A microperfusion investigation of bicarbonate secretion by the rat submaxillary gland. The action of a parasumpathomimetic drug on electrolyte transport

The effect of acetazolamide, amiloride, bumetanide and SITS on secretion of fluid and electrolytes by the parotid gland of common wombats, Vombatus ursinus

Mechanisms of saliva formation by wombat parotid glands were investigated in anaesthetized wombats at two levels of cholinergically-stimulated flow viz. mid-range (30-40% maximum flow) and maximum flow using ion-transport and carbonic-anhydrase inhibitors. Bumetanide (0.005-0.1 mmol l^-1 carotid plasma) progressively reduced mid-range flow by 52 ± 3.4% (mean ± SEM). Concurrently, saliva [Cl] decreased, [Na] and [HCO₃] increased but HCO₃ excretion was unaltered. Salivary flow during high-rate cholinergic stimulation was 31 ± 1.1% of the pre-bumetanide maximum. During mid-range stimulation, SITS (0.075 mmol l^-1) was without effect whereas 0.75 mmol l^-1 stimulated transient increases in fluid output. The higher SITS concentration caused no alterations to flow or electrolyte concentrations during maximal stimulation. Carotid plasma [amiloride] (0.05 mmol l^-1) caused immediate falls in flow rate of 20-30% followed by progressive recovery over 25 min to levels above pre-amiloride flow rates despite plasma [amiloride] increasing tenfold. Concurrently, salivary [Na] and [Cl] rose to equal plasma concentrations and [K] fell by 50% indicating blockade of acinar Na/H exchangers and luminal Na channels in the ducts. Increased salivary osmolarity caused the flow recovery. Saliva flow during maximum cholinergic stimulation was reduced by 38-46%. The depression of flow was interpreted as resulting from competition between amiloride and acetylcholine for access to the muscarinic receptors. Plasma [acetazolamide] (0.35-2.5 mmol l^-1) did not alter saliva outflow during mid-range or maximum flow regimes whereas salivary [Cl] increased and [HCO₃] decreased consistent with reduced anion exchange resulting from inhibition of carbonic anhydrase. Combined with bumetanide, acetazolamide (1.5 mmol l^-1) reduced flow by an additional 18-22% relative to bumetanide alone thereby demonstrating that acinar HCO₃ synthesis supported a limited proportion of saliva formation and that some HCO₃ secretion was independent of carbonic anhydrase activity.

The apical Na+ -HCO3 - cotransporter Slc4a7 (NBCn1) does not contribute to bicarbonate transport by mouse salivary gland ducts

The HCO₃ ^- secretion mechanism in salivary glands is unclear but is thought to rely on the co-ordinated activity of multiple ion transport proteins including members of the Slc4 family of bicarbonate transporters. Slc4a7 was immunolocalized to the apical membrane of mouse submandibular duct cells. In contrast, Slc4a7 was not detected in acinar cells, and correspondingly, Slc4a7 disruption did not affect fluid secretion in response to cholinergic or β-adrenergic stimulation in the submandibular gland (SMG). Much of the Na ⁺ -dependent intracellular pH (pH _i ) regulation in SMG duct cells was insensitive to 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid, S0859, and to the removal of extracellular HCO ₃ ^- . Consistent with these latter observations, the Slc4a7 null mutation had no impact on HCO ₃ ^- secretion nor on pH _i regulation in duct cells. Taken together, our results revealed that Slc4a7 targets to the apical membrane of mouse SMG duct cells where it contributes little if any to pH _i regulation or stimulated HCO ₃ ^- secretion.

Role of calcium signaling in epithelial bicarbonate secretion

Transepithelial bicarbonate secretion plays a key role in the maintenance of fluid and protein secretion from epithelial cells and the protection of the epithelial cell surface from various pathogens. Epithelial bicarbonate secretion is mainly under the control of cAMP and calcium signaling. While the physiological roles and molecular mechanisms of cAMP-induced bicarbonate secretion are relatively well defined, those induced by calcium signaling remain poorly understood in most epithelia. The present review summarizes the current status of knowledge on the role of calcium signaling in epithelial bicarbonate secretion. Specifically, this review introduces how cytosolic calcium signaling can increase bicarbonate secretion by regulating membrane transport proteins and how it synergizes with cAMP-induced mechanisms in epithelial cells. In addition, tissue-specific variations in the pancreas, salivary glands, intestines, bile ducts, and airways are discussed. We hope that the present report will stimulate further research into this important topic. These studies will provide the basis for future medicines for a wide spectrum of epithelial disorders including cystic fibrosis, Sjögren's syndrome, and chronic pancreatitis.

