Glucose-specific regulation of aldose reductase in human retinal pigment epithelial cells in vitro

PURPOSE

To test the hypothesis that pathophysiological levels of glucose regulate aldose reductase (AR2) gene expression, protein production, and activity in human retinal pigment epithelial (RPE) cells in vitro.

METHODS

Primary cultures of human RPE cells were grown for up to 72 hours in media supplemented with various concentrations of glucose (5, 20, or 75 mM), or in 5 mM glucose containing media supplemented with one of the following: galactose, the transported but nonmetabolized glucose analogue 3-O-methylglucose (3-OMG), or the impermeant hexitol mannitol-so that the final hexose concentrations were equimolar to those of the various glucose concentrations used. Changes in the transcript levels for AR2 mRNA, AR2 protein content, and AR2 enzyme activity were determined. RPE glucose utilization and lactate production were determined in media containing 5 and 20 mM glucose.

RESULTS

Glucose utilization and lactate production increased 4.8-fold and 4.4-fold, respectively, when RPE cells were grown in media containing 20 mM versus 5 mM glucose. Glucose was more effective than any other hexose in the induction of AR2 mRNA or increased AR2 protein expression. When RPE cells were grown in media containing 20 mM mannitol, 3-OMG, or galactose they had lower levels of AR2 mRNA expression than when cells were grown in medium containing 5 mM glucose. RPE cells grown in medium supplemented with 20 or 75 mM galactose did not show a greater increase in AR2 protein expression than cells grown in medium containing 5 mM glucose. Hyperosmotic induction of AR2 mRNA was the same in medium containing 75 mM glucose or 75 mM mannitol, but was at least 50% lower when RPE cells were grown in 75 mM galactose or 3-OMG. CONCLUSIONS. These data indicate that elevations in ambient glucose result in greater metabolism of glucose through glycolysis and polyol metabolism. Induction of AR2 was greatest when RPE cells were grown in pathophysiological concentrations of glucose. Hyperosmolar stress is not a necessary determinant of AR2 mRNA, AR2 protein, or AR2 protein activity in cells that form the outer blood-retinal barrier. Increased facilitative glucose transport or glucose metabolism appears to be requisite for glucose-specific and nonosmotic regulation of AR2 in the RPE cell in vitro.

Sodium, potassium-activated adenosine triphosphatase activity is impaired in the guinea pig pancreatic duct system in streptozotocin-induced diabetes

In patients with type I diabetes mellitus, clinical studies have demonstrated decreased secretion of pancreatic juice by the pancreatic excretory duct system. The cause of this decrease is unknown, but could involve changes in initial signal transduction pathways or one or more of the electrolyte transport components that subserve regulated fluid secretion. We have compared responsiveness to secretin in pancreatic ducts isolated from healthy and diabetic Hartley guinea pigs and also have compared the expression of CFTR and Na+, K(+)-ATPase in these two groups, as the activities of these two proteins are essential for secretion of pancreatic juice. The increases in cyclic AMP levels evoked by exposure to either 0.1 nM or 0.1 microM secretin were not significantly different in pancreatic ducts isolated from healthy and diabetic guinea pigs nor were levels of CFTR or Na+, K(+)-ATPase expression. By contrast, Na+, K(+)-ATPase activity in pancreatic ducts isolated from diabetic guinea pigs was decreased by 70%, suggesting a change in the enzyme's catalytic properties in the diabetic tissues. The observed decrease would be expected to seriously compromise the production of pancreatic juice.

Glucose-specific regulation of aldose reductase in capan-1 human pancreatic duct cells In vitro

Impaired pancreatic duct secretion is frequently observed in insulin-dependent diabetes mellitus (IDDM), although the cellular mechanism(s) of dysfunction remains unknown. Studies in other tissues have suggested that a hyperglycemia-induced decrease in Na, K-ATPase activity could contribute to the metabolic complications of IDDM and that increased polyol metabolism is involved in this response. The present studies examined the effects of glucose on Na, K-ATPase activity and on expression and activity of aldose reductase (AR), a primary enzyme of polyol metabolism, in Capan-1 human pancreatic duct cells. Increasing medium glucose from 5.5 to 22 mM caused a 29% decrease in Na,K-ATPase activity. The decrease was corrected by 100 microM sorbinil, a specific AR inhibitor. Increasing glucose from 5.5 to 110 mM also resulted in concentration-dependent increases in AR mRNA and enzyme activity that could be resolved into two components, one that was glucose specific and observed at pathophysiological concentrations (< 55 mM) and a second that was osmotically induced at high concentrations (> 55 mM) and which was not glucose specific. The present study demonstrates that pathophysiological levels of glucose specifically activate polyol metabolism with a consequent decrease in Na,K-ATPase activity in pancreatic duct epithelial cells, and that this response to hyperglycemia could contribute to decreased pancreatic secretion observed in IDDM. This is the first report of AR regulation in the pancreatic duct epithelium.

Confocal microscopic analysis of intracellular pH regulation in isolated guinea pig pancreatic ducts

pH regulation in isolated guinea pig pancreatic interlobular duct segments loaded with the pH-sensitive fluorophore, 5-(6)-carboxy-SNARF-1-acetoxymethyl ester (SNARF-1), was characterized by laser-scanning confocal microscopy. In HCO3(-)-free medium, intracellular pH (pHi) of duct epithelial cells fell by 0.32 +/- 0.06 pH units in the presence of 0.5 mM amiloride and by 0.36 +/- 0.08 pH units in the absence of Na+. In the presence of extracellular HCO3-, pHi acidified in Na(-)-free medium but not in amiloride-containing medium. Superfusion with Cl(-)-free buffers or with buffers containing 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid produced a cytosolic alkalinization of 0.13-0.22 pH units. These observations demonstrate the presence of Na+/H+ exchange, Na(+)-HCO3- cotransport, and Cl-/HCO3- exchange in guinea pig pancreatic ducts. pHi recovered significantly from an NH4Cl pulse in HCO3(-)-free buffers containing amiloride and carbachol (50.4%) or amiloride and secretin (40.6%). This recovery was blocked by the H(+)-adenosinetriphosphatase (H(+)-ATPase) inhibitor bafilomycin A1 and by preincubation of ducts with nocodazole or cytochalasin D. These observations suggest that a vesicular H(+)-ATPase augments Na(+)-dependent H+ extrusion during agonist-stimulated bicarbonate secretion and that activation of this transport mechanism involves cytoskeletal elements.

