Na+ influx is mediated by Na(+)-K(+)-2Cl- cotransport and Na(+)-H+ exchange in sublingual mucous acini

Ae4 (Slc4a9) is an electroneutral monovalent cation-dependent Cl-/HCO3- exchanger

Ae4 (Slc4a9) belongs to the Slc4a family of Cl(-)/HCO3 (-) exchangers and Na(+)-HCO3 (-) cotransporters, but its ion transport cycle is poorly understood. In this study, we find that native Ae4 activity in mouse salivary gland acinar cells supports Na(+)-dependent Cl(-)/HCO3 (-) exchange that is comparable with that obtained upon heterologous expression of mouse Ae4 and human AE4 in CHO-K1 cells. Additionally, whole cell recordings and ion concentration measurements demonstrate that Na(+) is transported by Ae4 in the same direction as HCO3 (-) (and opposite to that of Cl(-)) and that ion transport is not associated with changes in membrane potential. We also find that Ae4 can mediate Na(+)-HCO3 (-) cotransport-like activity under Cl(-)-free conditions. However, whole cell recordings show that this apparent Na(+)-HCO3 (-) cotransport activity is in fact electroneutral HCO3 (-)/Na(+)-HCO3 (-) exchange. Although the Ae4 anion exchanger is thought to regulate intracellular Cl(-) concentration in exocrine gland acinar cells, our thermodynamic calculations predict that the intracellular Na(+), Cl(-), and HCO3 (-) concentrations required for Ae4-mediated Cl(-) influx differ markedly from those reported for acinar secretory cells at rest or under sustained stimulation. Given that K(+) ions share many properties with Na(+) ions and reach intracellular concentrations of 140-150 mM (essentially the same as extracellular [Na(+)]), we hypothesize that Ae4 could mediate K(+)-dependent Cl(-)/HCO3 (-) exchange. Indeed, we find that Ae4 mediates Cl(-)/HCO3 (-) exchange activity in the presence of K(+) as well as Cs(+), Li(+), and Rb(+) In summary, our results strongly suggest that Ae4 is an electroneutral Cl(-)/nonselective cation-HCO3 (-) exchanger. We postulate that the physiological role of Ae4 in secretory cells is to promote Cl(-) influx in exchange for K(+)(Na(+)) and HCO3 (-) ions.

Normalization of hemoglobin-based oxygen carrier-201 induced vasoconstriction: targeting nitric oxide and endothelin

Hemoglobin-based oxygen carrier (HBOC)-201 is a cell-free modified hemoglobin solution potentially facilitating oxygen uptake and delivery in cardiovascular disorders and hemorrhagic shock. Clinical use has been hampered by vasoconstriction in the systemic and pulmonary beds. Therefore, we aimed to 1) determine the possibility of counteracting HBOC-201-induced pressor effects with either adenosine (ADO) or nitroglycerin (NTG); 2) assess the potential roles of nitric oxide (NO) scavenging, reactive oxygen species (ROS), and endothelin (ET) in mediating the observed vasoconstriction; and 3) compare these effects in resting and exercising swine. Chronically instrumented swine were studied at rest and during exercise after administration of HBOC-201 alone or in combination with ADO. The role of NO was assessed by supplementation with NTG or administration of the eNOS inhibitor N^ω-nitro-l-arginine. Alternative vasoactive pathways were investigated via intravenous administration of the ET_A/ET_B receptor blocker tezosentan or a mixture of ROS scavengers. The systemic and to a lesser extent the pulmonary pressor effects of HBOC-201 could be counteracted by ADO; however, dosage titration was very important to avoid systemic hypotension. Similarly, supplementation of NO with NTG negated the pressor effects but also required titration of the dose. The pressor response to HBOC-201 was reduced after eNOS inhibition and abolished by simultaneous ET_A/ET_B receptor blockade, while ROS scavenging had no effect. In conclusion, the pressor response to HBOC-201 is mediated by vasoconstriction due to NO scavenging and production of ET. Further research should explore the effect of longer-acting ET receptor blockers to counteract the side effect of hemoglobin-based oxygen carriers.NEW & NOTEWORTHY Hemoglobin-based oxygen carrier (HBOC)-201 can disrupt hemodynamic homeostasis, mimicking some aspects of endothelial dysfunction, resulting in elevated systemic and pulmonary blood pressures. HBOC-201-induced vasoconstriction is mediated by scavenging nitric oxide (NO) and by upregulating endothelin (ET) production. Pressor effects can be prevented by adjuvant treatment with NO donors or direct vasodilators, such as nitroglycerin or adenosine, but dosages must be carefully monitored to avoid hypotension. However, hemodynamic normalization is more easily achieved via administration of an ET receptor blocker.

Two-photon microscopy and fluorescence lifetime imaging reveal stimulus-induced intracellular Na+ and Cl− changes in cockroach salivary acinar cells

