'-disulphonic acid stilbene), an inhibitor of Cl-/HCO3- antiports , did not cause any change in salivary HCO3- concentration. Unexpectedly, it induced a significant increase in salivary secretory rate. The results show that salivary secretion depends on two independent transport systems. One is a Cl- -dependent, furosemide-sensitive system, probably a Na+/Cl- symport. The other is an HCO3- -dependent, methazolamide-sensitive system, and is probably an Na+/H+ antiport.

Perfusion of isolated tubules of the shark rectal gland. Electrical characteristics and response to hormones

Both the mammalian thick ascending limb of Henle's loop and the shark rectal gland actively transport Cl against an electrochemical gradient by mechanisms involving hormone-sensitive NaCl transport. In contrast to mammalian renal tubules, individual tubules of the shark rectal gland previously have not been perfused in vitro. Using a combination of renal slice and microdissection techniques we were able to isolate and perfuse single rectal gland tubules without the use of enzyme treatment. Single tubules consistently generated lumen-negative transepithelial voltages (Vt) of -1.8 mV when perfused and bathed with identical shark Ringer's solution. The addition of cyclic AMP, vasoactive intestinal peptide (VIP), and adenosine to the bath increased Vt to -7.5, -9.0, and -4.3 mV, respectively (all P less than 0.02 compared with paired controls). Each stimulation could be reversed by addition by furosemide to the bath. The adenosine response was inhibited by theophylline, a specific inhibitor of adenosine receptors. The tubules had a low transepithelial electrical resistance of 12-26 omega X cm2 and exhibited a transepithelial permselectivity for small cations. These results indicate that tubules of the rectal gland can be perfused in vitro and have receptors for VIP and adenosine. Cyclic AMP and secretagogues hyperpolarize the membrane consistent with electrogenic chloride transport, and these effects are reversed by furosemide, an inhibitor of coupled sodium-potassium-chloride co-transport. The response of Vt to cyclic AMP and furosemide, the transepithelial electrical resistance, and the cation selective permeability of tubules are remarkably similar to measurements in perfused mammalian thick ascending limbs.

Intracellular chloride activities in canine tracheal epithelium. Direct evidence for sodium-coupled intracellular chloride accumulation in a chloride-secreting epithelium

Canine tracheal epithelium secretes Cl via an electrogenic transport process that appears to apply to a wide variety of secretory epithelia. To examine the mechanisms involved, intracellular chloride activity, acCl, was measured with Cl-selective intracellular microelectrodes. The results indicate that when the rate of secretion was minimal acCl was 37 mM; with stimulation of secretion the intracellular voltage depolarized, but acCl was not significantly altered, at 39 mM. These findings indicate that: (a) Cl is accumulated across the basolateral membrane under nonsecreting and secreting conditions at an activity 3.8 and 2.4 times, respectively, that predicted for an equilibrium distribution; (b) Cl exit across the apical membrane may be passive with an electrochemical driving force of 22 mV; and (c) stimulation of secretion enhanced the rate of Cl entry across the basolateral membrane, since Cl transport increased without a change in acCl. In the absence of Na in the extracellular fluid, acCl approached the value expected for an equilibrium distribution. This finding suggests that "uphill" entry of Cl into the cell against its electrochemical gradient is dependent upon, and energized by, the entry of Na down its gradient. Submucosal bumetanide, a loop diuretic, also decreased the rate of Cl secretion and decreased acCl, indicating an inhibition of Cl entry. These findings indicate that Cl entry into the cell is directed against its electrochemical gradient and is mediated by a Na-coupled, bumetanide-inhibitable, transport process at the basolateral membrane and that Cl may exit passively down a favorable electrochemical gradient across the apical membrane.