Regulation of cyclic AMP levels in guinea pig pancreatic ducts and cultured duct epithelial monolayers

Pancreatic duct bicarbonate secretion is mediated primarily by secretin-induced elevation of intracellular cyclic AMP, although little is known of the effects of other physiological regulators on pancreatic duct cyclic AMP metabolism. We investigated the effects of secretin and several other potential agonists on cyclic AMP levels in isolated guinea pig main and interlobular pancreatic duct segments and in cultured duct epithelial monolayers. Secretin (0.1 microM) caused a five- to eightfold elevation of cyclic AMP in both isolated ducts and cultured monolayers (EC50 = 0.15 nM). Main duct segments, while responsive, were less so than segments of interlobular duct. In isolated duct segments, carbachol, bombesin, cholecystokinin, substance P, calcitonin gene-related peptide, glucagon, insulin, isoproterenol, neurotensin, and prostaglandin E2 did not significantly alter resting or secretin-stimulated cyclic AMP levels. In contrast, 0.1 microM vasoactive intestinal peptide significantly increased cyclic AMP to a level comparable to that evoked by an equal concentration of secretin. Somatostatin significantly attenuated the effects of a submaximal (physiological) dose of secretin on duct cyclic AMP levels without altering resting cyclic AMP levels, suggesting that somatostatin's effects on pancreatic duct fluid secretion are mediated by inhibition of adenylyl cyclase activity.

Insulin, transforming growth factors, and substrates modulate growth of guinea pig pancreatic duct cells in vitro

BACKGROUND & AIMS

Little is known of the physiological mechanisms that control cellular renewal in the pancreatic excretory duct system. This study investigated the effects of potential regulatory substances on the growth of cultured guinea pig pancreatic duct epithelial monolayers.

METHODS

Pancreatic duct explants were cultured for 3 days on plastic and on permeable filters in the presence and absence of different substances. Growth of epithelial monolayers from these explants was measured by 5-bromo-2'-deoxyuridine incorporation and morphometric procedures.

RESULTS

Epidermal growth factor and insulin both enhanced monolayer growth and together had an additive effect. Transforming growth factor alpha enhanced and transforming growth factor beta inhibited growth, whereas glucagon, somatostatin, pancreatic polypeptide, secretin, cerulein, bombesin, and dexamethasone had no significant effects. Monolayer growth on type 1 collagen-coated filters was enhanced when compared with that of monolayers grown on tissue culture plastic. Cell growth from explants on filters coated with type IV collagen and fibronectin was comparable with that on plastic, whereas growth on Matrigel- or laminin-coated filters was reduced.

CONCLUSIONS

Insulin, transforming growth factors, and substrate components modulate growth of pancreatic duct epithelial cells in vitro, suggesting that they are important regulators of cell division in the excretory duct system of the intact pancreas.

Regulation of goblet cell degranulation in isolated pancreatic ducts

Neurohumoral control of goblet cell degranulation in isolated segments of the guinea pig main pancreatic duct was examined using morphometric procedures. Goblet cells represent 25-30% of the epithelial cell population at the head of the main pancreatic duct, a percentage that decreases to 5-10% as the distance from the ampulla increases. Carbachol, bombesin, and vasoactive intestinal peptide (VIP) each stimulated degranulation of duct goblet cells, although cholecystokinin octapeptide, secretin, and histamine did not. The stimulatory effects of carbachol on goblet cell degranulation in isolated pancreatic ducts were blocked by atropine and enhanced by simultaneous exposure to VIP. These observations indicate that goblet cells in guinea pig pancreatic ducts express bombesin, VIP, and muscarinic cholinergic receptors and that multiple intracellular signaling pathways are involved in the regulation of goblet cell degranulation.

Intracellular mediators of goblet cell degranulation in isolated pancreatic ducts

The involvement of particular intracellular signalling pathways in agonist-evoked degranulation of guinea pig pancreatic duct goblet cells was investigated. Carbachol, vasoactive intestinal peptide (VIP), calcium ionophore A23187, phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA), cyclic AMP analogue Sp-5,6-DCl-cBIMPS and forskolin each caused degranulation of goblet cells in isolated ducts. Degranulation induced by carbachol was not inhibited by okadaic acid, cytochalasin-D or nocodazole. These results indicate that at least two major signalling pathways are involved in pancreatic duct goblet cell secretion.

Downregulation of hepatocyte P2-purinoceptor binding capacity after trauma-hemorrhage/resuscitation

Although P2-purinoceptors play an important role in the regulation of liver metabolism under normal conditions, it is not known if trauma-hemorrhage and resuscitation have any effects on such receptors. To study this, we performed a 5-cm midline laparotomy (i.e., trauma induced) on rats and then bled them to and maintained them at a mean arterial pressure of 40 mmHg until 40% of maximum bleedout volume was returned in the form of Ringer lactate (RL). The animals were then resuscitated with 3x the volume of shed blood with RL over 45 min followed by 2x RL over 95 min. Hepatocytes were isolated at the time of maximum bleedout or at 0, 4, 17, and 27 h after the completion of crystalloid resuscitation. P2-purinoceptor binding characteristics were determined in the isolated hepatocytes by using [alpha-35S]ATP. Scatchard analysis revealed high- and low-affinity components of P2-purinoceptors in hepatocytes from sham-operated as well as hemorrhaged and resuscitated animals. The maximum binding capacity (Bmax) of the high-affinity receptor component decreased at the time of maximum bleedout and at 4, 17, and 27 h after resuscitation. In addition to this, the Bmax of low-affinity receptor components also decreased at 4-27 h after resuscitation. In contrast, the dissociation constants of both receptor components were not altered. Because hemorrhagic shock produces abnormalities in glucose metabolism, the downregulation of hepatocyte P2-purinoceptor Bmax may be responsible for the altered glucose homeostasis under such conditions.

ATP-MgCl2 treatment after trauma-hemorrhage/resuscitation increases hepatocyte P2-purinoceptor binding capacity

Although our studies indicate that P2-purinoceptor binding capacity decreases after hemorrhage and resuscitation, it is not known whether ATP-MgCl2 administration after hemorrhage has any beneficial effects on the receptor dynamics. To study this, we performed laparotomy (i.e., trauma induced) on rats and bled them to and maintained them at a mean arterial pressure of 40 mmHg until 40% of maximum bleedout volume was returned in the form of Ringer lactate (RL). The animals were then resuscitated with 3 times the volume of maximum bleedout with RL over 45 min followed by 2 times RL along with ATP-MgCl2 (50 mumol/kg body wt) over 95 min. Hepatocytes were isolated at 4, 17, and 27 h after resuscitation. P2-purinoceptor binding characteristics were determined by using [alpha-35S]ATP. Scatchard analysis revealed high-affinity and low-affinity receptor components in the hepatocytes isolated from sham-operated or hemorrhaged animals with or without ATP-MgCl2 infusion. ATP-MgCl2 ameliorated and subsequently restored the decreased maximum binding capacity (Bmax) of the high-affinity receptor component and significantly improved Bmax of the low-affinity receptor component. ATP-MgCl2 administration also produced a progressive enhancement in the affinity of the low-affinity receptor component. Thus the beneficial effects of ATP-MgCl2 observed after trauma-hemorrhage and resuscitation may be, in part, due to the restoration of P2-purinoceptor binding capacity and the enhancement of the receptor affinity.