The intracellular ion homeostasis in cockroach salivary acinar cells during salivation is not satisfactorily understood. This is mainly due to technical problems regarding strong tissue autofluorescence and ineffective ion concentration quantification. For minimizing these problems, we describe the successful application of two-photon (2P) microscopy partly in combination with fluorescence lifetime imaging microscopy (FLIM) to record intracellular Na+ and Cl− concentrations ([Na+]i, [Cl−]i) in cockroach salivary acinar cells. Quantitative 2P-FLIM Cl− measurements with the dye N-(ethoxycarbonylmethyl)-6-methoxy-quinolinium bromide indicate that the resting [Cl−]i is 1.6 times above the Cl− electrochemical equilibrium but is not influenced by pharmacological inhibition of the Na+-K+-2Cl− cotransporter (NKCC) and anion exchanger using bumetanide and 4,4′-diisothiocyanatodihydrostilbene-2,2′-disulfonic acid disodium salt. In contrast, rapid Cl− reuptake after extracellular Cl− removal is almost totally NKCC mediated both in the absence and presence of dopamine. However, in physiological saline [Cl−]i does not change during dopamine stimulation although dopamine stimulates fluid secretion in these glands. On the other hand, dopamine causes a decrease in the sodium-binding benzofuran isophthalate tetra-ammonium salt (SBFI) fluorescence and an increase in the Sodium Green fluorescence after 2P excitation. This opposite behavior of both dyes suggests a dopamine-induced [Na+]i rise in the acinar cells, which is supported by the determined 2P-action cross sections of SBFI. The [Na+]i rise is Cl− dependent and inhibited by bumetanide. The Ca2+-ionophore ionomycin also causes a bumetanide-sensitive [Na+]i rise. We propose that a Ca2+-mediated NKCC activity in acinar peripheral cells attributable to dopamine stimulation serves for basolateral Na+ uptake during saliva secretion and that the concomitantly transported Cl− is recycled back to the bath.

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.

Characterisation of neurotransmitter-induced electrolyte transport in cockroach salivary glands by intracellular Ca2+, Na+ and pH measurements in duct cells

Ion-transporting acinar peripheral cells in cockroach salivary glands are innervated by dopaminergic and serotonergic fibres, but saliva-modifying duct cells are innervated only by dopaminergic fibres. We used microfluorometry to record intracellular Na+, Ca2+ and H+concentrations ([Na+]i, [Ca2+]iand pHi) in duct cells of two types of preparation, viz`lobes' consisting of acini with their duct system and `isolated ducts'without acini, in order to obtain information about the transporters involved in saliva secretion and/or modification. Our results indicate that (1)stimulation of lobes by dopamine (DA) causes a strong drop of pHiand increases in [Na+]i and[Ca2+]i in duct cells; (2) in contrast, DA stimulation of isolated ducts produces only a small pHi drop and no changes in[Na+]i and [Ca2+]i; (3)pHi and [Ca2+]i changes are also induced in duct cells by serotonin (5-HT) stimulation of lobes, but not isolated ducts;(4) in the absence of CO2/HCO3–, the DA-induced pHi drop is strongly reduced by removal of extracellular Cl– or inhibition of the Na+–K+–2Cl– cotransporter(NKCC); (5) in the presence of CO2/HCO3–, the DA-induced pHi drop is not reduced by NKCC inhibition, but rather by inhibition of the Cl–/HCO3–exchanger (AE), Na+/H+ exchanger (NHE) or carbonic anhydrase. We suggest that DA and 5-HT act predominantly on acinar peripheral cells. Their activity (secretion of primary saliva) seems to cause changes in ion concentrations in duct cells. NKCC and/or AE/NHE activities are necessary for pHi changes in duct cells; we consider that these transporters are involved in the secretion of the NaCl-rich primary saliva.

Muscarinic activation of Na+-dependent ion transporters and modulation by bicarbonate in rat submandibular gland acinus

To investigate the interaction between the ion channels and transporters in the salivary fluid secretion, we measured the membrane voltage (V(m)) and intracellular concentrations of Ca(2+), Na(+) ([Na(+)](c)), Cl(-), and H(+) (pH(i)) in rat submandibular gland acini (RSMGA). After a transient depolarization induced by a short application of acetylcholine (ACh; 5 muM, 20 s), RSMGA showed strong delayed hyperpolarization (V(h,ACh); -95 +/- 1.8 mV) that was abolished by ouabain. In the HCO(3)(-)-free condition, the V(h,ACh) was also blocked by bumetanide, a blocker of Na(+)-K(+)-2Cl(-) cotransporter (NKCC). In the presence of HCO(3)(-) (24 meq, bubbled with 5% CO(2)), however, the V(h,ACh) was not blocked by bumetanide, but it was suppressed by ethylisopropylamiloride (EIPA), a Na(+)/H(+) exchanger (NHE) inhibitor. Similarly, the ACh-induced increase in [Na(+)](c) was totally blocked by bumetanide in the absence of HCO(3)(-), but only by one-half in the presence of HCO(3)(-). ACh induced a prominent acidification of pH(i) in the presence of HCO(3)(-), and the acidification was further increased by EIPA treatment. Without HCO(3)(-), an application of ACh strongly accelerated the NKCC activity that was measured from the decay of pH(i) during the application of NH(4)(+) (20 mM). Notably, the ACh-induced activation of NKCC was largely suppressed in the presence of HCO(3)(-). In summary, the ACh-induced anion secretion in RSMGA is followed by the activation of NKCC and NHE, resulting an increase in [Na(+)](c). The intracellular Na(+)-induced activation of electrogenic Na(+)/K(+)-ATPase causes V(h,ACh). The regulation of NKCC and NHE by ACh is strongly affected by the physiological level of HCO(3)(-).

