Effects of Ca2+ and furosemide on Cl- transport and O2 uptake in rat parotid acini

Function and Regulation of the Calcium-Activated Chloride Channel Anoctamin 1 (TMEM16A)

Various human tissues express the calcium-activated chloride channel Anoctamin 1 (ANO1), also known as TMEM16A. ANO1 allows the passive chloride flux that controls different physiological functions ranging from muscle contraction, fluid and hormone secretion, gastrointestinal motility, and electrical excitability. Overexpression of ANO1 is associated with pathological conditions such as hypertension and cancer. The molecular cloning of ANO1 has led to a surge in structural, functional, and physiological studies of the channel in several tissues. ANO1 is a homodimer channel harboring two pores - one in each monomer - that work independently. Each pore is activated by voltage-dependent binding of two intracellular calcium ions to a high-affinity-binding site. In addition, the binding of phosphatidylinositol 4,5-bisphosphate to sites scattered throughout the cytosolic side of the protein aids the calcium activation process. Furthermore, many pharmacological studies have established ANO1 as a target of promising compounds that could treat several illnesses. This chapter describes our current understanding of the physiological roles of ANO1 and its regulation under physiological conditions as well as new pharmacological compounds with potential therapeutic applications.

Ca2+-CaMKKβ pathway is required for adiponectin-induced secretion in rat submandibular gland

Adiponectin functions as a promoter of saliva secretion in rat submandibular gland via activation of adenosine monophosphate-activated protein kinase (AMPK) and increased paracellular permeability. Ca²⁺ mobilization is the primary signal for fluid secretion in salivary acinar cells. However, whether intracellular Ca²⁺ mobilization is involved in adiponectin-induced salivary secretion is unknown. Here, we found that full-length adiponectin (fAd) increased intracellular Ca²⁺ and saliva secretion in submandibular glands. Pre-perfusion with ethylene glycol-bis (2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA) combined with thapsigargin (TG), an endoplasmic reticulum Ca²⁺-ATPase inhibitor, abolished fAd-induced salivary secretion, AMPK phosphorylation, and enlarged tight junction (TJ) width. Furthermore, in cultured SMG-C6 cells, co-pretreatment with EGTA and TG suppressed fAd-decreased transepithelial electrical resistance and increased 4-kDa FITC-dextran flux responses. Moreover, fAd increased phosphorylation of calcium/calmodulin-dependent protein kinase (CaMKKβ), a major kinase that is activated by elevated levels of intracellular Ca²⁺, but not liver kinase B1 phosphorylation. Pre-perfusion of the isolated gland with STO-609, an inhibitor of CaMKKβ, abolished fAd-induced salivary secretion, AMPK activation, and enlarged TJ width. CaMKKβ shRNA suppressed, whereas CaMKKβ re-expression rescued fAd-increased paracellular permeability. Taken together, these results indicate that adiponectin induced Ca²⁺ modulation in rat submandibular gland acinar cells. Ca²⁺-CaMKKβ pathway is required for adiponectin-induced secretion through mediating AMPK activation and increase in paracellular permeability in rat submandibular glands.

World Workshop on Oral Medicine VI: a systematic review of medication-induced salivary gland dysfunction

The aim of this paper was to perform a systematic review of the pathogenesis of medication-induced salivary gland dysfunction (MISGD). Review of the identified papers was based on the standards regarding the methodology for systematic reviews set forth by the World Workshop on Oral Medicine IV and the PRISMA statement. Eligible papers were assessed for both the degree and strength of relevance to the pathogenesis of MISGD as well as on the appropriateness of the study design and sample size. A total of 99 papers were retained for the final analysis. MISGD in human studies was generally reported as xerostomia (the sensation of oral dryness) without measurements of salivary secretion rate. Medications may act on the central nervous system (CNS) and/or at the neuroglandular junction on muscarinic, α-and β-adrenergic receptors and certain peptidergic receptors. The types of medications that were most commonly implicated for inducing salivary gland dysfunction were those acting on the nervous, cardiovascular, genitourinary, musculoskeletal, respiratory, and alimentary systems. Although many medications may affect the salivary flow rate and composition, most of the studies considered only xerostomia. Thus, further human studies are necessary to improve our understanding of the association between MISGD and the underlying pathophysiology.

Selective localization of diacylglycerol kinase (DGK)ζ in the terminal tubule cells in the submandibular glands of early postnatal mice

The present immunohistochemical study was attempted to localize in the submandibular glands of mice at various postnatal stages a diacylglycerol kinase (DGK) isoform termed DGKζ which is characterized by a nuclear localization signal and a nuclear export signal. This attempt was based on following facts: the continuous postnatal differentiation of glandular cells in the rodent submandibular gland, the regulatory role of DGK in the activity of protein kinase C (PKC) through attenuation of diacylglycerol (DAG), and the possible involvement of PKC in various cellular activities including the saliva secretion as well as the cell differentiation. As a result, a selective localization of immunoreactivity for DGKζ was detected in terminal tubule (TT) cells which comprise a majority of the newborn acinar structure and differentiate into the intercalated duct cells and/or the acinar cells. The immunoreactivity was deposited in portions of the cytoplasm lateral and basal to the nucleus, but not in the nuclei themselves. Although the immunoreactive TT cells remained until later stages in female specimen than in male, they eventually disappeared in both sexes by young adult stages. The present finding suggests that the regulatory involvement of DGKζ in PKC functions via control of DAG is exerted in the differentiation of the TT cells. In addition, another possible involvement of DGKζ in the regulation of secretion of the TT cells as well as its functional significance of its nuclear localization in the submandibular ganglion cells was also discussed.