Muscarinic receptors in isolated guinea pig pancreatic ducts

Biochemical and pharmacological characteristics of muscarinic cholinergic receptors in isolated guinea pig pancreatic ducts were determined in the present study. Duct homogenates bound 6.82 +/- 0.69 fmol of [3H]N-methylscopolamine ([3H]NMS)/micrograms of DNA with a Kd of 0.73 +/- 0.05 nM. The density of [3H]NMS binding sites in the excretory ducts was seven times greater than that in acini from the same pancreases. Competition binding studies with atropine, pirenzepine, 11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H- pyrido[2,3-b] [1,4]benzodiazepine-6-one (AF-DX 116), and 4-diphenylacetoxy-N-methyl piperidine methiodide (4-DAMP) indicated that both M2 and M3 subtypes of muscarinic receptors are present in these preparations of isolated pancreatic ducts. Electrophoretic analysis of [3H]propylbenzilylcholine mustard-labeled unreduced and reduced duct muscarinic receptors provided molecular mass estimates of 62.6 +/- 2.5 and 58.0 +/- 1.6 kDa, respectively. Deglycosylation of ductal muscarinic receptors with N-glycanase decreased their apparent molecular mass by approximately 4 kDa. These results demonstrate that isolated pancreatic ducts express both M2 and M3 muscarinic receptors, with the former subtype predominating.

Overview of pancreatic duct physiology and pathophysiology

The epithelium of the excretory duct system of the exocrine pancreas secretes bicarbonate ions and mucins. Epithelial cells of the duct system also constitute primary sites of dysfunction in cystic fibrosis, pancreatitis and pancreatic cancer. The present work provides an overview of the current state of understanding of the physiology and pathophysiology of the pancreatic duct system and suggests approaches that will provide continued progress in exploration of the basic physiological processes operating in this tissue.

Mechanism of agonist-induced downregulation of muscarinic receptors in rat pancreatic acini

Exposure of pancreatic acini to cholinergic agonists reduces their muscarinic acetylcholine receptor content. To investigate the mechanism of this reduction, rat pancreatic acini were cultured in the presence of carbachol and its effects on binding of [N-methyl-3H] scopolamine (NMS), which labels only cell surface receptors, and [3H]scopolamine, which also accesses intracellular receptors, were determined. Carbachol (0.1 mM) caused disappearance of 90% of binding sites for [3H]NMS and [3H]-scopolamine with half-time values of 3.9 +/- 0.4 and 5.7 +/- 0.7 h, respectively. Nocodazole and cytochalasin D, ouabain, ionophore A23187, 12-O-tetradecanoylphorbol 13-acetate (TPA), bombesin, cholecystokinin octapeptide (CCK-8), secretin, and vasoactive intestinal peptide (VIP) had no effect on the carbachol-induced decrease in binding sites for either antagonist, although staurosporine and the calmodulin inhibitor W-7 each caused slight inhibition. By contrast, the lysosomotropic agents methylamine and ammonium chloride caused 80% inhibition of disappearance of binding sites for [3H]scopolamine, although they only slightly inhibited disappearance of [3H]NMS binding sites. These results implicate the endosomal/lysosomal pathway in cholinergic agonist-evoked downregulation of muscarinic receptors in the pancreatic acinar cell.

Effects of cycloheximide and tunicamycin on cell surface expression of pancreatic muscarinic acetylcholine receptors

The importance of glycosylation in cell surface expression of muscarinic receptors in cultured guinea pig pancreatic acini was investigated. Recovery of the muscarinic receptor population after carbachol-induced down regulation was blocked by cycloheximide but not by tunicamycin, although tunicamycin reduced [3H]mannose incorporation into acinar macromolecules by up to 90%. Tunicamycin treatment also failed to alter carbachol stimulation of amylase secretion from cultured acini. These results indicate that glycosylation of the glandular subtype of muscarinic receptor in the pancreatic acinar cell is not necessary for its insertion in the plasma membrane or for its functional activity.

Secretagogue effects on intracellular calcium in pancreatic duct cells

Regulation of intracellular free calcium ([Ca2+]i) in single epithelial duct cells of isolated rat and guinea pig pancreatic interlobular ducts by secretin, carbachol and cholecystokinin was studied by microspectrofluorometry using the Ca2(+)-sensitive, fluorescent probe Fura-2. Rat and guinea pig duct cells exhibited mean resting [Ca2+]i of 84 nM and 61 nM, respectively, which increased by 50%-100% in response to carbachol stimulation, thus demonstrating the presence of physiologically responsive cholinergic receptors in pancreatic ducts of both species. The carbachol-induced increase in [Ca2+]i involved both mobilization of Ca2+ from intracellular stores and stimulation of influx of extracellular Ca2+. In contrast, neither cholecystokinin nor secretin showed reproducible or sizeable increases in [Ca2+]i. Both rat and guinea pig duct cells showed considerable resting Ca2+ permeability. Lowering or raising the extracellular [Ca2+]i led, respectively, to a decrease or increase in the resting [Ca2+]i. Application of Mn2+ resulted in a quenching of the fluorescence signal indicating its entry into the cell. The resting Ca2+ and Mn2+ permeability could be blocked by La3+ suggesting that it is mediated by a Ca2+ channel.