Vasodilator responses of coronary conduit and resistance arteries to continuous nitroglycerin infusion in humans: a Doppler guide wire study

To examine the responses of coronary conduit and resistance arteries to the continuous i.v. administration of nitroglycerin in 15 patients with atypical chest pain, we measured coronary blood flow velocity in the left anterior descending coronary artery using a Doppler guide wire and the lumen diameter and cross-sectional area by quantitative coronary angiography. Systolic flow, diastolic flow, total coronary flow, and coronary vascular resistance were calculated. Stepwise increases in dose of nitroglycerin resulted in significant dose-dependent decrease in mean aortic pressure (p < 0.01) and increase in lumen diameter (p < 0.05). After nitroglycerin administration of 0.5 microg/kg/min, systolic flow decreased significantly by 89.9+/-15.7% (p < 0.01), and diastolic flow increased significantly by 74.2+/-37.1% (p < 0.05). Total coronary flow did not change significantly with the various doses of nitroglycerin. However, coronary vascular resistance decreased significantly at concentrations greater than 0.5 microg/kg/min nitroglycerin. Continuous nitroglycerin infusion did not reduce either diastolic or total coronary blood flow despite a significant reduction in coronary perfusion pressure. These results indicate that subendocardial blood flow might be maintained during continuous i.v. infusion of nitroglycerin within the clinical dose range.

Metabolic coronary-flow regulation and exogenous nitric oxide in human coronary artery disease: assessment by intravenous administration of nitroglycerin

We sought to evaluate the effect of intravenous administration of the nitric oxide--donor substance nitroglycerin (NTG) on metabolic coronary-flow regulation in patients with coronary artery disease (CAD). In 12 patients with stable CAD, we measured coronary sinus blood flow and myocardial oxygen supply and consumption (MVO2) at sinus rhythm and during atrial pacing (30 beats/min above sinus rate), both at control and during infusion of NTG, 1 microg/kg/min, and NTG, 2 microg/kg/min. To study metabolic coronary vasodilation, changes in myocardial oxygen supply were related to pacing-induced changes in MVO2, by using standard regression analysis. The myocardial oxygen supply/consumption ratio (i.e., the slope of the regression line at control, characterizing physiological metabolic coronary flow regulation) was compared with the ratios obtained during infusion of NTG. Compared with control measurements, NTG, 1 microg/kg/min, and NTG, 2 microg/kg/min, attenuated pacing-induced increases in MVO2 by 29 and 60%, respectively, whereas coronary blood flow during pacing remained unchanged. At control, normal metabolic coronary-flow regulation resulted in a myocardial oxygen supply/demand ratio of 1.39 (95% CI, 1.29-1.49). This ratio did not change during NTG, 1 microg/kg/min: 1.44 (95% CI, 1.33-1.56). However, during NTG, 2 microg/kg/min, this ratio significantly increased to 1.84 (95% CI, 1.63-2.05; p<0.01). Intravenous administration of high-dose NTG, a donor of exogenous NO, blunts pacing-induced increases in MVO2 and may increase metabolic coronary vasodilation in patients with CAD.

Targeted disruption of the Nhe1 gene prevents muscarinic agonist-induced up-regulation of Na(+)/H(+) exchange in mouse parotid acinar cells

The onset of salivary gland fluid secretion in response to muscarinic stimulation is accompanied by up-regulation of Na(+)/H(+) exchanger (NHE) activity. Although multiple NHE isoforms (NHE1, NHE2, and NHE3) have been identified in salivary glands, little is known about their specific function(s) in resting and secreting acinar cells. Mice with targeted disruptions of the Nhe1, Nhe2, and Nhe3 genes were used to investigate the contribution of these proteins to the stimulation-induced up-regulation of NHE activity in mouse parotid acinar cells. The lack of NHE1, but not NHE2 or NHE3, prevented intracellular pH recovery from an acid load in resting acinar cells, in acini stimulated to secrete with the muscarinic agonist carbachol, and in acini shrunken by hypertonic addition of sucrose. In HCO(3)(-)-containing solution, the rate of intracellular pH recovery from a muscarinic agonist-stimulated acid load was significantly inhibited in acinar cells from mice lacking NHE1, but not in cells from NHE2- or NHE3-deficient mice. These data demonstrate that NHE1 is the major regulator of intracellular pH in both resting and muscarinic agonist-stimulated acinar cells and suggest that up-regulation of NHE1 activity has an important role in modulating saliva production in vivo.

Pulmonary natriuretic peptide system during rat development

Maturational changes in the rat lung natriuretic peptide system were studied postnatally in 1-, 4-, and 22-day-old rats. Lung atrial natriuretic factor (ANF) content increased significantly from day 1 to day 4 (712+/-188 vs. 1905+/-520 pg/mg protein; p<0.01) but decreased to 532+/-41 pg/mg protein, on day 22. These changes paralleled ANF messenger RNA (mRNA) detected by reverse transcribed polymerase chain reaction (RT-PCR). Rat pulmonary development also was associated with quantitative and qualitative alterations in ANF receptors. Competitive-binding radioreceptor assays of lung membranes with 125I-ANF and increasing concentrations of unlabeled ANF revealed that the natriuretic peptide receptor-binding sites (Bmax) progressively increased with age from 112 +/-21 fmol/mg protein at day 1 to 211+/-16 (p<0.02) and 326+/-62 fmol/mg protein (p<0.04) in 4- and 22-day-old rats, respectively. Autoradiographic studies of 125I-tyr(0)CNP binding to lung sections revealed that the levels of the natriuretic peptide receptor B (NPR-B) were undetectable. On the other hand, binding of 125I-ANF increased with age, and the higher binding at 4 days was mainly due to increased density of the clearance receptor-C (NPR-C), and at 22 days due to increased natriuretic peptide receptor-A (NPR-A). The increase in natriuretic peptide binding was confirmed at the level of synthesis, where RT-PCR revealed that NPR-A mRNA significantly increased (p<0.01) in 22-day-old rats. In conclusion, these studies demonstrate that the rat pulmonary natriuretic peptide system is altered during development. The altered synthesis of lung natriuretic peptides and their receptors may play a role in the postnatal adaptation of pulmonary circulation.