Ca²⁺-dependent K⁺ channels in exocrine salivary glands

In the last 15 years, remarkable progress has been realized in identifying the genes that encode the ion-transporting proteins involved in exocrine gland function, including salivary glands. Among these proteins, Ca(2+)-dependent K(+) channels take part in key functions including membrane potential regulation, fluid movement and K(+) secretion in exocrine glands. Two K(+) channels have been identified in exocrine salivary glands: (1) a Ca(2+)-activated K(+) channel of intermediate single channel conductance encoded by the KCNN4 gene, and (2) a voltage- and Ca(2+)-dependent K(+) channel of large single channel conductance encoded by the KCNMA1 gene. This review focuses on the physiological roles of Ca(2+)-dependent K(+) channels in exocrine salivary glands. We also discuss interesting recent findings on the regulation of Ca(2+)-dependent K(+) channels by protein-protein interactions that may significantly impact exocrine gland physiology.

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.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Measurement of Ca2+ signaling dynamics in exocrine cells with total internal reflection microscopy

In nonexcitable cells, such as exocrine cells from the pancreas and salivary glands, agonist-stimulated Ca2+ signals consist of both Ca2+ release and Ca2+ influx. We have investigated the contribution of these processes to membrane-localized Ca2+ signals in pancreatic and parotid acinar cells using total internal reflection fluorescence (TIRF) microscopy (TIRFM). This technique allows imaging with unsurpassed resolution in a limited zone at the interface of the plasma membrane and the coverslip. In TIRFM mode, physiological agonist stimulation resulted in Ca2+ oscillations in both pancreas and parotid with qualitatively similar characteristics to those reported using conventional wide-field microscopy (WFM). Because local Ca2+ release in the TIRF zone would be expected to saturate the Ca2+ indicator (Fluo-4), these data suggest that Ca2+ release is occurring some distance from the area subjected to the measurement. When acini were stimulated with supermaximal concentrations of agonists, an initial peak, largely due to Ca2+ release, followed by a substantial, maintained plateau phase indicative of Ca2+ entry, was observed. The contribution of Ca2+ influx and Ca2+ release in isolation to these near-plasma membrane Ca2+ signals was investigated by using a Ca2+ readmission protocol. In the absence of extracellular Ca2+, the profile and magnitude of the initial Ca2+ release following stimulation with maximal concentrations of agonist or after SERCA pump inhibition were similar to those obtained with WFM in both pancreas and parotid acini. In contrast, when Ca2+ influx was isolated by subsequent Ca2+ readmission, the Ca2+ signals evoked were more robust than those measured with WFM. Furthermore, in parotid acinar cells, Ca2+ readdition often resulted in the apparent saturation of Fluo-4 but not of the low-affinity dye Fluo-4-FF. Interestingly, Ca2+ influx as measured by this protocol in parotid acinar cells was substantially greater than that initiated in pancreatic acinar cells. Indeed, robust Ca2+ influx was observed in parotid acinar cells even at low physiological concentrations of agonist. These data indicate that TIRFM is a useful tool to monitor agonist-stimulated near-membrane Ca2+ signals mediated by Ca2+ influx in exocrine acinar cells. In addition, TIRFM reveals that the extent of Ca2+ influx in parotid acinar cells is greater than pancreatic acinar cells when compared using identical methodologies.

Regulation of fluid and electrolyte secretion in salivary gland acinar cells

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

Effects of furosemide and bendroflumethiazide on saliva flow rate and composition

Aim of this study was to evaluate the effect on saliva flow rate and composition and on perceived xerostomia. The study used a Latin square design, all subjects being once daily (at 7.00 a.m.) taking the bendroflumethiazide (2.5 mg), furosemide (40 mg), or placebo, in a randomised order. Each treatment period of 7 days was separated by wash-out periods of 14 days. Unstimulated and paraffin chewing stimulated whole saliva, and 3% citric acid stimulated parotid and submandibular-sublingual secretion were collected twice daily, at 7.30 a.m., with the patients in a fasting condition (morning values), and at 10.30 a.m., about 2 h after intake of a standard breakfast (lunchtime values), on day 0 (baseline), day 1 (acute treatment), and day 7 (chronic treatment). Saliva flow rates were measured and all four secretions were analysed for the concentration of sodium, potassium, chloride, and total protein. Xerostomia was assessed by means of a Visual Analogue Scale. Statistical analysis used the Wilcoxon signed rank test. For flow rate, only that of submandibular-sublingual secretion was affected, significantly so in the morning during chronic treatment with both drugs. In resting whole saliva the output of both sodium and chloride tended to decrease especially during treatment with bendroflumethiazide, while in submandibular-sublingual secretion the output of all the electrolytes was decreased, especially for potassium and chloride and during treatment with furosemide. Further, xerostomia tended to increase during treatment with furosemide, statistically significant at lunchtime during chronic treatment. In conclusion, this study has demonstrated a modest effect on salivary flow rate and a more pronounced effect on saliva composition, especially in submandibular-sublingual secretion during treatment of healthy volunteers with therapeutic doses of two different diuretics, encouraging clinical studies in hypertensive patients and basic research as to the presence of a thiazide sensitive Na-Cl cotransporter in human salivary glands.