Regulation of pancreatic duct epithelial growth in vitro

Effects of a number of possible trophic factors on growth of guinea pig pancreatic duct epithelial monolayers were investigated. Isolated fragments of main and interlobular ducts were prepared and explanted onto both tissue culture plastic and thick gels of type I collagen. Monolayers growing out from explants were first cultured in a basal medium for 3 or 4 days. Next, the medium was supplemented individually with bombesin, carbachol, caerulein, epidermal growth factor (EGF), secretin, 12-O-tetradecanoylphorbol 13-acetate (TPA), or vasoactive intestinal peptide (VIP). Cells were cultured in the absence or presence of these possible trophic factors, and monolayer areas were determined morphometrically at 0, 2, and 4 days. Rate of growth was determined from increase in area over each 2-day period. Monolayers grown in basal medium alone on plastic increased to 479% of initial area over the 4-day test period; those grown on collagen increased to 523%. Explants cultured in presence of bombesin, carbachol, caerulein, secretin, TPA, and VIP on either substrate grew at rates not significantly different from those cultured in basal medium. By contrast, duct monolayers grown on plastic or collagen in presence of 10 nM EGF expanded in area to 722 and 1,070%, respectively, of their initial areas. The EC50 for this trophic effect was approximately 1 nM. These results show that EGF exerts a potent trophic effect on guinea pig pancreatic duct cells in vitro but also indicate that cell division in the pancreatic main and interlobular ducts is not regulated by caerulein and related peptide hormones that have been reported to have growth-promoting effects on exocrine pancreas in vivo.

Effects of staurosporine on protein kinase C and amylase secretion from pancreatic acini

The effects of staurosporine, a recently isolated microbial alkaloid, on amylase secretion and protein kinase C activity of guinea pig pancreatic acini were investigated. Staurosporine at a concentration of 1 microM completely inhibited both acinar protein kinase C activity (IC50 = 5.5 +/- 1.4 nM) and amylase secretion induced by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) (IC50 = 4.1 +/- 0.4 nM). At this concentration, staurosporine reduced amylase secretion elicited by maximally effective concentrations of carbachol and cholecystokinin by approximately 50% but did not appreciably alter the potencies of the two secretagogues. In the presence of staurosporine, amylase secretion induced by carbachol was linear for at least 60 min. Staurosporine had no effect on amylase release elicited by the Ca2+ ionophore A23187. It did, however, inhibit secretion induced by vasoactive intestinal peptide, although with a reduced potency relative to its effects on amylase release stimulated by TPA, carbachol, and cholecystokinin (IC50 = 34 +/- 17 nM). These results indicate that staurosporine is a potent inhibitor of protein kinase C activity in pancreatic acini and that protein kinase C has an important role as an intracellular mediator of digestive enzyme secretion induced by cholecystokinin and carbachol in the acinar cell. In addition, a separate staurosporine-insensitive coupling pathway, most likely involving Ca2+, appears to be equally important and can maintain long-term secretion in the absence of functional protein kinase C activity.

Muscarinic receptor desensitization and downregulation induced by full and partial cholinergic agonists in rat pancreatic acini

Preincubation of rat pancreatic acini with carbachol, a full cholinergic agonist, for 30 min at 37 degrees C caused a several-fold decrease in the potency of this agonist to elicit amylase release from acini in a subsequent 30-min incubation, but did not reduce its maximal efficacy. Acini preincubated with carbachol were slightly less responsive to cholecystokinin, which utilizes the same intracellular messengers, but were almost completely unresponsive to the partial cholinergic agonist pilocarpine in a second incubation. In contrast, preincubation of acini with pilocarpine for up to 120 min at 37 degrees C failed to alter the subsequent secretory response either to itself or to carbachol. Incubation of pancreatic acini for up to 24 h in a hormonally defined culture medium containing either carbachol or pilocarpine caused a time- and concentration-dependent decrease in cell surface muscarinic cholinergic receptors, as assessed by binding of the labeled muscarinic antagonist [3H]N-methylscopolamine. These results suggest that cholinergic desensitization of secretory responsiveness in the pancreatic acinus is caused by a change in receptor properties distinct from that which elicits removal of the occupied receptor from the cell surface.

Monoclonal antibodies as probes for plasma membrane domains in the exocrine pancreas

Monoclonal antibodies (mAb) were generated as probes for the plasma membrane domains of pancreatic acinar cells. Primary monolayer cultures of mouse pancreatic acinar cells, which have an expanded apical surface relative to normal pancreas, were used to immunize rats. With conventional immunization and fusion protocols, 3% of the hybridomas were positive against the acinar lumen by indirect immunofluorescence of mouse pancreas cryosections. Culturing of spleen cells from an immunized rat on the apical surface of acinar cell monolayer cultures before fusion with the myeloma (an in vitro boost) doubled the percentage of hybridomas producing apical membrane-specific mAb. Monoclonal antibodies were characterized by immunofluorescence, ultrastructural immunoperoxidase cytochemistry, immunoprecipitation, and immunoblotting. One antibody, acinar-1 (IgG2a), labeled the apical membranes of pancreatic acinar cells, hepatocytes, salivary and lacrimal gland acinar cells, and the brush border of small intestine enterocytes. This mAb precipitated and blotted a protein of 94 KD. Acinar-2 (IgM) also labeled pancreatic acinar cell apical membranes but did not label other tissues and did not precipitate or blot. Acinar-3 labeled pancreatic acinar cell lateral membranes. Duct-1 (IgM) labeled pancreatic duct apical membrane and ducts in liver and salivary glands but did not precipitate or blot. These domain-specific mAb demonstrate that common antigenic determinants occur in the apical surfaces of several exocrine epithelia and may be important in secretion.

Isolation and monolayer culture of guinea pig pancreatic duct epithelial cells

Monolayers of cultured epithelial cells have been prepared from fragments of guinea pig pancreatic excretory ducts isolated by a simple procedure employing collagenase digestion and manual selection, through which virtually all of the ductal system can be recovered. The isolated fragments were cultured in enriched Waymouth's medium on extracellular matrices of various composition and thickness, including: thin (less than 5 micron) and thick (0.5 mm) layers of rat tail collagen; thin layers of human placental collagen; thin layers of Matrigel (a reconstituted basement membrane material); uncoated tissue culture plastic; and the cellulose ester membranes of Millipore Millicells. Cells spread rapidly from duct fragments cultured on uncoated plastic or on plastic coated with thin layers of rat tail collagen or human placental collagen and formed epithelial monolayers. However, these cells were squamous and lacked the abundant basolateral membrane amplification and apical microvilli characteristic of freshly isolated duct epithelial cells. Cells did not spread from duct fragments cultured on Matrigel. In contrast, when fragments of pancreatic ducts were explanted onto either a thick layer of rat tail collagen or onto Millicell membranes, cells readily spread and formed confluent monolayers of cuboidal epithelial cells characterized by abundant mitochondria, apical microvilli, and basolateral plasma membrane elaboration. These results demonstrate that different forms of extracellular matrix modulate the growth and differentiation of pancreatic duct epithelial cells, and that culture on a permeable substrate markedly enhances the maintenance of differentiated characteristics in this cell type. The monolayers formed on Millicell membranes should provide a useful model system for physiologic analysis of the regulation of electrolyte secretion by this epithelium.