Isosmotic modulation of Ca2+-regulated exocytosis in guinea-pig antral mucous cells: role of cell volume

1. Exocytotic events and changes of cell volume in mucous cells from guinea-pig antrum were examined by video-enhanced optical microscopy. 2. Acetylcholine (ACh) evoked exocytotic events following cell shrinkage, the frequency and extent of which depended on the ACh concentration. ACh actions were mimicked by ionomycin and thapsigargin, and inhibited by Ca2+-free solution and Ca2+ channel blockers (Ni2+, Cd2+ and nifedipine). Application of 100 microM W-7, a calmodulin inhibitor, also inhibited the ACh-induced exocytotic events. These results indicate that ACh actions are mediated by intracellular Ca2+ concentration ([Ca2+]i) in antral mucous cells. 3. The effects of ion channel blockers on exocytotic events and cell shrinkage evoked by ACh were examined. Inhibition of KCl release (quinine, Ba2+, NPPB or KCl solution) suppressed both the exocytotic events and cell shrinkage evoked by ACh. 4. Bumetanide (inhibition of NaCl entry) or Cl--free solution (increasing Cl- release and inhibition of NaCl entry) evoked exocytotic events following cell shrinkage in unstimulated antral mucous cells and caused further cell shrinkage and increases in the frequency of exocytotic events in ACh-stimulated cells. However, Cl--free solution did not evoke exocytotic events in unstimulated cells in the absence of extracellular Ca2+, although cell shrinkage occurred. 5. To examine the effects of cell volume on ACh-evoked exocytosis, the cell volume was altered by increasing the extracellular K+ concentration. The results showed that cell shrinkage increases the frequency of ACh-evoked exocytotic events and cell swelling decreases them. 6. Osmotic shrinkage or swelling caused the frequency of ACh-evoked exocytotic events to increase. This suggests that the effects of cell volume on ACh-evoked exocytosis under anisosmotic conditions may not be the same as those under isosmotic conditions. 7. In antral mucous cells, Ca2+-regulated exocytosis is modulated by cell shrinkage under isosmotic conditions.

The effect of nitroglycerin on pacing-induced changes in myocardial oxygen consumption and metabolic coronary vasodilation in patients with coronary artery disease

In the present study, we assessed the potential effect of nitroglycerin IV (NTG), a donor of exogenous nitric oxide, on metabolic coronary flow control in patients with coronary artery disease. In 12 patients scheduled for coronary artery surgery, arterial blood pressure, pulmonary capillary wedge pressure, coronary sinus blood flow (continuous thermodilution), myocardial oxygen supply (DVO2), and myocardial oxygen consumption (MVO2) were measured at sinus rhythm and in response to atrial pacing at 30 bpm greater than baseline sinus rate. These measurements were repeated during infusion of NTG 1 and 2 microg x kg(-1) x min(-1). At control, in the absence of NTG, MVO2 increased from 13.7 +/- 3.4 mL O2/min during sinus rhythm to 19.3 +/- 5.5 mL O2/min during pacing. NTG 1 and 2 microg x kg(-1) x min(-1) blunted the pacing-induced increase in MVO2 dose-dependently. During NTG 1 microg x kg(-1) x min(-1), MVO2 increased from 12.9 +/- 3.3 mL O2/min at sinus rhythm to 17.3 +/- 4.7 mL O2/min during pacing (P = 0.01 versus control pacing); during NTG 2 microg x kg(-1) x min(-1), MVO2 increased from 13.4 +/- 3.3 mL O2/min to 15.9 +/- 3.7 mL O2/min (P = 0.008 versus control pacing). However, the pacing-induced increase in DVO2 per mL O2/min increase in MVO2 (delta DVO2/delta MVO2), was significantly greater during the infusion of NTG 2 microg x kg(-1) x min(-1) (1.85 +/- 0.56; P = 0.023) compared with control (1.51 +/- 0.22). This was associated with an increase in coronary sinus hemoglobin oxygen saturation (30% +/- 5% at control pacing and 34% +/- 6% during pacing with NTG 2 microg x kg(-1) x min(-1); P = 0.018), which indicates that during the infusion of NTG, there was more metabolic coronary vasodilation than achievable solely on the basis of the metabolic stimulus.

IMPLICATIONS

Our findings suggest that nitroglycerin, a donor of exogenous nitric oxide, reduces pacing-induced increases in myocardial oxygen consumption and enhances metabolic coronary vasodilation in patients with coronary artery disease, in whom endogenous nitric oxide activity may be reduced.