Submandibular secretory and vascular responses to stimulation of the parasympathetic innervation in anesthetized cats

Submandibular secretory responses to stimulation of the parasympathetic chorda-lingual nerve in anaesthetized cats have been investigated before, during, and after intracarotid infusion of endothelin-1 (ET-1), which reduced blood flow through the gland by 64+/-7%. Stimulation at different frequencies (2, 4, 8, and 16 Hz) evoked a frequency-dependent increase in the flow of submandibular saliva, sodium concentration and output, and output of both potassium and protein. The reduction in submandibular blood flow, which occurred in response to the infusion of ET-1, was associated with a decreased flow of saliva and a diminished output of both sodium and protein. The flow of saliva was linearly related to submandibular blood flow both in the presence and absence of ET-1. It is concluded that submandibular secretory responses to electrical stimulation of the parasympathetic innervation can be significantly attenuated by reducing the blood flow through the gland by ET-1 infusion, just as it is when the blood flow is reduced by hypotension.

The buffer capacity and buffer systems of human whole saliva measured without loss of CO2

The buffer capacity of unstimulated (UWS) and stimulated (SWS) whole-mouth saliva involves three major buffer systems. The aim was to determine the buffer capacity of UWS and SWS at specific pH in the interval from pH 7.5 down to pH 3.0. The contribution of each of the buffer systems was also determined under conditions resembling those in the mouth. UWS and SWS were collected from 20 healthy volunteers; the saliva was collected under paraffin oil in order to avoid loss of CO2. The buffer capacity of UWS and SWS in samples with and without bicarbonate (HCO3-) and CO2 were measured at various pH by acid titration in a closed system at 36 C. The mean concentrations of the buffer systems in UWS (mean flow rate 0.55 ml/min) were 4.4 mmol/l HCO3-, 4.5 mmol/l phosphate (of which 1.3 mmol/l was present in the form of HPO4(2-)), 1876 microg/ml protein; the saliva pH was 6.8 and the P(CO2) 29.3 mmHg. The corresponding mean concentrations in SWS (mean flow rate 1.66 ml/min) were 9.7 mmol/l HCO3-, 3.8 mmol/l phosphate (of which 1.9 mmol/l was present in the form of HPO4(2-)), 1955 microg/ml protein; pH 7.2 and P(CO2) 25.7 mmHg, The highest buffer capacity of UWS and SWS was 6.0 and 8.5 mmol H+ /(1 saliva*pH unit) at pH 6.25, respectively. At saliva pH in the range from pH 7 down to pH 5, the following had significant impact on buffer capacity: the HCO3- concentration (p < 0.001), the flow rate (p < 0.01), and the pH of the saliva (p < 0.05). At acidic pH in the range from pH 5 down to pH 4, however, only the protein concentration had a significant impact on buffer capacity (p < 0.01).

Carbachol-induced oxygen consumption in slices from developing rat submandibular and parotid glands

In contrast to the submandibular gland, the developing rat parotid gland shows refractoriness to cholinergic secretagogues until 2 wks of age. To assess the underlining mechanism of this refractoriness, I investigated changes in oxygen consumption as a function of animal age in slices from rat submandibular and parotid glands, measuring both basal and carbachol-stimulated levels. The oxygen consumption was determined by a direct manometric method in the Warburg apparatus. Carbachol-induced oxygen uptake in submandibular gland slices was observed by 1 day of age and reached the adult level of stimulation by 3 wks of age. In the parotid gland, carbachol failed to stimulate oxygen uptake in the early post-natal period, and the first response was detected at 2 wks of age, reaching the adult level at 4 wks of age. Para-fluorohexahydro-sila-diphenidol (pFHHSiD), a selective M3 antagonist, inhibited carbachol-induced oxygen uptake in both glands, while pirenzepine, a selective M1 antagonist, had no effect, suggesting that the M3 muscarinic receptors are involved in this process. The respiratory effect of carbachol, in both glands, was inhibitable by ouabain and, to a lesser extent, by furosemide, indicating that carbachol-enhanced oxygen uptake is due to Na,K-ATPase and that the furosemide-sensitive co-transport of Na+ entry is underdeveloped in immature cells. The ouabain-sensitive Na,K-ATPase activity in the parotid gland increased from birth until 28 days of age. At the time of parotid gland refractoriness to carbachol, Ca2+ ionophore A23187 caused an increase of oxygen uptake only in the presence of extracellular Ca2+. In the presence of carbachol, the effect of ionophore was significantly higher than that of ionophore alone. These results raise the possibility that the refractoriness of the parotid gland to carbachol is due to the inability of carbachol to increase Ca2+ uptake rather than to the lack of distal limb, which resides on the pathway from receptor stimulation to Na,K-ATPase activation.

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

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

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.

Mediation of the depolarization-induced [Ca(2+)]i increase in rat sublingual acini by acetylcholine released from nerve terminals

In sublingual mucous acini, membrane depolarization induces a threefold transient increase in cytosolic free Ca(2+) concentration [(Ca(2+))i]. The underlying mechanism was examined by using the Ca(2+) sensitive fluorescent indicator fura-2. Membrane depolarization with high K+ induced a transient [Ca(2+)]i increase in acini, but not in single acinar cells. Atropine, pirenzepine and 4-diphenylacetoxy-N-methylpiperidine methiodide prevented the[Ca(2)+]i increase, suggesting the involvement of muscarinic receptor activation. Inhibition of the inositol trisphosphate (IP3)-sensitive Ca(2+) release pathway with S-(diethylamino)-octyl-3,4,5-trimethoxybenzoate prevented the depolarization-induced increase in [Ca(2+)]i. Blockade of nicotinic receptors and L-, N-, and P-type voltage-dependent Ca(2+) channels (hexamethonium, nifedipine, diltiazem, (omega-conotoxin GVIA and omega-agatoxin IVA) did not inhibit the increase in [Ca(2+)]i. However, Cd(2)+ (0.2 mM) blocked >85 percent of the [Ca2+]i increase. The depolarization-induced [Ca(2+)]i increase was also extracellular Ca(2+)-dependent. These results suggest that the membrane depolarization-induced Ca(2+) increase in sublingual acini is mediated by activating Cd(2+)-sensitive, voltage-dependent Ca(2+) channels in nerve terminals associated with the dispersed acini and stimulating release of acetylcholine, which then triggers the [Ca(2+)]i increase in acinar cells.