Downregulation of protein kinase C in guinea pig pancreatic acini: effects on secretion

Pretreatment of guinea pig pancreatic acini with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) induced a time- and concentration-dependent down-regulation of protein kinase C. In control acini almost all of the protein kinase C activity was present in a cytosolic fraction. Incubation with TPA initially shifted protein kinase C activity to a particular fraction which then disappeared over the following 24-h incubation with TPA. To study the role of protein kinase C in stimulus-secretion coupling, acini were pretreated with TPA and then amylase release was studied in response to various secretagogues. Preincubation of acini with TPA led to a time- and concentration-dependent decrease in TPA-stimulated amylase release that correlated with protein kinase C downregulation. Preincubation of acini with 1 microM TPA for 24 h, resulting in complete loss of protein kinase C activity, abolished the secretory effect of subsequently added TPA. By contrast, the secretory effects of cholecystokinin octapeptide (CCK-8) and carbamylcholine chloride (CCh) were only inhibited by 44 and 34%, respectively, and amylase release stimulated by the Ca2+ ionophore A23187 and an adenosine 3',5'-cyclic monophosphate-mediated agonist, vasoactive intestinal peptide, was unaffected. Dose-response curves for CCK-8- or CCh-stimulated amylase release in TPA-pretreated acini revealed attenuation of both maximal efficacy and sensitivity. However, the CCh-stimulated intracellular Ca2+ increase as determined by use of the fluorescent probe fura-2 was not affected by the long-term TPA pretreatment of acini. This study strongly suggests that both protein kinase C and intracellular Ca2+ play a significant role in CCK-8- and CCh-stimulated amylase release.

Phorbol esters and A23187 regulate Na+-K+-pump activity in pancreatic acinar cells

To clarify the subcellular mechanisms that mediate stimulation of Na+-K+-pump activity in pancreatic acinar cells by cholinergic agonists, we examined the effects of the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA) and the Ca2+ ionophore A23187 on [3H]ouabain binding to dispersed guinea pig pancreatic acinar cells under conditions in which binding reflects the average rate of pump cycling. The phorbol ester more than doubled Na+-K+-pump activity as did the diacylglycerol analogue, 1-oleoyl-2-acetolyl-sn-3-glycerol. A23187 increased pump activity by a maximum of 31% at 0.3 microM but was progressively inhibitory at higher concentrations. The stimulatory effects of TPA and A23187 were additive, although either secretagogue elicited a less than additive response when added together with a maximally effective concentration of the cholinergic agonist, carbachol. Removal of extracellular Ca2+ had little effect on the pump response to TPA and did not reduce the maximal effect of A23187 but abolished the inhibitory effect seen at high ionophore concentrations in Ca2+-containing medium. These results indicate that both Ca2+ and protein kinase c are involved in regulating Na+-K+-pump activity in the pancreatic acinar cell.

Regulation of muscarinic acetylcholine receptors in cultured guinea pig pancreatic acini

Regulation of muscarinic receptors in cultured guinea pig pancreatic acini was investigated by assessing the effects of cholinergic agonists on binding of [N-methyl-3H]scopolamine [( 3H]NMS) and on amylase release. Freshly dispersed acini bound [3H]NMS with a Kd of 74 pM and a maximal binding level (Bmax) of 908 fmol/mg DNA. Carbachol (CCh) stimulated amylase secretion and inhibited [3H]NMS binding. Incubation of acini for 30 min with 0.1 mM CCh decreased the subsequent efficacy of CCh in stimulating amylase release by threefold but had no effect on its potency. In contrast, amylase release in response to cholecystokinin octapeptide (CCK-8) was not altered by CCh preincubation. [3H]NMS binding to acini was decreased only 15-20% after 30-min incubation with CCh. However, culture of acini with 0.1 mM CCh decreased [3H]NMS binding by 50% at 3-4 h and by 85-90% at 24 h. This decrease was attributable primarily to a reduction in Bmax. [3H]NMS binding also was decreased to a similar extent by the cholinergic agonists bethanechol and methacholine but not by other secretagogues. The decrease in antagonist binding induced by CCh was dose dependent, with the IC50, 5.8 microM, approximating the EC50 for amylase release, 4.3 microM. Culture of acini for 24 h with CCh abolished subsequent amylase release in response to CCh but not to CCK-8. When CCh was removed from the culture medium after 24 h and acini recultured in its absence, [3H]NMS binding increased with a half-time for recovery of 20-24 h; this recovery was blocked by cycloheximide.(ABSTRACT TRUNCATED AT 250 WORDS)

Antimuscarinic effects of chloroquine in rat pancreatic acini

Chloroquine inhibited carbachol-induced amylase release in a dose-dependent fashion in rat pancreatic acini; cholecystokinin- and bombesin-induced secretory responses were almost unchanged by the antimalarial drug. The inhibition of carbachol-induced amylase release by chloroquine was competitive in nature with a Ki of 11.7 microM. Chloroquine also inhibited [3H]N-methylscopolamine binding to acinar muscarinic receptors. The IC50 for chloroquine inhibition of [3H]N-methylscopolamine binding was lower than that for carbachol or the other antimalarial drugs, quinine and quinidine. These results demonstrate that chloroquine is a muscarinic receptor antagonist in the exocrine pancreas.

Regulation of protein phosphorylation in pancreatic acini. Distinct effects of Ca2+ ionophore A23187 and 12-O-tetradecanoylphorbol 13-acetate

Regulation of protein phosphorylation in isolated pancreatic acini by the intracellular messengers Ca2+ and diacylglycerol was studied by using the Ca2+ ionophore A23187 and the tumour-promoting phorbol ester 12-O-tetradecanoylphorbol 13-acetate. As assessed by two-dimensional polyacrylamide-gel electrophoresis, the phorbol ester (1 microM) and Ca2+ ionophore (2 microM) altered the phosphorylation of distinct sets of proteins between Mr 83,000 and 23,000 in mouse and guinea-pig acini. The phorbol ester increased the phosphorylation of four proteins, whereas the ionophore increased the phosphorylation of two proteins and, in mouse acini, decreased the phosphorylation of one other protein. In addition, the phorbol ester and ionophore each caused the dephosphorylation of two proteins, of Mr 20,000 and 20,500. Administered together, these agents reproduced the changes in phosphorylation induced by the cholinergic agonist carbamoylcholine. The effects of the phorbol ester and ionophore on acinar amylase release were also studied. In mouse pancreatic acini, a maximally effective concentration of phorbol ester (1 microM) produced a secretory response that was only 28% of that produced by a maximally effective concentration of carbamoylcholine, whereas the ionophore (0.3 microM) stimulated amylase release to two-thirds of the maximal response to carbamoylcholine. In contrast, in guinea-pig acini, the phorbol ester and carbamoylcholine evoked similar maximal secretory responses, whereas the maximal secretory response to the ionophore was only 35% of that to carbamoylcholine. Combination of phorbol ester and ionophore resulted in a modest synergistic effect on amylase release in both species. It is concluded that cholinergic agonists act via both diacylglycerol and Ca2+ to regulate pancreatic protein phosphorylation, but that synergism between these intracellular messengers is of limited importance in stimulating enzyme secretion.