Molecular mechanism of pancreatic and salivary gland fluid and HCO3 secretion

Fluid and HCO(3)(-) secretion is a vital function of all epithelia and is required for the survival of the tissue. Aberrant fluid and HCO(3)(-) secretion is associated with many epithelial diseases, such as cystic fibrosis, pancreatitis, Sjögren's syndrome, and other epithelial inflammatory and autoimmune diseases. Significant progress has been made over the last 20 years in our understanding of epithelial fluid and HCO(3)(-) secretion, in particular by secretory glands. Fluid and HCO(3)(-) secretion by secretory glands is a two-step process. Acinar cells secrete isotonic fluid in which the major salt is NaCl. Subsequently, the duct modifies the volume and electrolyte composition of the fluid to absorb the Cl(-) and secrete HCO(3)(-). The relative volume secreted by acinar and duct cells and modification of electrolyte composition of the secreted fluids varies among secretory glands to meet their physiological functions. In the pancreas, acinar cells secrete a small amount of NaCl-rich fluid, while the duct absorbs the Cl(-) and secretes HCO(3)(-) and the bulk of the fluid in the pancreatic juice. Fluid secretion appears to be driven by active HCO(3)(-) secretion. In the salivary glands, acinar cells secrete the bulk of the fluid in the saliva that is driven by active Cl(-) secretion and contains high concentrations of Na(+) and Cl(-). The salivary glands duct absorbs both the Na(+) and Cl(-) and secretes K(+) and HCO(3)(-). In this review, we focus on the molecular mechanism of fluid and HCO(3)(-) secretion by the pancreas and salivary glands, to highlight the similarities of the fundamental mechanisms of acinar and duct cell functions, and to point out the differences to meet gland-specific secretions.

Channels and transporters in salivary glands

According to the two-stage hypothesis, primary saliva, a NaCl-rich plasma-like isotonic fluid is secreted by salivary acinar cells and its ionic composition becomes modified in the duct system. The ducts secrete K(+) and HCO (3) (-) and reabsorb Na(+) and Cl(-) without any water movement, thus establishing a hypotonic final saliva. Salivary secretion depends on the coordinated action of several channels and transporters localized in the apical and basolateral membrane of acinar and duct cells. Early functional studies in perfused glands, followed by the molecular cloning of several transport proteins and the subsequent analysis of mutant mice, have greatly contributed to our understanding of salivary fluid and the electrolyte secretion process. With a few exceptions, most of the key channels and transporters involved in salivary secretion have now been identified and characterized. However, the picture that has emerged from all these studies is one of a complex molecular network characterized by redundancy for several transport proteins, compensatory mechanisms, and adaptive changes in health and disease. Current research is directed to the molecular interactions between the determinants and the ways in which they are regulated by extracellular signals and intracellular mediators. This review focuses on the functionally and molecularly best-characterized channels and transporters that are considered to be involved in transepithelial fluid and electrolyte transport in salivary glands.

H+ and HCO3- transporters in human salivary ducts. An immunohistochemical study

The presence and cellular distribution of key H+ and HCO3- transport proteins was studied in human salivary ducts. Immunofluorescence and immunoperoxidase light microscopy was applied, using specific antibodies against the NHE1 and NHE3 isoforms of the Na+/H+ exchanger, against the 31 and 70 kDa subunits of the vacuolar H+-ATPase and against the electrogenic Na+-HCO3- cotransporter. The results show basolateral NHE1 and apical NHE3 in human submandibular, parotid and sublingual duct cells. Vacuolar H+-ATPase was found predominantly in the apical membrane of parotid, submandibular and sublingual duct cells, although it was absent in certain parotid striated duct cells. The Na+-HCO3- cotransporter was predominantly expressed in the apical membrane of parotid and sublingual striated ducts, and intracellularly distributed in the distal parts of the gland tree and in submandibular ducts. The results indicate that HCO3- transport properties of salivary ducts may vary not only between gland and species, but even in different duct segments of the same gland as well.