Nitroglycerin fails to dilate coronary collateral vessels during exercise

This study was performed to determine whether nitroglycerin can increase blood flow to collateral-dependent myocardium during exercise. Intermittent repetitive occlusions of the left circumflex coronary artery (LCX) were used to stimulate growth of coronary collateral vessels in seven adult mongrel dogs. Coronary pressure distal to the occluder was measured with a long-term implanted microcatheter. When sufficient collateral growth had occurred to increase distal coronary pressure to >40 mm Hg during occlusion, the artery was permanently occluded. Dogs were returned to the laboratory 1 week later for study. Measurements were obtained at rest and during treadmill exercise during control conditions and after a 300-microg bolus of nitroglycerin (i.v.). Aortic and coronary pressures were measured with fluid-filled catheters, whereas myocardial blood flow was measured with radioactive microspheres. During control conditions, exercise caused significant increases of blood flow in the normal and collateral zones with significant decreases in vascular resistance. However, nitroglycerin failed to cause a further increase in blood flow to either the normal or the collateral-dependent myocardial regions during exercise. Furthermore, neither calculated transcollateral resistance (TCR) nor small-vessel resistance (SVR) changed significantly in response to nitroglycerin (TCR, 27 +/- 9 mm Hg/ml/min/g before nitroglycerin and 27 +/- 6 mm Hg/ml/min/g after; SVR, 43 +/- 5 mm Hg/ml/min/g before nitroglycerin and 49 +/- 7 mm Hg/ml/min/g after). The finding that the collateral vessels failed to dilate in response to nitroglycerin suggests that the nitric oxide system is already maximally recruited during exercise.

Influence of chitosan microspheres on the transport of prednisolone sodium phosphate across HT-29 cell monolayers

PURPOSE

The present study was performed to investigate the influence of chitosan microspheres on transport of the hydrophilic, antiinflammatory drug prednisolone sodium phosphate (PSP) across the epithelial barrier.

METHODS

Microspheres were prepared using a precipitation method and loaded with PSP. Transport studies were performed in a diffusion cell chamber using the polarized human cell line HT-29B6. Porcine small intestine and fluorescence-labeled microspheres were used to investigate penetration ability of microspheres.

RESULTS

It was shown that transport of PSP drug solution was not saturable across the cell monolayers (P = 8.68 +/- 8.24 x 10(-6) cm sec-1) and no sodium dependency could be established. EGTA treatment resulted in an increased permeability (P = 18.69 +/- 1.09 x 10(-6) cm sec-1). After binding of prednisolone to chitosan microspheres its permeability was enhanced drastically compared with the drug solution (P = 35.37 +/- 3.21 x 10(-6) cm sec-1). This effect was prevented by EGTA treatment (P = 15.11 +/- 2.57 x 10(-6) cm sec-1). Furthermore the supporting effect of chitosan microspheres was impaired by pH and ion composition of the medium, whereas the effect remained unchanged in cells treated with bacterial lipopolysaccharides. In vitro incubation of fluorescence-labeled microspheres in the lumen of freshly excised intestine revealed a significant amount of the spheres in the submucosa.

CONCLUSIONS

Chitosan microspheres are a useful tool to improve the uptake of hydrophilic substances like PSP across epithelial layers. The effect is dependent on the integrity of the intercellular cell contact zones and the microparticles are able to pass the epithelial layer. Their potential benefit under inflammatory conditions like in inflammatory bowel disease, in order to establish high drug doses at the region of interest, remains to be shown.

Membrane-specific regulation of Cl- channels by purinergic receptors in rat submandibular gland acinar and duct cells

Measurement of [Cl-]i and the Cl- current in the rat salivary submandibular gland (SMG) acinar and duct cells was used to evaluate the role of Cl- channels in the regulation of [Cl-]i during purinergic stimulation. Under resting conditions [Cl-]i averaged 56 +/- 8 and 26 +/- 7 mM in acinar and duct cells, respectively. In both cells, stimulation with 1 mM ATP resulted in Cl- efflux and subsequent influx. Inhibition of NaKCl2 cotransport had no effect on [Cl-]i changes in duct cells and inhibited only about 50% of Cl- uptake in acinar cells. Accordingly, low levels of expression of NaKCl2 cotransporter protein were found in duct cells. Acinar cells expressed high levels of the cotransporter. Measurement of Cl- current under selective conditions revealed that acinar and duct cells express at least five distinct Cl- channels; a ClCO-like, volume-sensitive, inward rectifying, Ca2+-activated and CFTR-like Cl- currents. ATP acting on both cell types activated at least two channels, the Ca2+-activated Cl- channel and a Ca2+-independent glibenclamide-sensitive Cl--current, possibly cystic fibrosis transmembrane regulator (CFTR). Of the many nucleotides tested only 2'-3'-benzoylbenzoyl (Bz)-ATP and UTP activated Cl- channels in SMG cells. Despite their relative potency in increasing [Ca2+]i, BzATP in both SMG cell types largely activated the Ca2+-independent, glibenclamide-sensitive Cl- current, whereas UTP activated only the Ca2+-dependent Cl- current. We interpret this to suggest that BzATP and UTP largely activate Cl- channels residing in the membrane expressing the receptor for the active nucleotide. The present studies reveal a potentially new mechanism for transcellular Cl- transport in a CFTR-expressing tissue, the SMG. Coordinated action of the P2z (luminal) and P2u (basolateral) receptors can mediate part of the transcellular Cl- transport by acinar and duct cells to determine the final electrolyte composition of salivary fluid.

Extracellular Mg2+ regulates the intracellular Na+ concentration in rat sublingual acini

The intracellular free Na+ concentration ([Na+]i) increases during muscarinic stimulation in salivary acinar cells. The present study examined in rat sublingual acini the role of extracellular Mg2+ in the regulation of the stimulated [Na+]i increase using the fluorescent sodium indicator benzofuran isophthalate (SBFI). The muscarinic induced rise in [Na+]i was approximately 4-fold greater in the absence of extracellular Mg2+. When Na+ efflux was blocked by the Na+,K+-ATPase inhibitor ouabain, the stimulated [Na+]i increase was comparable to that seen in an Mg2+-free medium. Moreover, ouabain did not add further to the stimulated [Na+]i increase in an Mg2+-free medium suggesting that removal of extracellular Mg2+ may inhibit the Na+ pump. In agreement with this assumption, ouabain-sensitive Na+ efflux and rubidium uptake were reduced by extracellular Mg2+ depletion. Our results suggest that extracellular Mg2+ may regulate [Na+]i in sublingual salivary acinar cells by modulating Na+ pump activity.