The distribution of intracellular ions in the avian salt gland

To investigate the mechanism of salt secretion in the avian salt gland, we used quantitative electron probe microanalysis to measure the intracellular elemental concentrations in dry cryosections of unspecialized and partially specialized secretory epithelial cells from fresh water- and salt water-adapted ducklings, respectively. In conjunction with this, human and duckling erythrocytes were also analyzed, since these provided the experimental basis for using in situ erythrocytes as standards for determining the local water content of epithelia from the analysis of dried cryosections. The microprobe results from both types of erythrocytes compared favorably with chemical determinations of elemental concentrations. The nucleated avian erythrocytes, whose wet-weight elemental concentrations were determined by a compartmental analysis that required neither a peripheral standard nor a measure of the local mass, revealed a marked accumulation of P and K in the nucleus (388 and 190 mmol/kg wet wt, respectively) relative to the cytoplasm (67 and 85 mmol/kg wet wt). In both developmental states of the epithelial cells, the nucleus and apical cytoplasm had essentially similar and unremarkable concentrations of Na (76 and 83 mmol/kg dry wt, respectively, in the adapted cells vs. 72 and 81 mmol/kg dry wt in the control cells) and K (602 and 423 mmol/kg dry wt vs. 451 and 442 mmol/kg dry wt). Chloride, however, which was in general rather high, was significantly depressed in the apical cytoplasm of adapted cells only (164 and 124 mmol/kg dry wt in the nucleus and cytoplasm, respectively, of adapted cells (P less than 0.05) vs. 138 and 157 mmol/kg dry wt for control cells (P less than 0.05). Cation concentrations (Na + K) were elevated approximately 15% in the basal regions of adapted cells as compared with apical cytoplasm. When tissue water variations are accounted for, the results suggest that: (a) an active, energy-requiring process is responsible for chloride accumulation in this cell; (b) the apical membrane is a regulatory site for secretion; and (c) there are regional distinctions in the distribution of ions and water, particularly in the salt water-adapted cell. These conclusions are consistent with active chloride transport as the basis for salt secretion in this tissue.

Presence of a sodium-potassium chloride cotransport system in the rectal gland of Squalus acanthias

In order to investigate whether the loop diuretic sensitive, sodium-chloride cotransport system described previously in shark rectal gland is in fact a sodium-potassium chloride cotransport system, plasma membrane vesicles were isolated from rectal glands of Squalus acanthias and sodium and rubidium uptake were measured by a rapid filtration technique. In addition, the binding of N-methylfurosemide to the membranes was investigated. Sodium uptake into the vesicles in the presence of a 170 mM KCl gradient was initially about five-fold higher than in the presence of a 170 mM KNO3 gradient. In the presence of chloride, sodium uptake was inhibited 56% by 0.4 mM bumetanide and 40% by 0.8 mM N-methylfurosemide. When potassium chloride was replaced by choline chloride or lithium chloride, sodium uptake decreased to the values observed in the presence of potassium nitrate. Replacement of potassium chloride by rubidium chloride, however, did not change sodium uptake. Initial rubidium uptake into the membrane vesicles was about 2.5-fold higher in the presence of a 170 mM NaCl gradient than in the presence of a 170 mM NaNO3 gradient. The effect of chloride was completely abolished by 0.4 mM bumetanide. Replacement of the sodium chloride gradient by a lithium chloride gradient decreased rubidium uptake by about 40%; replacement by a choline chloride gradient reduced the uptake even further. Rubidium uptake was also strongly inhibited by potassium. Sodium chloride dependence and bumetanide inhibition of rubidium flux were also found in tracer exchange experiments in the absence of salt gradients. The isolated plasma membranes bound 3[H]-N-methylfurosemide in a dose-dependent manner. In Scatchard plots, one saturable component could be detected with an apparent KD of 3.5 x 10(-6) M and a number of sites n of 104 pmol/mg protein. At 0.8 microM, N-methylfurosemide binding decreased 51% when sodium-free or low-potassium media were used. The same decrease was observed when the chloride concentration was increased from 200 to 600 mM or when 600 1 mM bumetanide or furosemide was added to the incubation medium. These studies indicate that the sodium-chloride cotransport system described previously in the rectal gland is in fact a sodium-potassium chloride cotransport system. It is postulated that this transport system plays an essential role in the secondary active chloride secretion of the rectal gland.

Chloride secretion by canine tracheal epithelium: I. Role of intracellular c AMP levels