Immunohistochemical localization of carbonic anhydrases I, II, and VI in the developing rat sublingual and submandibular glands

Carbonic anhydrase has been localized to the acini and ducts of mature rat salivary glands. This enzyme has been associated with ion transport, a prominent function of striated and excretory ducts in salivary glands, suggesting that it might be used as a marker of ductal differentiation. The purpose of this study was to immunohistochemically document developmental changes in carbonic anhydrase in the ducts of the rat sublingual and submandibular glands. Immunohistochemistry was performed with antibodies to human carbonic anhydrase isoenzymes I, II and VI on sections of sublingual and submandibular glands from rats at representative postnatal developmental ages. Reactions were weak in the ducts of both glands at 1 day, then progressively increased. By 42 days, reactions had the adult pattern of virtually none in the mucous or seromucous acini, moderate to strong in the striated and excretory ducts, and none to weak in the intercalated ducts. Weak to moderate reactions were observed in the granular convoluted tubules of the submandibular gland as they became recognizable at age 42 days. Reactions to carbonic anhydrase I and II antibodies also increased from none (1 day) to modest (42 days) in the demilunes of the sublingual gland. The order of reaction intensity of the antibodies was II > I > VI. When localized via these anti-human antibodies, carbonic anhydrase is a useful marker of the functional differentiation of the striated and excretory ducts of the developing rat sublingual and submandibular glands.

Immunolocalization of vacuolar-type H+-ATPase in rat submandibular gland and adaptive changes induced by acid-base disturbances

Using antibodies against the 31-kD and 70-kD subunits of vacuolar type H+-ATPase (V-ATPase) and light microscopic immunocytochemistry, we have demonstrated the presence of this V-ATPase in rat submandibular gland. We have also investigated the adaptive changes of this transporter during acid-base disturbances such as acute and chronic metabolic acidosis or alkalosis. Our results show intracellularly distributed V-ATPase in striated, granular, and main excretory duct cells in controls, but no V-ATPase immunoreaction in acinar cells. Both acute and chronic metabolic acidosis caused a shift in V-ATPase away from diffuse distribution towards apical localization in striated and granular duct cells, suggesting that a V-ATPase could be involved in the regulation of acid-base homeostasis. In contrast, during acidosis the main excretory duct cells showed no changes in the V-ATPase distribution compared to controls. With acute and chronic metabolic alkalosis, no changes in the V-ATPase distribution occurred. (J Histochem Cytochem 46:91-100, 1998)

The effect of transport-blocking drugs on secretion of fluid and electrolytes by the mandibular gland of red kangaroos, Macropus rufus

Mechanisms of primary fluid formation by macropodine mandibular glands were investigated in anaesthetized red kangaroos using ion-transport and carbonic anhydrase inhibitors. Bumetanide at carotid plasma concentrations of 0.005-0.1 mmol/l progressively reduced a stable, acetylcholine-evoked flow rate of 1.02 +/- 0.024 ml/min to 0.16 +/- 0.016 ml/min (mean +/- SEM). Concurrently, saliva [Na], [Cl] and osmolality decreased, [K] and [HCO3] increased and HCO3 excretion was unaffected. High-rate cholinergic stimulation was unable to increase salivary flow above 12 +/- 1.5% of that for equivalent pre-bumetanide stimulation. Furosemide (1.0 mmol/l) and ethacrynate (0.5 mmol/l) caused depression of salivary flow and qualitatively similar effects on ion concentrations to those of bumetanide. Amiloride (up to 0.5 mmol/l) caused no reduction in salivary flow rates or [Na] but decreased [K] and [Cl] and increased [HCO3]. When compared with bumetanide alone, amiloride combined with bumetanide further augmented [K] and [HCO3] and lowered [Cl], but had no additional effects on Na or flow. At the higher level, 4-acetamido-4'- isothiocyanatostilbene-2,2'disulphonic acid (SITS) (0.05 and 0.5 mmol/l) stimulated fluid output, increased [HCO3] and [protein], and depressed [Na], [K] and [Cl]. Relative to bumetanide alone, SITS given with bumetanide had no additional effects on salivary flow or electrolytes. Methazolamide (0.5 mmol/l) in combination with bumetanide curtailed the decrease in [Cl] and the increases in [K] and [HCO3] associated with bumetanide. The residual methazolamide-resistant HCO3 excretion was sufficient to support 2-6% of primary fluid secretion. It was concluded that secretion of primary fluid by the kangaroo mandibular gland is initiated mainly (> 90%) by Cl transport resulting from Na-K-2Cl symport activity. A small proportion of the fluid secretion (up to 6%) appears to be supported by HCO3 secretion. No evidence was found for fluid secretion being dependent on Cl transport involving Na/H and Cl/HCO3 antiports or on HCO3 synthesis involving carbonic anhydrase.