Heterogeneity in the vasorelaxing effect of nicorandil on dog epicardial coronary arteries: comparison with other NO donors

The relaxation responses to nicorandil, nitroglycerin (NTG), and cromakalim were compared in isolated dog large (>1.5 mm inside diameter) and small (<0.3 mm inside diameter) epicardial coronary arteries. Nicorandil and NTG produced more potent relaxing effects in large coronary arteries. In contrast, cromakalim produced greater relaxation in small arteries. No significant differences were observed in the nitric oxide (NO)-induced response after treatment with superoxide dismutase. The responses to 8-bromo-cyclic guanosine monophosphate (cGMP), SIN-1, and atrial natriuretic peptide did not differ in arteries of different sizes. Treatment with L-cysteine had no significant effect on the relaxation responses to NTG in both large and small coronary arteries. Oxyhemoglobin and glibenclamide inhibited relaxation induced by nicorandil in large and small coronary arteries. Oxyhemoglobin had a greater suppressive effect on the response to nicorandil in large coronary arteries than in small coronary arteries. Methylene blue inhibited the response to nicorandil in large coronary arteries. These findings suggest that nicorandil behaves predominantly as a nitrate in large epicardial coronary arteries rather than small epicardial arteries and that this difference between large and small coronary arteries with regard to the nitrate action of nicorandil may be the result of a pathway in which nicorandil is converted to NO.

Evidence for a Ca2+ pool associated with secretory granules in rat submandibular acinar cells

Intracellular Ca2+ stores in rat submandibular acinar cells were characterized using the Ca(2+)-sensitive fluorescent indicator fura 2 and the radiotracer 45Ca2+. Acetylcholine induced a rapid Ca2+ release from a store sensitive to inositol 1,4,5-trisphosphate (IP3) and to thapsigargin (TG). After this store was presumably depleted, ionomycin caused a further increase in cytosolic free Ca2+ concentration ([Ca2+]i), suggesting the presence of an IP3-insensitive Ca2+ release from a store that is more extensive and heterogeneous than the IP3-sensitive one and includes a small mitochondrial component. After both of these stores had been discharged, exposure to monensin caused an additional release of Ca2+ from a third store. This store appears to be associated with secretory granules, since Ca2+ release was significantly reduced when degranulation was induced by isoprenaline. This third store appears to be insensitive to IP3, discharges Ca2+ when the pH gradient across the limiting membrane is collapsed with monensin and only in the presence of both ionomycin and monensin. Ca2+ release from this store is not by Na+/Ca2+ exchange, since simply altering [Na+]i did not cause significant Ca2+ release. In permeabilized cells, IP3 and TG released approx. 35% of 45Ca2+, and ionomycin released an additional 57%, whereas monensin only caused a small additional release, suggesting that only IP3- and ionomycin-sensitive stores are loaded with 45Ca2+ under these conditions. The absence of significant isotope uptake into the ionomycin+monensin-sensitive store may result from a low rate of tracer accumulation or from the lack of Ca2+ pumps in the store. The pattern of response was similar in the presence and absence of mitochondrial inhibitors, indicating that the store is not located in mitochondria. In summary, these results suggest that a substantial IP3-insensitive Ca2+ store is present in secretory granules in rat submandibular acinar cells.

Na+-dependent release of Mg2+ from an intracellular pool in rat sublingual mucous acini

Muscarinic stimulation induces release of Mg2+ from an intracellular pool in rat sublingual mucous acini (Zhang, G. H., and Melvin, J. E. (1992) J. Biol. Chem. 267, 20721-20727). In the present study we examined the interdependence of Mg2+ mobilization on intracellular Na+ and Ca2+ by monitoring the intracellular free concentrations of Na+ ([Na+]i), Mg2+ ([Mg2+]i), and Ca2+ ([Ca2+]i) using ion-sensitive fluorescent indicators. Gramicidin increased the intracellular concentrations of all three ions. Comparable to agonist-stimulated mobilization of Mg2+, the gramicidin-induced [Mg2+]i increase was independent of extracellular Mg2+ indicating release of Mg2+ from an intracellular pool. Clamping the [Ca2+]i near 30 nM with the Ca2+-selective chelator BAPTA failed to alter the [Na+]i or [Mg2+]i increases generated by gramicidin. In contrast, depletion of intracellular Na+ markedly suppressed the muscarinic-stimulated [Mg2+]i increase, whereas the [Ca2+]i increase was similar to that seen in physiological extracellular Na+. These results revealed that intracellular Mg2+ mobilization did not directly relate to the [Ca2+]i, but required an increase in [Na+]i. Consistent with this hypothesis, increasing [Na+]i by activating Na+ influx via the Na+/H+ exchanger also increased the [Mg2+]i. The Na+/Mg2+ exchange inhibitor quinidine suppressed both the gramicidin- and muscarinic-induced discharge of internal Mg2+. These results suggest that release of Mg2+ from an intracellular pool is mediated by a Na+-dependent Mg2+ transport mechanism in salivary acinar cells.