Maxi K+ channels in the basolateral membrane of the exocrine frog skin gland regulated by intracellular calcium and pH

With the single-channel patch-clamp technique we have identified Ca2+-sensitive, high-conductance (maxi) K+ channels in the basolateral membrane (BLM) of exocrine gland cells in frog skin. Under resting conditions, maxi K+ channels were normally quiescent, but they were activated by muscarinic agonists or by high serosal K+. In excised inside-out patches and with symmetrical 140mmol/l K+, single-channel conductance was 200pS and the channel exhibited a high selectivity for K+ over Na+. Depolarization of the BLM increased maxi K+ channel activity. Increasing cytosolic free Ca2+ (by addition of 100nmol/l thapsigargin to the bathing solution of cell-attached patches also increased channel activity, whereas thapsigargin had no effect when added to excised inside-out patches. An increase in cytosolic free Ca2+ directly activated channel activity in a voltage-dependent manner. Maxi K+ channel activity was sensitive to changes in intracellular pH, with maximal activity at pH 7.4 and decreasing activities following acidification and alkalinization. Maxi K+ channel outward current was reversibly blocked by micromolar concentrations of Ba2+ from the cytosolic and extracellular site, and was irreversibly blocked by micromolar concentrations of charybdotoxin and kaliotoxin from the extracellular site in outside-out patches.

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.

Quantitative estimation of the area of luminal and basolateral membranes of rat parotid acinar cells: some physiological applications

Knowledge of luminal and basolateral acinar cell membrane areas of the secretory endpieces is a prerequisite for a detailed quantitative analysis of the ion transport involved in secretion of the primary saliva. In the present study, these areas were estimated in rat parotid acinar cells using standard stereological methods. A total of 480 micrographs--obtained by random sampling from eight glands from four rats--were analysed at a final magnification of 40000x. Expressed per unit cell volume, the area of the luminal acinar cell membrane was: 0.125 micron 2.micron-3 (SEM = 0.027 micron 2.micron-3, n = 4 animals) and the area of the basolateral membrane was: 1.54 microns 2.micron-3 (SEM = 0.085 micron 2.micron-3, n = 4 animals). These figures make it possible to perform a synthesis based upon different categories of experimental data, e.g. on ion fluxes, membrane potentials and single-channel conductances. Thus, we have estimated the density of open, low-conductance Cl- channels in the luminal membrane--which are not readily accessible for direct, patch-clamp analysis--to be approximately 18 channels per microns 2 in the stimulated state.

Peptide-evoked release of amylase from isolated acini of the rat parotid gland

Investigations of the effects of the neuropeptides, substance P (SP), neurokinin A (NKA), neuropeptide K (NPK), gastrin releasing peptide (GRP), calcitonin gene related peptide (CGRP) and vasoactive intestinal peptide (VIP), and of acetylcholine on amylase secretion have been carried out on isolated acini of the rat parotid gland. Furthermore, the occurrence and location of the peptides in the gland was studied. Finally, binding of 125I-BH-SP to isolated acini were studied in order to characterize their tachykinin receptor(s) and their binding kinetics. Only SP, NKA, NPK and VIP stimulated amylase release. VIP, however, with a rather low potency (EC50 at 155 nmol/l). Simultaneous stimulation with two compounds elicited additive responses, except for VIP and acetylcholine which elicited an effect significantly above additive response. Only SP, NKA, VIP and CGRP could be identified in extracts of the gland. The immunoreactivity of these peptides could be located to varicose nerve fibers in the gland. Binding of labeled SP to the isolated acini exhibited the characteristics of a genuine agonist/receptor interaction, and the rank order of displacement potencies indicated the presence of NK1-receptors. Thus, the results of the present study support previous suggestions that the tachykinins and VIP are likely to be involved in amylase secretion in the rat parotid gland.

A review of electron probe X-ray microanalysis studies of salivary gland cells

Electron probe X-ray microanalysis (EPXMA) has now been successfully applied to several salivary gland preparations. This paper briefly reviews the principles underlying this technique and the specific sample preparation procedures which permit accurate measurement of elemental concentrations in the various intracellular spaces. Findings from salivary gland studies indicate that cytoplasmic and nuclear spaces of nonstimulated acinar cells have high concentrations of K and P, and low concentrations of Mg, Ca, and S; and that mature secretory granules have high concentrations of Ca and S, and relatively low concentrations of K and P. No consistent differences have been found between the elemental concentrations of mucous and serous secretory granules. In vivo and in vitro EPXMA studies of the elemental changes associated with secretory granule maturation indicate there are at least two stages in this process: an early stage during which granule S concentration increases in parallel with mass density as condensing vacuoles mature into secretory granules, and a late stage during which granule mass density and protein content increase with no further elemental concentration changes. Findings from other in vivo and in vitro studies indicate that secretory granule membranes are permeable to Na, K, and Cl ions because the granular concentrations of these elements are altered by electrochemical gradients. Recent EPXMA results indicate that cells stimulated with parasympathomimetic agonists have decreased K and Cl concentrations, and increased Na concentrations. Furthermore, the magnitude of these changes are quantitatively consistent with changes measured using radio-isotope equilibration and other techniques. In contrast, cells stimulated with the beta-adrenergic agonist, isoproterenol, have increased concentrations of Na and Cl, but unchanged K concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Signaling mechanisms that regulate saliva formation

The precipitating event in the formation of saliva is the binding of neurotransmitter molecules to cell surface receptor proteins. The principal neurotransmitters involved are acetylcholine and norepinephrine that bind, respectively, to muscarinic-cholinergic, and alpha- and beta-adrenergic receptors. The transduction of the extracellular signal requires an integral membrane protein capable of binding GTP, a G protein, that specifically interacts with the receptor. The components of G protein transduction systems are fairly well studied, but the pathways by which signals are routed are just being recognized. Delineation of such routing pathways is essential to understanding the regulation of saliva formation.