Neuroendocrine control of secretion in pancreatic and parotid gland acini and the role of Na+,K+-ATPase activity

The results of our investigations into the localization of Na+,K+-pump activity in pancreatic and parotid acinar cells and the effects of hormones and neurotransmitters on pump turnover can be integrated with data on other aspects of stimulus-response coupling to construct models of the neurohumoral control of protein, fluid, and electrolyte secretion (Fig. 23). In both tissues, Ca2+ and cyclic AMP serve as intracellular messengers. In pancreatic acinar cells, the Ca2+-dependent pathway activated by the occupation of CCK or cholinergic receptors provides the primary stimulus for digestive enzyme secretion. Cyclic AMP plays a comparatively minor role; VIP and secretin are much less effective stimulators of protein secretion. Conversely, cyclic AMP levels in parotid acinar cells, which are modulated primarily through occupation of beta-adrenergic receptors, are a major determinant of enzyme secretion. Activation of the Ca2+-dependent pathway by cholinergic or alpha-adrenergic agonists or substance P is less important. The presence of dual control processes in each gland suggests that the observed differences in effectiveness of cyclic AMP- versus Ca2+-dependent secretagogues may reflect not different mechanisms, but rather a shift in the relative emphasis placed on each pathway. This emphasis could conceivably result from subtle variations in the interaction between cellular protein kinases and phosphatases and their phosphoprotein substrates. Electrolyte secretion, on the other hand, appears to involve both discrete and common entities. In pancreatic acinar cells from rodent species, cholinergic or CCK receptor occupancy elicits a Ca2+-dependent increase in the open-state probability of nonselective cation channels in the basolateral plasma membrane. The resultant influx of Na+ and efflux of K+ is most probably the factor which activates Na+, K+-pumps. Based on electron probe studies of the effects of cholinergic agonists on acinar cell Na+ and K+ contents discussed earlier, a transient reduction in the intracellular K+/Na+ ratio of up to 4-fold may occur. A shift of this magnitude in the cytoplasmic microenvironment of the Na+, K+-pump clearly would have a stimulatory influence (see discussion by Jorgensen, 1980). In addition, Ca2+ itself may have direct effects on Na+,K+-pump activity. Calcium at levels much above 1 microM progressively inhibits Na+,K+-ATPase activity (Tobin et al., 1973; Yingst and Polasek, 1985). In unstimulated guinea pig pancreatic acinar cells, Ca2+i measured by quin-2 fluorescence was 161 +/- 13 nM (Hootman et al., 1985a) which increased to a maximal concentration of 803 +/- 122 nM following CCh stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular mediators of Na+-K+ pump activity in guinea pig pancreatic acinar cells

The involvement of Ca2+ and cyclic nucleotides in neurohormonal regulation of Na+-K+-ATPase (Na+-K+ pump) activity in guinea pig pancreatic acinar cells was investigated. Changes in Na+-K+ pump activity elicited by secretagogues were assessed by [3H]ouabain binding and by ouabain-sensitive 86Rb+ uptake. Carbachol (CCh) and cholecystokinin octapeptide (CCK-8) each stimulated both ouabain-sensitive 86Rb+ uptake and equilibrium binding of [3H]ouabain by approximately 60%. Secretin increased both indicators of Na+-K+ pump activity by approximately 40% as did forskolin, 8-bromo- and dibutyryl cAMP, theophylline, and isobutylmethylxanthine. Incubation of acinar cells in Ca2+-free HEPES-buffered Ringer (HR) with 0.5 mM EGTA reduced the stimulatory effects of CCh and CCK-8 by up to 90% but caused only a small reduction in the effects of secretin, forskolin, and cAMP analogues. In addition, CCh, CCK-8, secretin, and forskolin each stimulated ouabain-insensitive 86Rb+ uptake by acinar cells. The increase elicited by CCh and CCK-8 was greatly reduced in the absence of extracellular Ca2+, while that caused by the latter two agents was not substantially altered. The effects of secretagogues on free Ca2+ levels in pancreatic acinar cells also were investigated with quin-2, a fluorescent Ca2+ chelator. Basal intracellular Ca2+ concentration ([Ca2+]i) was 161 nM in resting cells and increased to 713 and 803 nM within 15 s after addition of 100 microM CCh or 10 nM CCK-8, respectively. Forskolin, secretin, and cAMP analogues had no effect on [Ca2+]i, nor did they either reduce or potentiate the rise in [Ca2+]i evoked by CCh.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscarinic acetylcholine receptor structure in acinar cells of mammalian exocrine glands

Characterization of muscarinic acetylcholine receptors in acinar cells from rat pancreas and lacrimal and parotid glands was achieved by binding of the reversible muscarinic antagonist [3H]quinuclidinyl benzilate (QNB) and the specific alkylating reagent [3H]propylbenzilylcholine mustard (PrBCM) to intact acini or dispersed acinar cells. Binding studies with [3H]QNB showed that acinar cells from pancreas contain 26,400, from parotid 21,400, and from lacrimal gland 25,700 binding sites/cell. To assess molecular size of the receptor in each gland, acini were prepared by digestion with purified collagenase and singly dispersed acinar cells were prepared by a combination of digestion with crude collagenase, hyaluronidase, and alpha-chymotrypsin and divalent cation chelation using EDTA. Muscarinic receptors on acini or dispersed cells were covalently labeled with 5 nM [3H]PrBCM, solubilized directly in hot sodium dodecyl sulfate buffer, and resolved by polyacrylamide gel electrophoresis. When solubilized acini were electrophoresed, a major labeled peak was observed on gels along with a smaller peak of lower apparent molecular weight. For pancreatic acini, the apparent molecular weights of these peaks were 117,600 and 85,700; for parotid acini, 104,800 and 74,500; and for lacrimal acini, 87,200 and 63,100. Addition of muscarinic antagonists to the labeling medium abolished both peaks. When dispersed acinar cells were labeled, the larger peak was eliminated, and all radioactivity was concentrated in a single peak: 87,600 for pancreas, 78,000 for parotid gland, and 62,800 for lacrimal gland. Digestion of prelabeled acini with the mixture of enzymes used to produce dispersed acinar cells similarly shifted all radioactivity into this second peak. Limited digestion of acini or dispersed cells with 1 mg/ml of papain resulted in the disappearance of these higher molecular weight peaks and the appearance of a broad peak at Mr = 40,000. Cells of nonepithelial origin, IM-9 lymphocytes and NG108 neuroblastoma X glioma hybrids, also were labeled with [3H]PrBCM and electrophoresed.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium-potassium pump in guinea-pig parotid gland: secretagogue stimulation of ouabain binding to dispersed acini