Na+,K(+)-ATPase activity in the cochlear lateral wall of the gerbil

We examined Na+,K(+)-ATPase activity in four regions of the gerbil cochlear lateral wall by measuring intracellular Na+ concentration ([Na+]i). The application of 1 mM ouabain and removal of external K+ caused a most striking increase in [Na+]i of the suprastrial fibrocytes, followed by the stria vascularis and the type II fibrocytes, but no detectable response in the type I fibrocytes. These results may imply the potency of the Na+,K(+)-ATPase activity. The high values of the resting [Na+]i in these cells under perilymph-like solution returned to the [Na+]i within physiological values after exposure to low-Na+ conditions similar to endolymph. Our results are interpreted in the light of recent morphological findings with regard to the roles in the secretion of K+ into endolymph of several cell types within the lateral wall.

Regulation of changes in cytosolic Ca2+ and Na+ concentrations in rat submandibular gland acini exposed to carbachol and ATP

The relationship between cytosolic concentrations of Ca2+ (Ca2i) and Na+ (Na+i) were studied in preparations of rat submandibular and pancreatic acini loaded with the Ca(2+)-sensitive dye Fura-2 or the Na(+)-sensitive dye SBFI. Pancreatic acini showed no changes in Na+i during either transient or persistent changes in Ca2+i. Increases in Ca2+i produced by exposure of submandibular gland acini to carbachol, a muscarinic cholinergic agonist, were followed by an increase in Na+i after a delay of 5-10 s. When Ca2+ stores were mobilized without Ca2+ influx Na+i also increased, but in acini loaded with BAPTA, a nonfluorescent Ca2+ chelator, the transient increase in Ca2+ caused by mobilization of stored Ca2+ was virtually abolished, as was the increase in Na+i. In the presence of inomycin, increases in Ca2+i were followed by increases in Na+i. Ca(2+)-dependent increases in Na+i were abolished in Na(+)-free buffer and by the presence of furosemide, a blocker of Na(+)-K(+)-2Cl- cotransport. In other studies, extracellular ATP (ATPo) produced an increase in Ca2+i and Na+i. The steady-state increase in Ca(i)2+ was reduced by increasing extracellular Na+ concentrations (Na+o in dose-dependent fashion (IC50 = 16.4 +/- 4.7 mM Na+). Likewise, increasing Na+o reduced ATPo-stimulated 45Ca2+ uptake at steady state (IC50 = 15.8 +/- 9.2 mM Na+). Changing Na+o had no effect on carbachol-stimulated increases in Ca2+i. We conclude that, in rat submandibular gland acini, ATPo promotes an increase in Ca2+i and Na+i via a common influx pathway and that, under physiologic conditions, Na+ significantly limits the ATPo-stimulated increase in Ca2+i. In the presence of carbachol, however, Na+i rises in Ca2+i-dependent fashion in submandibular gland acini via stimulation of Na(+)-K(+)-2Cl- cotransport.

Regulation of the renal response to atrial natriuretic peptide by sodium intake in preweaned rats

The neonate conserves sodium avidly, and sodium intake is normally limited to that present in maternal milk. To evaluate the role of atrial natriuretic peptide (ANP) in this adaptation, preweaned rat pups were artificially reared and fed a formula with either normal sodium (25 mEq/L) or high sodium (145 mEq/L) for 7-8 d. To determine whether increased dietary sodium decreases ANP clearance receptor activity, animals were anesthetized, and the plasma ANP concentration (ANPp), urine flow (V), urinary sodium (UNa V), and cGMP excretion (UcGMP V) were measured before and after infusion of ANF(4-23), an ANP clearance receptor inhibitor (C-ANF), at 50 micrograms/kg/ min. Infusion of C-ANF increased ANPp 10-fold in both normal and high sodium groups, but V, UNa V, and UcGMP V increased only in animals receiving the high sodium diet (p < 0.05). Incubation of isolated glomeruli with 0.1 microM ANP increased extracellular cGMP more in high sodium than normal sodium groups (p < 0.05). We conclude that ANP clearance receptors in the neonate are highly activated regardless of sodium intake. Increased dietary sodium increases the renal diuretic and natriuretic response to circulating ANP through enhanced generation of cGMP.

Atrial natriuretic peptide suppresses the transcription of its guanylyl cyclase-linked receptor

Atrial natriuretic peptide (ANP) treatment of rat aortic smooth muscle cells suppressed both 125I-ANP binding and ANP-dependent cGMP accumulation, suggesting reductions in the type C (NPR-C) and type A (NPR-A) natriuretic peptide receptor populations, respectively. NPR-A, but not NPR-C, mRNA levels were reduced in a dose-dependent fashion by ANP. The latter effect appeared to be due, at least in part, to suppression of NPR-A gene promoter activity. ANP effected a dose- and time-dependent reduction in a transiently transfected NPR-A luciferase reporter (-1575LUC). Analysis of 5' deletion mutants of the NPR-A promoter demonstrated that the ANP-dependent sequence lies between -1575 and -1290 relative to the transcription start site. Inhibition of the ANP promoter was also effected by brain natriuretic peptide, type C natriuretic peptide, and 8-bromo-cGMP, but not by the NPR-C-selective ligand cANF. In the case of 8-bromo-cGMP, the responsive element(s) was localized to the same 285-base pair region linked to the ANP effect above. These findings indicate that ANP autoregulates its own receptors in these cells and, at least in the case of NPR-A, it does so through suppression of receptor gene expression and receptor synthesis. This suppression may operate through a cGMP-dependent element located more than a kilobase upstream from the transcription start site.