Characterization, localization and axial distribution of Ca2+ signalling receptors in the rat submandibular salivary gland ducts

1. To characterize [Ca2+]i signalling in salivary duct cells a procedure was developed for the rapid preparation and isolation of intralobular ducts, some of which had attached intercalated ducts. The isolated ducts retained agonist-induced Ca2+ signalling after permeabilization with streptolysin O (SLO). 2. The improved cell preparation technique was reflected in the repertoire and intensity of agonist responsiveness of the cells. Measurements of [Ca2+]i in intact cells showed that all agonists previously reported to affect electrolyte transport by the submandibular salivary gland (adrenaline, carbachol, isoprenaline and forskolin) mobilized Ca2+ from internal stores and increased Ca2+ influx across the plasma membrane. 3. The use of the SLO-permeabilized ducts showed that all agonists, including isoprenaline and forskolin, mobilized Ca2+ exclusively from the inositol 1,4,5 trisphosphate (IP3)-sensitive pool. However, in granular ducts only adrenaline mobilized the entire IP3-sensitive pool whereas all other agonists mobilized only part of the pool. 4. All regions of the duct responded to substance P and the luminally secreted agonist ATP. Interestingly, the intercalated duct was most responsive to ATP and demonstrated only a minimal response to all other agonists. The granular region of the same duct and the extralobular duct always responded best to stimulation by adrenaline. 5. The perfused extralobular duct was used to show that adrenaline and carbachol stimulated the duct through the basolateral membrane whereas the receptors for ATP were localized in the luminal membrane of the duct. This suggests the presence of an ATP-dependent positive feedback loop in salivary duct with decreased activity along the ductal tree.

The effect of carbonic anhydrase inhibitors on secretion by the parotid and mandibular glands of red kangaroos Macropus rufus

The effects of carbonic anhydrase inhibitors on secretion by macropodine parotid and mandibular glands were investigated using anaesthetized red kangaroos. In the parotid gland, acetazolamide (500 mumol.l-1) reduced a stable acetylcholine-evoked, half-maximal flow rate of 2.02 +/- 0.034 to 0.27 +/- 0.023 ml.min-1 (87% reduction). Concurrently, salivary bicarbonate concentration and secretion fell (129.4 +/- 1.46 to 80.9 +/- 1.63 mmol.l-1 and 264.8 +/- 7.96 to 22.3 +/- 2.30 mumol.min-1, respectively), phosphate and chloride concentrations rose (14.0 +/- 0.79 to 27.6 +/- 0.85 mmol.l-1 and 5.6 +/- 0.25 to 27.5 +/- 1.32 mmol.l-1, respectively), sodium concentration and osmolality were unaltered, and potassium concentration fell (8.8 +/- 0.33 to 6.4 +/- 0.29 mmol.l-1). High-rate cholinergic stimulation during acetazolamide blockade was unable to increase salivary flow beyond 11 +/- 0.9% of that for equivalent unblocked control stimulation. However, superimposition of isoprenaline infusion on the acetylcholine stimulation caused a three-fold increase in the blocked flow rate. These treatments were accompanied by small increases in salivary phosphate and chloride concentrations but not bicarbonate concentration. Methazolamide infusion caused similar changes in parotid secretion. In the mandibular gland, acetazolamide infusion had no effect on salivary flow rate during either low- or high-level acetylcholine stimulation. Acetazolamide caused no alterations in salivary electrolyte secretion at low flow rates, but curtailed the rise in bicarbonate concentration associated with high-level acetylcholine stimulation. Acetazolamide administration did not affect the increase in salivary flow rate associated with isoprenaline infusion, but did block the concomitant increase in bicarbonate concentration and secretion substantially.(ABSTRACT TRUNCATED AT 250 WORDS)

Carbonic anhydrase III in the salivary glands and kidney of the Japanese monkey (Macaca fuscata)

Carbonic anhydrase III (CA-III) was found in muscles of the Japanese monkey by the double immunodiffusion test and western blotting using antiserum raised against equine CA-III. Immunocytochemical localization of CA-III in the salivary glands and kidney of the monkey was studied using an avidin-biotinylated glucose oxidase complex. CA-III was found mainly in the striated duct and interlobular duct cells of the parotid glands. In the submandibular glands, striated duct, interlobular duct, and excretory duct cells were strongly stained for CA-III. In the kidney of the monkey, CA-III was localized mainly in the dark cells of the collecting duct at the medulla and in the epithelial cells of thick limb of Henle's loop.