Altered responses to agonists after chronic in vivo atropine administration in rat parotid acini

Salivary gland hypofunction, resulting from a variety of perturbations including prescribed medications, is associated with adverse effects on the health of the oral cavity. In the present study, we investigated the in vivo effects of chronic administration of atropine, a muscarinic antagonist, on the acute response of rat parotid acini to alpha-adrenergic and muscarinic stimulation. The regulation of intracellular pH (pHi) and cytosolic free Ca2+ ([Ca2+]i) were monitored using dual wavelength microfluorometry of the ion-sensitive fluorescent dyes, BCECF and fura-2, respectively. Chronic atropine treatment (40 mg/kg/d for 4 weeks) significantly increased the magnitude of the initial (< 30 s) agonist-induced rise in [Ca2+]i, but did not alter the sustained increase in [Ca2+]i (> 2 min). The generation of inositol trisphosphates and inositol tetrakisphosphates after 30 s of muscarinic stimulation was not significantly altered. The resting Cl- content as well as the stimulated Cl- loss, were reduced in parotid acini after chronic atropine administration. In addition, the muscarinic- and alpha-adrenergic-induced intracellular acidification was blunted, suggesting that reduced HCO3- efflux occurs in acini isolated from atropine-treated animals. Our results indicate (1) that chronic atropine treatment does not inhibit the receptor-coupled generation of inositol phosphates or the resulting rise in [Ca2+]i and (2) chronic treatment may prevent the production of saliva either by reducing the driving force for anion-dependent fluid secretion or by preventing the activation of the anion efflux pathway.

Regulation of cytoplasmic pH in rat sublingual mucous acini at rest and during muscarinic stimulation

The regulation of intracellular pH (pHi) in rat sublingual mucous acini was monitored using dual-wavelength microfluorometry of the pH-sensitive dye BCECF (2',7'-biscarboxyethyl-5(6)-carboxyfluorescein). Acini attached to coverslips and continuously superfused with HCO3(-)-containing medium (25 mM NaHCO3/5% CO2; pH 7.4) have a steady-state pHi of 7.25 +/- 0.02. Acid loading of acinar cells using the NH4+/NH3 prepulse technique resulted in a Na(+)-dependent, MIBA-inhibitable (5-(N-methyl-N-isobutyl) amiloride, Ki approximately 0.42 microM) pHi recovery, the kinetics of which were not influenced by the absence of extracellular Cl-. The rate and magnitude of the pHi recovery were dependent on the extracellular Na+ concentration, indicating that Na+/H+ exchange plays a critical role in maintaining pHi above the pH predicted for electrochemical equilibrium. When the NH4+/NH3 concentration was varied, the rate of pHi recovery was enhanced as the extent of the intracellular acidification increased, demonstrating that the activity of the Na+/H+ exchanger is regulated by the concentration of intracellular protons. Switching BCECF-loaded acini to a Cl(-)-free medium did not significantly alter resting pHi, suggesting the absence of Cl-/HCO3- exchange activity. Muscarinic stimulation resulted in a rapid and sustained cytosolic acidification (t 1/2 < 30 sec; 0.16 +/- 0.02 pH unit), the magnitude of which was amplified greater than two-fold in the presence of MIBA (0.37 +/- 0.05 pH unit) or in the absence of extracellular Na+ (0.34 +/- 0.03 pH unit). The agonist-induced intracellular acidification was blunted in HCO3(-)-free media and was inhibited by DPC (diphenylamine-2-carboxylate), an anion channel blocker. In contrast, the acidification was not influenced by removal of extracellular Cl-. The Ca2+ ionophore, ionomycin, mimicked the effects of stimulation, whereas preloading acini with BAPTA (bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetra-acetic acid) to chelate intracellular Ca2+ blocked the agonist-induced cytoplasmic acidification. The above results indicate that during muscarinic stimulation an intracellular acidification occurs which: (i) is partially buffered by increased Na+/H+ exchange activity; (ii) is most likely mediated by HCO3- efflux via an anion channel; and (iii) requires an increase in cytosolic free [Ca2+].

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

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Agonist-induced Ca2+ mobilization in the rat submandibular gland during aging and subsequent to chronic propranolol treatment

The effects of age and chronic propranolol treatment on the agonist-induced rise in intracellular free Ca2+ ([Ca2+]i), an index for the coupling of receptor-second messenger generation, was studied using a dispersed rat submandibular gland preparation. Muscarinic stimulation (10 microns carbachol) caused a rapid (T1/2 less than 2 s) and dramatic (approximately 4.5-fold) rise in [Ca2+]i followed by a lower sustained increase (approximately 3-fold) in [Ca2+]i as measured directly with the Ca(2+)-sensitive fluorescent probe, fura-2. The magnitude and the rate of increase of the initial rise in [Ca2+]i and the level of the sustained increase in [Ca2+]i were not different between 2- an 21-month-old rats. Stimulation in a Ca(2+)-free medium reduced the initial agonist-induced increase in [Ca2+]i by approximately 35-40%, while the sustained increase was abolished by the removal of extracellular Ca2+ from cells in both young and old rats. Chronic treatment for 30 days with 20 mg/kg propranolol, a beta-adrenergic antagonist, did not significantly alter the ability of dispersed submandibular cells in old rats to mobilize Ca2+ during agonist stimulation or influence the in vivo stimulated gland output. These results suggest that the agonist-induced rise in [Ca2+]i is not altered by aging or by chronic treatment of aged rats with propranolol and, therefore, receptor-second messenger coupling remains intact.