Dispersed acini were prepared from guinea-pig parotid glands and incubated at 37 degrees C in HEPES-buffered Ringer (HR) containing [3H]ouabain. Acini bound the Na+-K+-pump inhibitor with an estimated equilibrium dissociation constant (Kd) of 2.05 microM and a capacity of 2.9 X 10(6) ouabain-binding sites/cell. Carbachol and adrenaline each increased the equilibrium level of binding attained at a medium [3H]ouabain concentration of 0.1 microM by up to 250%, but had no effect on binding at a medium ouabain concentration of 10 microM, a near-saturating level of the glycoside. These results indicate that the two secretagogues elicit increases in Na+-K+-pump activity without increasing the number of pump sites available. The observed stimulation by carbachol was readily reversed by atropine and that evoked by adrenaline was reversed by phentolamine. Incubation of acini in Ca2+-free HR with 0.2 mM-EGTA did not alter [3H]ouabain binding in the absence of agonists, but decreased by over 80% the response to both carbachol and adrenaline. The full response to either could be restored by adding 1.5 mM-Ca2+ to the Ca2+-free medium after challenge with agonists. The response to a maximally effective dose of carbachol could not be augmented by adrenaline or vice versa in either the presence or absence of extracellular Ca2+. Carbachol and adrenaline also stimulated the ouabain-sensitive component of acinar oxygen uptake by 230-260%, but had no significant effect on ouabain-insensitive respiration. In the absence of extracellular Ca2+, stimulation of oxygen uptake by either agonist was reduced by over 80% and the stimulatory effects disappeared within 5 min. We conclude that alterations in equilibrium level of ouabain bound at 0.1 microM-[3H]ouabain and in ouabain-sensitive oxygen uptake by guinea-pig parotid acini both reflect changes in acinar Na+-K+-pump activity. Muscarinic cholinergic and alpha-adrenergic receptor occupancy strongly stimulates pump activity in a Ca2+-dependent manner. Na+-K+ pumps are the primary energy-requiring ion-transport component that is activated by these secretagogues.

Amylase secretion by isolated pancreatic acini after acute cholecystokinin treatment in vivo

A single dose of synthetic cholecystokinin octapeptide (CCK8, 5 micrograms/kg) in a depot carrier was injected subcutaneously into rats 2 and 14 h before the removal of the pancreas and the preparation of isolated pancreatic acini. CCK8 treatment induced no significant change in body weight or total amount of pancreatic DNA, but pancreatic weight, total pancreatic protein and amylase, and the concentration of amylase and total protein relative to DNA were significantly decreased. In acini prepared from CCK8-pretreated rats, responsiveness to maximal and supramaximal concentrations of CCK8 was significantly increased, irrespective of whether the amount of amylase released was expressed relative to DNA or calculated as a percentage of the acinar content. The dose-response curves for CCK8 were similarly shaped in both CCK8-pretreated and control rats but shifted threefold toward higher concentrations of CCK8 2 or 14 h after CCK8 treatment. Specific 125I-CCK binding was significantly increased only for high-affinity binding sites. Although these observations suggest that alterations in pancreatic amylase release could be due to changes at the cholecystokinin receptor, the secretory responsiveness to maximal and supramaximal concentrations of carbachol was also increased without any change in the sensitivity. Moreover, in contrast to the cholecystokinin receptor, there was no change in the number of muscarinic receptors or in their affinity for either agonists or antagonists measured with [3H]quinuclidinyl benzilate.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscarinic stimulation of phospholipid turnover in dissociated avian salt gland cells

Addition of carbamylcholine to 32P-prelabeled dissociated avian salt gland cells resulted in increased turnover of phosphatidic acid, phosphatidylinositol, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate, which could be prevented by the inclusion of atropine. Carbamylcholine had no discernable effect on protein phosphorylation, measured either in the total preparation or in subcellular fractions. It is concluded that for the avian salt gland, no obligatory link is indicated between protein phosphorylation and either phospholipid turnover or salt secretion.

Characterization of muscarinic acetylcholine receptors in the avian salt gland

Electrolyte and fluid secretion by the avian salt gland is regulated by activation of muscarinic acetylcholine receptors (R). In this study, these receptors were characterized and quantitated in homogenates of salt gland from domestic ducks adapted to conditions of low (freshwater, FW) and high (saltwater, SW) salt stress using the cholinergic antagonist [3H]-quinuclidinyl benzilate (QNB). Specific binding of the antagonist to receptors in both FW- and SW-adapted glands reveals a single population of high affinity binding sites (KdFW = 40.1 +/- 3.0 pM; KdSW = 35.1 +/- 2.1 pM). Binding is saturable; RLmaxFW = 1.73 +/- 0.10 fmol/micrograms DNA; RLmaxSW = 4.16 +/- 0.31 fmol/micrograms DNA (where L is [3H]QNB and RL the high affinity complex). Calculated average cellular receptor populations of 5,800 sites/cell in FW-adapted glands and 14,100 sites/cell in SW-adapted glands demonstrate that upward regulation of acetylcholine receptors in the secretory epithelium follows chronic salt stress. The receptor exhibits typical pharmacological specificities for muscarinic cholinergic antagonists (QNB, atropine, scopolamine) and agonists (oxotremorine, methacholine, carbachol). In addition, the loop diuretic furosemide, which interferes with ion transport processes in the salt gland, competitively inhibits [3H]QNB binding. Preliminary studies of furosemide effects on [3H]QNB binding to rat exorbital lacrimal gland membranes showed a similar inhibition, although the diuretic had no effect on antagonist binding to rat brain or atrial receptors.