The immunohistolocalization of carbonic anhydrase III in the submandibular gland of rats and hamsters

Carbonic anhydrase III has been localized using the avidin-biotin-glucose oxidase complex (ABC) method in the submandibular gland of the rat and hamster. This isozyme, which is predominant in skeletal muscle, was observed in intercalated duct, striated duct and excretory duct cells in the rat submandibular glands. In contrast, only some striated duct cells in hamster submandibular glands were stained.

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)

Presence of serum proteins in submaxillary duct perfusate

The secretory activity of the main excretory duct of rat submaxillary gland was investigated by the technique of luminal perfusion. Immunologic studies of the perfusate revealed the presence of serum antigens and the absence of intrinsic submaxillary gland antigens. It is suggested that the submaxillary duct permits passive transport of serum proteins to saliva from serum.

The relationship between localization of Na+, K+-ATPase and cellular fine structure in the rat parotid gland

A method for the ultrastructural localization of the Na+-pump enzyme, Na+, K+-ATPase has been applied to the rat parotid gland. A ouabain-sensitive final reaction product, dependent on the presence of K+ and Mg2+, was found to be evenly distributed along the basal and basolateral plasma membranes of acinar and striated duct cells. In both cases, it was localized predominantly in the cytoplasm of the extensive foldings of these membranes. It is concluded that the reaction product meets the criteria for valid localization of this enzyme and that potential Na+-pump sites have been demonstrated. This study supports previous findings in salivary glands and contributes to an increasing body of evidence that contraluminal Na+-pumps are common to both reabsorbing and secreting epithelial cells.

Na+ and K+ concentration of rat parotid saliva. Comparison of carbachol and auriculo-temporal stimulation

The effects of carbachol and auriculo-temporal stimulation on the Na+ and K+ concentrations of rat parotid saliva have been compared. The main duct perfused in situ, does not transport Na+ or K+ and is water impermeable. The Na+ concentration of secretion evoked by either stimulus is flow dependent, increasing with increasing flow rate and plateauing at near plasma Na+ levels. At low flow rate the carbachol evoked secretion has a higher Na+ concentration. This is not due to the release of catecholamines since neither sympathectomy nor adrenoceptors block altered the nature of the secretion. The K+ concentration, whilst flow dependent, decreasing with increasing flow rate, was the same for both stimuli.

A micropuncture investigation of electrolyte transport in the parotid glands of sodium-replete and sodium-depleted sheep

1. Parotid secretion has been studied by micropuncture in sodium-replete and sodium-deficient sheep. 2. The osmolality of unstimulated primary saliva was slightly higher than in plasma and fell following cholinergic nerve stimulation. In sodium-depleted animals the osmolality of final saliva was hypotonic and exhibited flow dependency, where as in sodium-replete animals it was always isotonic. 3. In sodium-replete sheep, the primary fluid sodium concentration was about 120-130 mmol l-1 but in final saliva it was about 167 mmol l-1 and showed little or no flow-dependency. In sodium-depleted sheep, the primary sodium concentration averaged only 82.2 mmol l-1 and it was concluded that sodium-depleted primary fluid contained some other unidentified solute that allowed it to remain approximately isotonic; in final saliva the unstimulated sodium concentration was about 40 mmol l-1 and it rose with increasing flow rate to a maximum of 114.9 mmol l-1. 4. The primary fluid potassium concentration in sodium-replete animals did not differ significantly from that seen in sodium-depleted animals and the values were uninfluenced by stimulation; the over-all mean value was 11.2 mmol l-1. In final saliva, in sodium-replete sheep, the potassium concentrations averaged 7.8 mmol l-1 but in sodium-depleted sheep the concentrations were between 5 and 10 times greater than in primary fluid. 5. It was calculated from the equilibrium pH that the primary bicarbonate concentration would have been about 35 mmol l-1. In final saliva, where bicarbonate was measured directly, the concentrations were much greater and increased with stimulation to about 115 mmol l-1. 6. The primary fluid phosphate and chloride concentrations were the same in both sodium-replete and sodium-depleted animals and were unchanged by stimulation; the mean concentration of phosphate was 1.30 mmol l-1 and of chloride, 53.0 mmol l-1. In final saliva the phosphate concentrations were little changed but the chloride concentrations fell to an average value of 20.0 mmol l-1. In final saliva it was found that the summed sodium and potassium concentrations exceeded the summed chloride, bicarbonate and phosphate (in mequiv l-1) concentrations, on average by 13.9 mequiv l-1, regardless of sodium status or flow rate. 7. The results indicate that secretion by the sheep parotid can be accounted for in terms of the standard two-state model. Phosphate seems to enter the saliva only in the primary fluid and potassium and bicarbonate appear to enter at both primary and secondary sites; sodium and chloride enter at the primary level and can be reabsorbed in the ducts. Salt depletion causes the primary fluid concentrations of sodium and chloride to fall and the content of an unidentified solute to rise markedly while, at the ductal level, it causes normally quiescent sodium and potassium transport processes to become activated.