In vitro methods for the assessment of the inhibitory effects of antidepressants in rat parotid glands

The present study evaluates suitable in vitro methods for the assessment of the inhibiting properties of four principally different antidepressant drugs. This was done by comparing the acute effects of antidepressants on autonomic receptor binding (homogenates) together with parallel tests evaluating the biological activities of the receptor systems in collagenase-isolated rat parotid acini. The responses were measured as receptor-activated changes in cyclic nucleotide formation and acinar oxygen consumption. Muscarinic acetylcholine receptor binding, carbachol-induced cGMP formation, and oxygen consumption all reflected the various inhibiting effects of the antidepressants tested. Measurements of the carbachol-induced O2 consumption was however, the most sensitive method and may be considered a well-suited and reliable parameter concerning the expected severity of anticholinergic side-effects caused by medication. The disturbing 'dry mouth' symptoms following treatment with amitriptyline or mianserin are however, also attributed to their substantial adrenoceptor-blocking effects, which are best demonstrated by alpha 1-adrenoceptor binding studies in combination with measurements of the adrenaline-induced O2 consumption in the rat parotid gland.

Effects of chloride deficiency on the pancreatic B-cell response to acetylcholine

Muscarinic stimulation of pancreatic B-cells markedly amplifies insulin secretion through complex mechanisms which involve changes in membrane potential and ionic fluxes. In this study, normal mouse islets were used to evaluate the role of Cl- ions in these effects of acetylcholine (ACh). Whatever the concentration of glucose, the rate of 36Cl- efflux from islet cells was unaffected by ACh. Replacement of Cl- by impermeant isethionate in a medium containing 15 mM glucose did not affect, or only slightly decreased, the ability of ACh to depolarize the B-cell membrane and increase electrical activity, to accelerate 45Ca2+ and 86Rb+ efflux from islet cells, and to amplify insulin release. In the absence of extracellular Ca2+, a high concentration of ACh (100 microM) mobilized intracellular Ca2+ and caused a transient release of insulin and a sustained acceleration of 86Rb+ efflux. None of these effects was affected by Cl- omission or by addition of furosemide, a blocker of the Na+, K+, 2Cl- cotransport. Isethionate substitution for Cl- in a medium containing a nonstimulatory concentration of glucose (3 mM) barely reduced the depolarization of B-cells by ACh, but inhibited the concomitant increase in 86Rb+ efflux. We have no explanation for the latter effect that was not mimicked by furosemide. In conclusion, ACh stimulation of pancreatic B-cells, unlike that of exocrine acinar cells, is largely independent of Cl- and is insensitive to furosemide. The acceleration of ionic fluxes produced by ACh does not involve the Na+, K+, 2Cl- cotransport system.

Comparative studies of D-myoinositol 1,4,5-trisphosphate and synthetic 6-deoxy D-myoinositol 1,4,5-trisphosphate: binding and calcium release activity in rat parotid microsomes

In this study, we report our data on the binding of D-myoinositol(1,4,5)P3 and of 6-deoxy D-myoinositol(1,4,5)P3 to a rat parotid microsomal fraction and their effect on Ca2+ release. The binding affinity and the potency of 6-deoxy Ins(1,4,5)P3 to induce Ca2+ release are about 100 times lower than those of Ins(1,4,5)P3. However, maximal concentrations of both inositol trisphosphates induce similar calcium efflux and present comparable displacement of radioligand binding. Experiments were performed to exclude that the microsomal preparations used display rapid metabolism of Ins(1,4,5)P3 or 6-deoxy Ins(1,4,5)P3 during binding and Ca2+ release. We also report that, in permeabilized rat parotid acini preparations, 6-deoxy Ins(1,4,5)P3 is about 100 times less potent than Ins(1,4,5)P3 in inducing Ca2+ release. These data indicate that removal of the hydroxyl group in position 6 of the Ins(1,4,5)P3 molecule severely reduces its binding affinity which seems, in a large part at least, responsible for the reported loss of potency in mobilizing Ca2+. Nevertheless, 6-deoxy Ins(1,4,5)P3 seems to be a full agonist for the release of Ca2+.

Patch-clamp studies of K+ and Cl- channel currents in canine pancreatic acinar cells

K+ and Cl- channel currents in the plasma membrane of isolated canine pancreatic acinar cells were studied by patch-clamp single-channel and whole-cell current recording techniques. In excised inside-out patches, we found a Ca(2+)-activated (control range 0.01-0.4 microM) and voltage-activated K(+)-selective channel with a unit conductance of approximately 40 pS in symmetrical K(+)-rich solutions. In intact cells, addition of acetylcholine (1 microM) or bombesin tetradecapeptide (0.1 nM) to the bath evoked an increase in frequency of K+ channel opening. In whole-cell recordings on cells dialyzed with K(+)-rich and Ca(2+)-free solution containing 0.5 mM EGTA, the resting potential was about -40 mV. Depolarizing voltage pulses activated outward K+ currents, which were blocked by 10 mM tetraethylammonium, whereas hyperpolarizing pulses evoked smaller inward currents. Acetylcholine or bombesin activated the K+ current and enhanced the inward current, which was reduced by a low Cl- (10 mM) intracellular solution at -90 mV holding potential. These results suggest that both Ca(2+)- and voltage-activated K+ channels and Ca(2+)-activated Cl- channels exist in the plasma membrane of canine pancreatic acinar cells.