We measured the short-circuit current (Isc) across canine tracheal epithelium and the intracellular cAMP levels of the surface epithelial cells in the same tissues to assess the role of cAMP as a mediator of electrogenic Cl secretion. Secretogogues fall into three classes: (i) epinephrine, prostaglandin (PG) E1, and theophylline decrease both Isc and cellular cAMP levels; (ii) PGF2 alpha and calcium ionophore, A23187, increase Isc without affecting cell cAMP levels at the doses employed; and (iii) acetylcholine, histamine, and phenylephrine do not alter either Isc or cAMP levels. These findings indicate that: (i) increases in cAMP or Ca activity stimulate electrogenic Cl secretion by the columnar cells of the surface epithelium; (ii) cAMP mediates the effects of PGE1 and beta-adrenergic agonists; (iii) a strict correlation between cAMP levels and Cl secretion rate is not apparent from spontaneous variations in these parameters or from dose-response relations of Isc and cAMP to epinephrine concentration; and (iv) acetylcholine, histamine, and phenylephrine, agents that stimulate electrically-neutral NaCl secretion by submucosal glands, do not evoke cAMP-mediated responses by the surface epithelium. Addition of 10(-6) M indomethacin (or other prostaglandin synthesis inhibitors) to the mucosal solution decreases Isc and cellular cAMP levels and reduces the release of PGE2 into the bathing media by 80%. Indomethacin does not interfere with the subsequent secretory response to PGE1. This suggests that endogenous prostaglandin production underlies the spontaneous secretion of Cl across canine tracheal epithelium under basal conditions.

Epithelial cell volume modulation and regulation

Epithelial cell volume is a sensitive indicator of the balance between solute entry into the cell and solute exit. Solute accumulation in the cell leads to cell swelling because the water permeability of the cell membranes is high. Similarly, solute depletion leads to cell shrinkage. The rate of volume change under a variety of experimental conditions may be utilized to study the rate and direction of solute transport by an epithelial cell. The pathways of water movement across an epithelium may also be deduced from the changes in cellular volume. A technique for the measurement of the volume of living epithelial cells is described, and a number of experiments are discussed in which cell volume determination provided significant new information about the dynamic behavior of epithelia. The mechanism of volume regulation of epithelial cells exposed to anisotonic bathing solution is discussed and shown to involve the transient stimulation of normally dormant ion exchangers in the cell membrane.

A pharmacological analysis of chloride transport across the amphibian cornea

1. Active Cl, but not Na, transport across the toad cornea was inhibited by mepacrine, which is a phospholipase A2 inhibitor; trifluoperazine, which blocks the action of calmodulin; and meclofenamic acid, which inhibits synthesis of prostaglandins. Bumetanide and DIDS (4,4'-diisothiocyano-2,2'-stilbene disulphonic acid) have previously been shown to inhibit this Cl transport. The interactions of these antagonists with several agonists that increase Cl transport were studied. 2. The effects of adrenaline, prostaglandin E2, dibutyryl cyclic AMP (DBcAMP) and the Ca ionophore A23187 were inhibited by mepacrine, trifluoperazine and bumetanide. 3. The inhibitory effects of high concentrations of DIDS, however, could be overcome by all of the agonists except DBcAMP. 4. Meclofenamic acid only blocked the effects of A23187. 5. A model is proposed to account for the observed actions and interactions of the various antagonists and agonists on Cl transport. This involves possible roles for Ca, calmodulin and phospholipase A2.

Lacrimal gland electrolyte and water secretion in the rabbit: localization and role of (Na+ + K+)-activated ATPase

1. The rate of acetylcholine (ACh)-induced fluid secretion was measured from the main secretory duct of rabbit lacrimal glands perfused in vivo with Krebs Henseleit bicarbonate solutions. 2. Perfusion with ouabain (10(-5) M) decreased the rate of lacrimal gland fluid secretion to 23% of the control value. 3. Perfusion with furosemide (10(-4) and 10(-3) M), which has been shown to inhibit the coupled transport of Na+ and Cl-, reversibly decreased the rate of secretion to 43 and 33% of the control value respectively. 4. (Na+ + K+)-activated ATPase was localized in slices of rabbit lacrimal gland using autoradiography with [3H]ouabain. 5. A high density of [3H]ouabain binding sites was present on ductal cells, whereas a very low density was found on acinar cells. For both types of cells the [3H]ouabain binding sites were located on the basolateral plasma membranes. 6. It is concluded that ACh-induced secretion of electrolytes and water is dependent upon (Na+ + K+)-activated ATPase. In addition, coupled transport of Na+ and Cl- appears to be involved in secretion. 7. Basolateral location of the (Na+ + K+)-activated ATPase implies that it plays an indirect role in electrolyte and water secretion. A possible role may be to energize a secondary active transport of Cl- that is mediated by a NaCl cotransport system.