Effect of methacholine on Na+ pump activity and ion content of dispersed avian salt gland cells

The effects of the cholinergic agonist methacholine chloride (MCh) on cellular ion content and Na+ pump activity of dissociated duck salt gland cells were studied. Dispersed salt gland cells regulate intracellular ion levels in a ouabain-sensitive manner. MCh (0.5 mM) caused no detectable change in cell Na+ levels over the first 10 min of exposure of cells to the agonist but elicited decreases of 23 and 13%, respectively, in intracellular Cl- and K+ content. The rate of turnover of salt gland cell plasmalemmal Na+ pumps, as measured by [3H]ouabain binding to the dissociated cells, was markedly stimulated by 0.5 mM MCh, although the total number of binding sites at equilibrium remained unchanged. Replacement of medium Na+ with choline abolished the MCh-stimulated increase in ouabain binding but had no effect on the rate of glycoside binding in the absence of the agonist. Substitution of Cl- in the medium by NO3-, SO42-, or benzene sulfonate- reduced the stimulated component of Na+ pump turnover by 85-90%. Addition of 1 mM furosemide to the medium abolished the increase in ouabain binding and ouabain-sensitive oxygen consumption observed after exposure of salt gland cells to MCh. These data are consistent with the hypothesis that cholinergic stimulation of salt gland cells triggers a Cl--dependent uptake of Na+, which elicits a compensatory increase in Na+ pump turnover. In addition, the decrease in cellular Cl- content caused by MCh suggests that the agonist either directly or indirectly mediates an efflux of Cl- from the cells.

Microscopical methods for the localization of Na+,K+-ATPase

Na+,K+-ATPase plays a central role in the ionic and osmotic homeostasis of cells and in the movements of electrolytes and water across epithelial boundaries. Microscopic localization of the enzyme is, therefore, of crucial importance in establishing the subcellular routes of electrolyte flow across structurally complex and functionally polarized epithelia. Recently developed approaches to the localization of Na+,K+-ATPase are reviewed. These methods rely on different properties of the enzyme and encompass cytochemical localization of the K+-dependent nitrophenylphosphatase component of the enzyme, autoradiographic localization of tritiated ouabain binding sites, and immunocytochemical localization of the holoenzyme and of its catalytic subunit. The rationales for each of these techniques are outlined as are the criteria that have been established to validate each method. The observed localization of NA+,K+-ATPase in various tissues is discussed, particularly as it relative to putative and hypothetical mechanisms that are currently thought to mediate reabsorptive and secretory electrolyte transport.

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.

Dissociation of avian salt gland: separation procedures and characterization of dissociated cells

A procedure for dissociation of the nasal salt glands of the domestic duck, Anas platyrhynchos, into suspensions of individual cells has been developed. This technique employs enzymatic digestion with collagenase, hyaluronidase, and chymotrypsin; divalent cation chelation with EDTA; and gentle mechanical dispersion. Average cellular yields of 39 and 26% based on DNA recovered were obtained from the glands of freshwater- and saline-adapted ducks, respectively. Epithelial secretory cells comprised 60-80% of the cell suspensions with the remainder of the populations consisting of endothelial cells, fibroblasts, and blood cells. The dissociated cells were viable as judged by trypan blue exclusion (80-100%, maintenance of ultrastructural integrity, and retention of responsiveness to secretagogues and metabolic inhibitors. Methacholine chloride (0.5 mM) stimulated oxygen consumption by suspensions of both freshwater- and saline-adapted cells, whereas ouabain (0.05 mM) abolished the methacholine-stimulated respiratory response. These cell suspensions provide a promising system for the in vitro study of secretory mechanisms in the avian salt gland.

Accessory cells in teleost branchial epithelium

Correlated morphological and cytochemical investigations of the branchial epithelium of the pinfish, Lagodon rhomboides, have revealed a cell type that is invariably associated with chloride cells. These cells, termed "accessory cells," have been described previously in the teleost pseudobranch (Dunel and Laurent. J. Microsc. Biol. Cell 16:53-74, 1973) but not in the gill proper. Accessory cells are more numerous in pinfish adapted to seawater than to 33% seawater, and in the former participate with chloride cells in the formation of apical crypts. Although accessory cells are much smaller than chloride cells, they possess numerous mitochondria and display an abbreviated labyrinth of plasma membrane-derived tubules. The labyrinth membranes of accessory cells are essentially unreactive, however, when processed for Na-K-ATPase localization by K-nitrophenylphosphatase cytochemistry, whereas chloride cell membranes exhibit copious, ouabain-sensitive reaction products. The zonulae occludentes between accessory cells and chloride cells also appear to be less extensive than those between either of these cells and the flanking pavement cells. These features suggest that accessory cells represent a population of partially differentiated chloride cells.

Ultracytochemical localization of Na+,K+-activated ATPase in chloride cells from the gills of a euryhaline teleost

The activity of the electrolyte transport enzyme, sodium, potassium-activated adenosine triphosphatase (Na+,K+-ATPase), in the gills of the pinfish, Lagodon rhomboides, increased markedly following transfer of fish from brackish water to seawater. Cytochemical localization of Na+,K+-ATPase via its potassium-dependent phosphatase (K+-NPPase) activity in the branchial epithelium of pinfish adapted to seawater demonstrated that chloride cells are the major sites for the enzyme. Subcellularly, the heaviest depositions of reaction product were observed lining the cytoplasmic membrane surfaces of the labyrinth of anastomosing plasma membrane tubules that ramifies throughout the chloride cell cytoplasm. Enzyme activity was demonstrated also on the cytoplasmic surface of the apical crypt membrane and on the cytoplasmic surfaces of vesicles in the cytoplasm subjacent to the crypt. Deletion of potassium from the cytochemical incubation medium or inclusion of 10 mM ouabain abolished the reaction products associated with these membranes. The significance of these cytochemical results is discussed with reference to current hypotheses of chloride cell function.

Rapid isolation of chloride cells from pinfish gill

Enriched fractions of chloride cells with good ultrastructural integrity have been obtained from gill filaments of the euyhaline teleost, Lagodon rhomboides. The branchial epithelium from seawater-adapted fish was dissociated by gentle mechanical means in a Ca++, Mg++-free balanced salt solution. Density gradient centrifugation of the mixed cell suspensions through a Ficoll gradient yielded a fraction containing between 50 and 70% chloride cells. This fraction showed a 3- to 4-fold enrichment over comparable gill homogenate values for sodium plus potassium-activated adenosinetriphosphatase, (Na+, K+ ATPase), an enzyme concentrated in chloride cells. Isolation of chloride cells from fish adapted to one-third seawater was less successful, due to the smaller size and reduced number of these cells, although fractions with at least a 2-fold enrichment of the enzyme were obtained. These results continue to support the belief that chloride cells are responsible for osmoregulatory activity associated with the branchial epithelium of teleosts and that this vital function is mediated through the activity of the transport associated enzyme, Na+, K+-ATPase, the specific activity of which increases with osmotic stress.