Spatial distribution of intracellular, free Ca2+ in isolated rat parotid acini

The spatial distribution of intracellular, free Ca2+ ([Ca2+]i) in rat parotid acini was measured by imaging fura-2 fluorescence from individual acinar cells by means of a digital imaging microscope. Upon cholinergic stimulation in a Krebs-Ringer bicarbonate buffer at (37 degrees C), [Ca2+]i increased synchronously at both the basolateral and luminal membranes as well as in all cells of the secretory endpiece, reaching peak [Ca2+]i levels 1 s after stimulation. Atropine addition caused a rapid down-regulation of [Ca2+]i, which, however, never reached prestimulatory levels. When acini were stimulated in a medium containing 5 nM Ca2+, the Ca2+ mobilization arising from internal pools caused an increase in [Ca2+]i predominantly near the basolateral area, where the endoplasmic reticulum is located, and standing Ca2+ gradients were observed for up to 10 s. A mathematical model is developed to simulate the time courses of the Ca2+ profiles through the cytoplasm using estimated values of the Ca2+ diffusion coefficients and the cytosolic Ca2+ buffering capacity. It is concluded that under physiological conditions, the Ca2+ release from the endoplasmic reticulum is responsible for the activation of the basolaterally located K+ channels. Furthermore, Ca2+ influx from the interstitium is responsible for much of the rise in [Ca2+]i near the luminal membranes, where the Cl- channels are supposed to be located.

Adrenoceptor-activation of oxygen consumption in rat parotid acini

The purpose of this study was to examine the effects of various adrenergic agonists and antagonists upon rat parotid oxygen consumption. The experiments were performed using collagenase-isolated acini, and the O2 consumption was determined using a Gilson Oxygraph 5/6 H with a Clark electrode. Stimulation with the alpha- and beta-adrenergic agonist adrenaline (10 microM) lead to a 65% increase in parotid O2 consumption in about 10 sec. Addition of adrenaline after preincubation with the beta-adrenergic antagonist propranolol or the alpha-adrenergic antagonist prazosin showed that about 2/3 of the adrenaline-induced O2 consumption originated in alpha-adrenergic activity, whereas the remaining 1/3 stemmed from beta-adrenergic activity. Correspondingly, it was found that stimulation by the beta-adrenergic agonist isoprenaline (10 microM) increased the O2 consumption with approximately 22%. Stimulation with the alpha-adrenergic agonist phenylephrine (10 microM) did however, only increase O2 consumption with 21%. This finding is probably not related to the existence of alpha-2-adrenoceptors stimulated by adrenaline and not by phenylephrine, since: (1) the adrenaline-induced response was unaffected by preincubation by pertussis toxin, (an activator of the Gi protein of the adenylate cyclase complex), and (2) the stimulating effect of clonidine (an alpha-2-adrenoceptor agonist) was inhibited by preincubation with prazosin, and (3) radioligand binding studies using [3H]-yohimbine was unsuccessful in demonstrating parotid alpha-2-adrenoceptors. Accordingly, a conclusion that accounts for the findings in this paper is that only beta- and alpha 1-adrenoceptors are functioning in the parotid acini and that phenylephrine acts as a partiel alpha 1-agonist.

Inhibitory effects of amitriptyline on the stimulation-induced Ca2+ increase in parotid acini

The present study demonstrates the effects of the antidepressant, amitriptyline, and the acetylcholine antagonist, atropine, on the stimulation-induced rise in cytosolic, free Ca2+ (Cai2+). The changes in Cai2+ of collagenase-isolated rat parotid acini were measured by means of the Ca2(+)-sensitive dye, fura-2. It was found that stimulation by carbachol resulted in a maximal increase of 582 +/- 34 nM (mean +/- S.E.) in Cai2+ with a ks of 5.8 +/- 1.3 microM. Adrenaline caused a rise of 380 +/- 22 nM in Cai2+ with a ks of 0.5 +/- 0.2 microM. Amitriptyline and atropine were found to inhibit the carbachol-induced rise in Cai2+ with dissociation constants (kI) of 105 and 1.25 nM, respectively, in the absence of agonist. The adrenergic-induced rise in Cai2+ was inhibited by amitriptyline with a kI of 45 nM. Amitriptyline was found to inhibit both receptor classes by a competitive or mixed type of inhibition. Similarly, atropine exerted the same type of inhibition on the acetylcholine receptor. Amitriptyline and atropine were found to be mutually exclusive for competing for substrate binding on the receptor. These findings are consistent with a common binding site for amitriptyline and atropine on the acetylcholine receptor, possibly in close proximity with, but different from the substrate binding site. The stimulation-induced cell shrinkage evoked by the loss of electrolytes and water from the acini was measured by a 90 degree light scattering signal. It was found that this method makes possible the detection of autonomic side-effects of antidepressants on acini suspended in protein-containing media.