The effect of carbonic anhydrase inhibitors on the anionic composition of sheep's parotid saliva. With an appendix on uncatalysed carbon dioxide-water kinetics by P. T. McTigue

1. The effects of the carbonic anhydrase inhibitors, acetazolamide, ethoxzolamide and benzolamide on the ionic composition of parotid saliva were studied in anaesthetized sheep with access to the parotid blood vessels. 2. The inhibitors were infused directly into the arterial blood supply to the gland to give blood concentrations in the range 10(-5) to 10(-2) M. 3. Mean anionic concentrations at basal flow rate before inhibitor infusion were, bicarbonate 98 m-mole/l., phosphate 15 m-mole/l. and chloride 26 m-mole/l. In the presence of inhibitors, bicarbonate concentration fell by 11 m-mole/ml. and phosphate and chloride concentrations rose. Secreto-motor nerve stimulation increased bicarbonate concentration by 13 m-mole/l. before infusion of inhibitors and the concentrations of the other anions fell. The bicarbonate rise was abolished by the inhibitors and the fall in phosphate concentration was balanced by a rise in chloride concentration. 4. These effects show that only a small component of the bicarbonate ion transfer system in the sheep parotid gland is sensitive to these inhibitors. 5. The relationship of these findings to a new enzyme with carbonic anhydrase action isolated from the sheep's parotid gland is discussed.

The action of physalaemin on electrolyte excretion by the mandibular and sublingual salivary glands of the rat

The effect of physalaemin, an undecapeptide belonging to a family known collectively as the tachykinins, on water and electrolyte excretion of the mandibular and sublingual salivary glands of the rat has been investigated and compared to that of acetylcholine. Drugs were administered intravenously or by close-arterial infusion. Physalaemin is a powerful stimulant of fluid secretion by both glands although less potent than acetylcholine. The Na and K excretion patterns in physalaemin-evoked saliva resembled, but were by no means identical to those evoked by acetylcholine and other parasympathomimetic drugs: saliva evoked by physalaemin was considerably poorer in Na and K at all secretory rates. The HCO3 excretion curves, on the other hand, seemed to be identical to those evoked by parasympathomimetic drugs. From an analysis of the Na and K excretion patterns, it can be concluded, both for the mandibular and the sublingual glands, that physalaemin stimulated Na reabsorption and K secretion across the gland duct epitheluim, whereas acetylcholine has the opposite effect. These findings agree nicely with what has previously been demonstrated in vitro in the isolated perfused main excretory duct of the rat mandibular gland.

Modification of salivary duct electrolyte transport in rat and rabbit by physalaemin, VIP, GIP and other enterohormones

The effects of various polypeptide enterohormones and the tachykinin secretogogue, physalaemin, on electrolyte transport by the main excretory duct of the mandibular gland of the rabbit were studied in vitro. Vasoactive intestinal peptide (VIP, 2 X 10(-11) mol 1(-1)) and gastric inhibitory polypeptide (GIP, 10(-11) mol 1(-1)) reduced nett Na+ movement from lumen to interstitium and VIP also reduced the transepithelial potential difference; the effective concentrations of the two hormones lay within the range of normal plasma concentrations. Gastrin (5 x 10(-7) mol 1(-1)) and synthetic secretin (2 x 10(-7) mol 1(-1)) had similar effects but only at concentrations well above the normal plasma levels. Caerulein, an analogue of the octapeptide of cholecystokinin, had no effect on duct function even at a concentration of 10(-6) mol 1(-1). The potent salivary secretogogue, physalaemin (4 x 10(-8) mol 1(-1)), which is an analogue of Substance P, a putative mammalian enterohormone and neurotransmitter substance, caused a marked increase in ductal Na transport (in rat as well as rabbit). It is concluded that VIP and GIP would normally play a role in determining salivary electrolyte composition and it is postulated that their action may be antagonized by a tachykinin such as Substance P.