Ion transport by canine tracheal mucosa: effect of elevation of cellular calcium

Calcium is considered to be modulator of paracellular shunt pathway as well as several cell functions such as ion transport. We used calcium ionophore A23187 to study effect of mobilization of calcium on chloride and sodium transport in canine tracheal epithelium. This epithelium secretes Cl and absorbs Na under short-circuit conditions. Sheets of canine tracheal mucosa were mounted in chambers, perfused with Krebs-Henseleit solution at 37 degrees C and oxygenated. Unidirectional fluxes of 22Na and 36Cl were measured in matched paired mucosae under short-circuit conditions. Net flux of each ion was calculated from these unidirectional fluxes, before and after addition of A23187 (10(-6) M) to submucosal bath. Short-circuit current and potential difference were measured. From these values conductance was calculated. Initially, short-circuit current, potential difference, and conductance were 31 +/- 8 microA/cm(2), 26 +/- 4 mV, and 1.2 +/- 0.2 mS/cm(2), respectively, and increased significantly after A23187 to 76 +/- 20 microA/cm(2), 41 +/- 5 mV, and 1.8 +/- 0.4 mS/cm(2), respectively (mean +/- SE for 10 tissues). Net Cl secretion increased from 0.83 +/- 0.23 muEq/cm(2) hr to 2.84 +/- 0.38 muEq/cm(2) hr (P less than .002) and net Na absorption of 0.50 +/- 0.14 muEq/cm(2) hr was abolished (P less than .02). These data show that increase in cellular levels of calcium increases chloride secretion and abolishes sodium transport. Intracellular calcium appears to regulate cell membrane permeability to chloride and sodium.U

Ion transport in 'tight' epithelial monolayers of MDCK cells

Cultured monolayers of MDCK cells grown upon filter supports display many features of in vivo epithelia. Previously reported values of transmonolayer resistance of 100 omega cm-2 (Misfeldt, Hamamoto & Pitelka, 1976; Cereijido, Robbins, Dolan, Rotunno & Sabatini, 1978) indicate a leaky epithelium. This paper describes the properties of a strain of MDCK cells which displays entirely different electrophysiological properties. The results show that (i) the mean transmonolayer resistance is 4.16 k omega cm-2, (ii) transmonolayer ion transport is of small magnitude since the mean spontaneous open circuit PD is only 2.17 mV basal surface positive and isotopic Na and Cl flux measurements fail to demonstrate a significant net flux, (iii) the action of ouabain, amiloride and ion substitutions are consistent with transmonolayer net Na movement being largely responsible for the spontaneous PD, and (iv) asymmetry in the localization of the Na-K ATPase is evident on the basis of 3H-ouabain binding to cell monolayer.

Freeze-fracture and morphometric analysis of occluding junctions in rectal glands of elasmobranch fish

The structure of occluding junctions in secretory and ductal epithelium of salt-secreting rectal glands from two species of elasmobranch fish, the spiny dogfish Squalus acanthias and the stingray Dasyatis sabina, was examined by thin-section and freeze-fracture electron microscopy. In both species, occluding junctions between secretory cells are shallow in their apical to basal extent and are characterized by closely juxtaposed parallel strands. Average strand number in the dogfish was 3.5 +/- 0.2. with a mean depth of 56 +/- 5 nm; in the stingray a mean of 2.0 +/- 0.2 strands encompassed an average depth of 18 +/- 3 nm. In contrast, the linear extent of these junctions was remarkably large due to the intermeshing of the narrow apices of the secretory cells to form the tubular lumen. Morphometric analysis gave values of 66. 8 +/- 2.5 and 74.9 +/- 4.6 m/cm2 for the length of junction per unit of luminal surface area in the dogfish and stingray, respectively. This junctional morphology is similar to that generally described for "leaky" epithelia. In comparison, the stratified ductal epithelium which carries the NaCl-rich secretion to the intestine is characterized by extensive occluding junctions which extend 0.6-0.8 mum in depth and consist of a mean of 12 strands arranged in an anastomosing network, an architectural pattern typical of "tight" epithelia. The length density of these junctions in the dogfish rectal gland was 7.6 +/- 0.1 m/cm2. The junctional architecture of the rectal gland secretory epithelium (few strands, large junctional length densities) is similar to that described for several other hypertonic secretory epithelia [20, 34] and is compatible with the recent model for salt secretion in rectal glands [39] and in other C1- secretory epithelia which posits a conductive paracellular pathway for trans-epithelial Na+ secretion from intercellular space to the lumen to form the NaCl-rich secretory product.