Central glucopenia induced by 2-deoxy-D-glucose stimulates somatostatin secretion in the rat

The mechanisms involved in 2-deoxy-D-glucose (2-DG)-induced growth hormone (GH) suppression in the rat were examined. Conscious male rats were given 2-DG by intracerebroventricular (icv) injection and the pulsatile GH secretion was monitored for 6 h. The single icv injection of 2-DG (8 mg/rat) eliminated pulsatile GH secretion in conscious rats. Pretreatment with somatostatin (SS) antiserum completely restored the suppressed GH secretion in the 2-DG treated rats. Hypothalamic GH-releasing hormone (GRH) and SS mRNA levels were not altered by single and multiple icv injections of 2-DG. These findings suggest that 2-DG-induced GH suppression is primarily due to hypersecretion of SS without a significant change at the transcription level in the rat.

C-natriuretic peptide but not atrial natriuretic peptide increases cyclic GMP in cerebral arterial smooth muscle cells

A-type natriuretic peptide (ANP) is found primarily in the heart and is released into the circulation. C-type (CNP) is found principally in the brain and has also been detected in the systemic circulation. When injected, both peptides produce vasodilatation most likely by elevation of guanosine 3'5'-cyclic monophosphate (cGMP) in smooth muscle cells via two distinct receptors, NPR-A and NPR-B. In this present study, we determined the effects of these two peptides on intracellular cGMP in smooth muscle cells cultured from pig cerebral and peripheral arteries. In smooth muscle cells cultured from the left anterior descending coronary artery, ANP and CNP increased cGMP with equal potency and efficacy (EC50 for ANP and CNP, 3.6 +/- 0.2 x 10(-8) M and 6.7 +/- 0.8 x 10(-8) M, respectively). In contrast, in smooth muscle cells from cerebral arteries, ANP was without effect while CNP increased cGMP in a concentration dependent manner (EC50: 9.6 +/- 1.7 x 10(-8) M). Stimulation of the soluble guanylyl cyclase with either nitroglycerin or nitroprusside was equivalent in the two cell types. The pattern of response of intracellular cGMP to CNP and ANP in isolated intact arteries from brain and heart was similar to that found in the cultured cells. These results suggest that smooth muscle cells in cerebral arteries express only NPR-B while cells from peripheral arteries can express both NPR-A and NPR-B.

HCO3(-)-dependent ACh-activated Na+ influx in sheep parotid secretory endpieces

In the present study we used the Na)+)-sensitive fluorescent dye SBFI and optical measurement of endpiece volume to investigate the transport of Na+ in sheep parotid secretory cells. Sheep parotid endpiece cells bathed in a HCO3(-)-free Cl(-)-rich solution had a resting intracellular Na+ concentration ([Na+]i) of 17 +/- 2 mmol/l (n = 39). Exposure of the cells to a 2-min pulse of acetylcholine (ACh) (3 x 10(-7) mol/l) in a HCO3(-)-free bathing solution produced no change in [Na+]i or in cell volume. Changing from a Cl(-)-containing HCO(3-)-free bath solution to a Cl- solution containing 25 mmol/l HCO3- caused the endpieces to swell by 8 +/- 2% (n = 11) and the [Na+]i to increase by 10 +/- 2 mmol/l (n = 14). Subsequent exposure of the cells to ACh led to shrinkage of the cells by 12 +/- 2% from the volume in the HCO3(-)-containing solution prior to ACh exposure, with the maximum decrease occurring after 29 +/- 7 s (n = 9). This shrinkage was accompanied by a rapid and transient increase in [Na+]i, the [Na+]i reaching a peak at 70 +/- 5 mmol/l above the unstimulated level (n = 9). Substitution of gluconate for Cl- did not significantly alter the effects of HCO3- on unstimulated [Na+]i or endpiece volume, nor did it significantly inhibit the effects of ACh on these two parameters when HCO3- was present.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by thiocyanate of muscarinic-induced cytosolic acidification and Ca2+ entry in rat sublingual acini

Thiocyanate (SCN-) plays a critical part in an oral antimicrobial system by acting as a substrate for peroxidases. Salivary glands concentrate SCN- from blood up to 5 mM in saliva; however, the influence of SCN- on salivary acinar-cell function is unknown. The present study examined the effects of SCN- on the regulation of cytosolic pH (pHi) and free Ca2+ concentration ([Ca2+]i) in rat sublingual mucous acini using the pH- and Ca(2+)-sensitive fluorescent indicators, 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein and fura-2, respectively. SCN- induced a concentration-dependent inhibition of the carbachol-stimulated cytosolic acidification (K1/2, approx. 1.4 mM SCN-). Cytosolic pH recovery from an acid load was not changed by substitution of Cl- by SCN-, suggesting that Na+/H+ exchange activity was not affected by SCN-. SCN- did not alter the initial carbachol-stimulated increase in [Ca2+]i; however, the sustained [Ca2+]i increase was inhibited by > 65% (K1/2, approx. 1.0 mM SCN-). Furthermore, SCN- prevented the carbachol-stimulated Mn2+ influx, indicating that it inhibits the divalent-cation entry pathway. Consistent with decreased Ca2+ mobilization being involved in the blockade of the agonist-induced acidification by SCN-, only total replacement of Cl- with SCN- significantly inhibited the acidification induced by the Ca2+ ionophore ionomycin. The permeability to SCN- through the Ca(2+)-dependent Cl- channels was 5.2-fold higher than the permeability to Cl-. These results suggest that inhibition of the agonist-induced cytosolic acidification by high-concentration SCN- may be mediated by both competitive inhibition of HCO3- efflux and by blockade of Ca2+ influx.