Technique of continuous collection of parotid saliva in the rat

A method is described, which is suitable to collect continuously and quantitatively saliva from both parotid glands in rats under in vivo conditions and which allows to investigate salivary electrolyte composition in relation to flow rate during several hours. This might be of expanding scientific interest in disturbances of transepithelial electrolyte transport in various pathophysiological situations (for instance different forms of experimental hypertension, endstage renal failure and so on).

Selective blockage of the Na+ transport path by Bay g 2821

A series of transport and electrophysiological experiments were carried out on rat salivery duct epithelium to study the mechanism of action of Bay g 2821 on electrolyte transport at the cellular level. On the basis of the reported data it is suggested that Bay g 2821 blocks Na+ entry into the cell at the luminal rather than at the intersitial membrane of the duct cell.

The effect of spironolactone on transport of Na+, K+ and H+. A microperfusion study in rat main submaxillary duct

The epithelium of the main excretory duct of the rat submaxillary gland was used as a target tissue for studies on the effect of a spironolactone on electrolyte transport. The spironolactone decreased net Na+ reabsorption by 27% and net K+ secretion by 23%. HCO-3 was found to be about 2-fold accumulated in the duct lumen, which was considered to result from decreased H+ion secretion. The results can be reconciled with an action of spironolactone on 1) the peritubular Na+-K+-exchange mechanism and 2) the functional coupling of Na+ entry from lumen to cell with K+ and H+ transfer from cell to lumen.

On the mechanism of action of triamterene: effects on transport of Na+, K+, and H+/HCO3- -ions

The rat salivary duct epithelium, which actively transports Na+, K+, and H+/HCO3- in a manner similar to renal distal tubules, was used as a model tissue to study the mechanism of action of triamterene on electrolyte transport. 10(-4) M triamterene completely blocked Na+ resorption and lowered net K+ secretion to half that of controls, whereas HCO3- accumlated in the lumen, probably due to a decrease in H+ secretion. The rates of K+ and H+/HCO3- transport in the presence of triamterene did not differ from those determined after omission of Na+ from the luminal fluid. This was considered to be evidence against a direct action of triamterene on transport of K+ and H+/HCO3-. Triamterene rapidly and reversibly reduced the transepithelial electrical potential difference. This was due to almost complete abolition of Na+ conductance of the luminal membrane at 10(-4) M triamterene, whereas K+ conductance was not altered. Triamterene, administered in vitro from the interstitial side of the isolated duct epithelium was ineffective even at the highest concentrations. The activities of the Na-K-ATPase, the Mg-ATPase and the microsomal HCO3-ATPase were influenced by 10(-4) M triameterene in a similiar fashion. These effects were clearly demonstrated only in the homogenate of the duct tissue and not in intact cells in the isolated duct preparation. Therefore they were considered unspecific. The transport studied demonstrate a primary effect of triamterene on Na+ entry from lumen to cell. Influences on net K+ and H+/HCO3 transport are secondary consequences of functional coupling between movement of Na+ and movement of K+ and H+ across the luminal cell membrane.

Non-specific inhibition of membrane-ATPase by amiloride: a comparative in vivo and in vitro study with ouabain

The submaxillary duct epithelium, which actively transports Na+ (rabbit) and, in addition, K+ and H+/HCO-/3 (rat), was used as a model epithelium to compare the effects of ouabain and amiloride on transport parameters. 1. Ouabain was only effective from the interstitial side, amiloride, however, only from the luminal side. Amiloride induced effects on transport of the ions were seen within less than 1 s, ouabain effects, however, only after minutes. 2. Ouabain inhibited in a parallel fashion the Na+ transport potential and the Na+-K+-ATPase activity. It had no effect on the Mg2+-ATPase and the HCO-/3-ATPase. 3. Amiloride also inhibited the Na+ transport potential and the Na+-K+-ATPase; however, the Na+ transport potential was significantly more sensitive to amiloride than the Na+-K+-ATPase. 4. Amiloride inhibited in a similar fashion the Na+-K+-ATPase, the Mg2+-ATPase and the HCO-/3-ATPase, but did not influence active HCO-/3 secretion. 5. It is concluded that the amiloride induced effects on the membrane ATPases are non-specific.