Passive sodium movements across the opercular epithelium: the paracellular shunt pathway and ionic conductance

The unidirectional Na+, Cl-, and urea fluxes across isolated opercular epithelia from seawater-adapted Fundulus heteroclitus were measured under different experimental conditions. The mean Na+, Cl0, and urea permeabilities were 9.30 x 10(-6) cm . sec-1, 1.24 x 10(-6) cm . sec-1, and 5.05 x 10(-7) cm . sec-1, respectively. The responses of the unidirectional Na+ fluxes and the Cl- influx (mucosa to serosa) to voltage clamping were characteristic of passively moving ions traversing only one rate-limiting barrier. The Na+ conductance varied linearly with, and comprised and mean 54% of, the total tissue ionic conductance. The Cl- influx and the urea fluxes were independent of the tissue conductance. Triaminopyrimidine (TAP) reduced the Na+ fluxes and tissue conductance over 70%, while having no effect on the Cl- influx of urea fluxes. Mucosal Na+ substitution reduced the Na+ permeability 60% and the tissue conductance 76%, but had no effect on the Cl- influx or the urea fluxes. Both the Na+ and Cl- influxes were unaffected by respective serosal substitutions, indicating the lack of any Na+/Na+ and Cl-/Cl- exchange diffusion. The results suggest that the unidirectional Na+ fluxes are simple passive fluxes proceeding extracelluarly (i.e., movement through a cation-selective paracellular shunt). This pathway is dependent on mucosal (external) Na+, independent of serosal (internal) Na+, and may be distinct from the transepithelial Cl- and urea pathways.U

Chloride efflux in single barnacle muscle fibres

1. A study has been made of the behaviour of the radiochloride efflux in single muscle fibres from the barnacle, Balanus nubilus. 2. In the majority of the fibres studied, the fractional rate constant for 36Cl efflux is a constant and unaffected by the injection of distilled water (approximately 0 . 3 microliter. in volume). 3. Acidification of the HCO3(-)-containing medium causes stimulation of the Cl efflux, the threshold value being pH 7 . 0. The magnitude of the response is a logarithmic function of the external H+ and HCO3 concentration over a wide concentration range. 4. (i) Total replacement of the external Cl and NO3 fails to alter the course of the Cl efflux. However, the magnitude of the response to acidification is reduced to a marked degree. (ii) Replacement of the external Na by Li reduces not only the Cl efflux but also the size of the response to acidification. 5. Injection of HCl, HCO3 or KCl fails to alter the Cl efflux. Injection, however, of 4 M-KCl or NaCl causes a fall in the efflux. 6. 10( 4)M-ouabain is ineffective. It also fails to alter the response of the Cl efflux to acidification. 7. (i) Injection of cyclic AMP stimulates the Cl efflux in a dose-dependent manner, but only transitorily. (ii) Preinjection of pure protein kinase inhibitor causes a marked reduction in the magnitude of the response to cyclic AMP. 8. Preinjection of pure protein kinase inhibitor fails to affect the response to external acidification. 9. (i) Pretreatment externally with ethacrynic acid reduces the response to external acidification. (ii) External application of 4-acetoamineo-4'-isothiocyano-2,2'-stilbene disulphonate (SITS) reduces the resting Cl efflux. It also abolishes completely the response to acidification. (iii) The effect of 4,4'-diisothiocyano-2,2'-stilbene disulphonate (DIDS) resembles that of SITS. (iv) Injection of H2DIDS fails to reduce the resting efflux but tends to reduce the magnitude of the response to acidification. 10. (i) 5 X 10(-4) M-benzolamide is without effect on the basal Cl efflux. (ii) Benzolamide in high concentration reduces the magnitude of the response to acidification. This occurs within a rather narrow concentration range. 11. (i) A sudden reduction in environmental temperature from 24 to 0 degrees C causes a marked fall in the Cl efflux. (ii) Acidification of the artificial sea water at 0 degrees C stimulates the efflux. 12. The present experiments have led to evidence which is consistent with the view that the Cl efflux is modulated by at least two distinct mechanisms: one is responsive to acidification when HCO3 as buffer is present and involves participation of a benzolamide-sensitive component presumably lying in the fibre membrane. The other is responsive to injection of cyclic AMP, and, and probably involves cyclic AMP-protein kinase.