Evidence for Na+ independent active secretion of K+ and HCO - 3 by rat salivary duct epithelium

In order to elucidate whether or not active secretion of potassium and bicarbonate by the rat submaxillary duct epithelium operates independently of sodium reabsorption, Na+ transport was blocked by amiloride, which is known to inhibit Na+ entry from lumen into cell. With 10(-4) M amiloride in HCO - 3 -Ringer at the luminal side, the transepithelial electrical potential difference approached zero, the Na+ conductance of the luminal cell membrane was drastically reduced, and the K+ conductance was significantly reduced. Net K+ secretion was reduced by 80%, whereas net HCO - 3 secretion was significantly increased. The remaining 20% of net K+ secretion proceeded at zero net Na+ transport and in the absence of significant chemical and electrical potential differences between lumen and interstitium of the duct. This active component of net K+ secretion was accompanied by an equal rate of active HCO - 3 secretion. These findings confirm the independence of this active secretion of K+ and HCO - 3 from Na+ transport. They indicate an electrically neutral secretion of K+ and HCO - 3, probably by the postulated luminal K+ -H+ -exchange mechanism. The 80% of net K+ secretion, which were abolished by amiloride together with Na+ reabsorption, seem to be functionally coupled with Na+ transport. The linkage of K+ -to- Na+ is probably mediated by a luminal carrier exchanging Na+ for K+ and H+.

H+ transport and membrane-bound HCO - 3 ATPase in salivary duct epithelium

An ATPase stimulated by HCO - ions and other oxybases and inhibited by SCN- has been found in main excretory duct of rat submaxillary gland, a tissue, capable of actively secreting HCO - 3 ions. No such ATPase was found in the rabbit duct, which normally does not secrete HCO - 3. The HCO - 3 ATPase was localized in the plasma membrane fraction of the homogenate, as evidenced by the marker 5'-nucleotidase. The activities of the HCO - 3 ATPase increased in metabolic alkalosis and decreased in metabolic acidosis in parallel to secretion of HCO - 3 and K+ ions by the duct epithelium. These findings provide further evidence that the membrane-bound HCO - 3 ATPase is involved in active H+/HCO - 3 transport.

pH and bicarbonate excretion in the rat parotid gland as a function of salivary rate

The bicarbonate concentration in rat parotid saliva increases with increasing flow rates and approximates plasma values at highest salivation. At lowest flow rates the bicarbonate concentration in the secretory fluid markedly exceeds the plasma levels. Intravenous administration of acetazolamide has no influence on the bicarbonate excretion of the parotid gland. Following retrograde application of acetazolamide into the gland duct the concentrations of both bicarbonate and sodium are elevated. The potassium concentrations in final saliva exceed 70 mEq/l at flow rates below 5 mul/min g gland weight. With increasing flow rates a precipitous decrease in potassium concentration below 10 mEq/l occurs. With further increase in flow rate the potassium concentration remains unchanged. The sodium concentrations increased with augmented salivation rate. At lowest flow rates the sodium concentrations showed an increase of modest degree. Our findings can best be explained by the existence of two independent ductular mechanism: a) bicarbonate reabsorption probably in the striated ducts of the parotid gland; b) secretion of potassium with concomitand secretion of bicarbonate in the main excretory duct.

Excretion of trypsin-like activity, electrolytes and protein in mixed and parotid saliva of patients with cystic fibrosis of the pancreas

The esterolytic activity of mixed and parotid saliva in cystic fibrosis (CF) patients and normal subjects was determined using BAEE (alpha-Benzoyl-1-arginine-ethylester) as the substrate. Using soybean-trypsin-inhibitor the trypsin-like activity (TLA) was measured and plotted as a function of parotid flow rate. In addition calcium, protein and pH were determined. Trypsin-like activity in mixed and parotid saliva showed large individual variations in CF and normal children. In parotid saliva we could not find any significant difference, whereas a reduction of TLA in mixed saliva of CF patients was observed. The fact that our normal values fell within the range of heterozygotes reported by Rao et al. (19), makes their hypothesis of a close relationship between reduced TLA and the genetic defect very doubtful. Protein, calcium and pH increased with augmented salivation and no difference between CF patients and normal age matched children could be found except for the pH at a flow rate above 0.75 ml/min per m2 body surface where significantly lower pH values resulted. The relevance of reduced TLA to the pathogenesis of cystic fibrosis is discussed.