Effects of extracellular ATP on ion transport systems and [Ca2+]i in rat parotid acinar cells. Comparison with the muscarinic agonist carbachol

The effects of extracellular ATP on ion fluxes and the intracellular free Ca2+ concentration ([Ca2+]i) were examined using a suspension of rat parotid acinar cells and were contrasted with the effects of the muscarinic agonist carbachol. Although ATP and carbachol both rapidly increased [Ca2+]i about threefold above the resting level (200-250 nM), the effect of ATP was due primarily to an influx of Ca2+ across the plasma membrane, while the initial response to carbachol was due to a release of Ca2+ from intracellular stores. Within 10 s, ATP (1 mM) and carbachol (20 microM) reduced the cellular Cl- content by 39-50% and cell volume by 15-25%. Both stimuli reduced the cytosolic K+ content by 57-65%, but there were marked differences in the rate and pattern of net K+ movement as well as the effects of K+ channel inhibitors on the effluxes initiated by the two stimuli. The maximum rate of the ATP-stimulated K+ efflux (approximately 2,200 nmol K+/mg protein per min) was about two-thirds that of the carbachol-initiated efflux rate, and was reduced by approximately 30% (vs. 60% for the carbachol-stimulated K+ efflux) by TEA (tetraethylammonium), an inhibitor of the large conductance (BK) K+ channel. Charybdotoxin, another K+ channel blocker, was markedly more effective than TEA on the effects of both agonists, and reduced the rate of K+ efflux initiated by both ATP and carbachol by approximately 80%. The removal of extracellular Ca2+ reduced the ATP- and the carbachol-stimulated rates of K+ efflux by 55 and 17%, respectively. The rate of K+ efflux initiated by either agonist was reduced by 78-95% in cells that were loaded with BAPTA to slow the elevation of [Ca2+]i. These results indicated that ATP and carbachol stimulated the efflux of K+ through multiple types of K(+)-permeable channels, and demonstrated that the relative proportion of efflux through the different pathways was different for the two stimuli. ATP and carbachol also stimulated the rapid entry of Na+ into the parotid cell, and elevated the intracellular Na+ content to 4.4 and 2.6 times the normal level, respectively. The rate of Na+ entry through Na(+)-K(+)-2Cl- cotransport and Na(+)-H+ exchange was similar whether stimulated by ATP, carbachol, or ionomycin, and uptake through these two carrier-mediated transporters accounted for 50% of the ATP-promoted Na+ influx. The remainder may be due to a nonselective cation channel and an ATP-gated cation channel that is also permeable to Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

Solubilization and partial purification of the rabbit parotid Na/K/Cl-dependent bumetanide binding site

We demonstrate that the high affinity bumetanide binding site of the rabbit parotid acinar cell can be extracted from a basolateral membrane fraction using relatively low concentrations (0.07%, wt/vol; 1 mg membrane protein/ml) of the nonionic detergent Triton X-100. This extracted site cannot be sedimented by ultracentrifugation at 100,000 x g x 1 hr. Bumetanide binding to this site retains the ionic characteristics of bumetanide binding to native membranes but shows a fivefold increase in binding affinity (Kd = 0.57 +/- 0.15 microM vs. Kd = 3.3 +/- 0.7 microM for native membranes). Inactivation of the extracted bumetanide binding site observed at detergent/protein ratios greater than 1 can be prevented or (partially) reversed by the addition of exogenous lipid (0.2% soybean phosphatidylcholine). When the 0.07% Triton extract is fractionated by sucrose density gradient centrifugation in 0.24% Triton X-100, 0.2% exogenous lipid and 200 mM salt, the high affinity bumetanide binding site sediments as a single band with S20,w = 8.8 +/- 0.8 S. This corresponds to a molecular weight approximately 200 kDa for the bumetanide binding protein-detergent-lipid complex and represents a sevenfold purification of this site relative to the starting membrane fraction. In contrast to previous attempts to purify Na/K/Cl cotransport proteins and their associated bumetanide binding sites, the present method avoids harsh detergent treatment as well as direct covalent modification (inactivation) of the transporter itself. As a consequence, one can follow the still active protein through a series of extraction and purification steps by directly monitoring its bumetanide binding properties.

Differentiation of alpha adrenoceptors mediating increase of oxygen consumption in rat submandibular salivary gland slices

Clonidine, noradrenaline and adrenaline (in the presence of propranolol), but not phenylephrine and methoxamine, stimulated an increase in the oxygen consumption of these slices that was blocked by yohimbine but not by prazosin. The stimulation was inhibited by ouabain and required the presence of Ca2+ in the incubation medium. The calcium ionophore A 23187 stimulated oxygen consumption in the tissue slices and enhanced the respiratory effect of clonidine. Atropine and (D-Pro2, D-Trp7.9)-substance P failed to block the respiratory response to clonidine in concentrations that inhibited the respiratory effects of carbachol and substance P, respectively. Release of acetylcholine from the unstimulated gland slices was reduced by clonidine or Ca2+ omission. Yohimbine prevented the clonidine effect and stimulated acetylcholine resting release. Nifedipine did not affect either the release of acetylcholine or the clonidine-induced reduction of acetylcholine release but blocked the oxygen uptake due to clonidine or to release acetylcholine.

Inactivation of the rabbit parotid Na/K/Cl cotransporter by N-ethylmaleimide

The inactivation of the rabbit parotid Na/K/Cl cotransporter by the irreversible sulfhydryl reagent N-ethylmaleimide (NEM) is studied by monitoring its effect on high affinity bumetanide binding to the carrier. NEM reduces the number of bumetanide binding sites with no significant change in the affinity of those remaining. NEM also reduces KCl-dependent 22Na flux via the cotransporter by the same factor as the reduction in bumetanide binding sites. Both bumetanide and its analogue furosemide can protect against the effect of NEM. The concentration range over which this protection occurs is in good agreement with affinities of these two compounds for the high affinity bumetanide binding site (2.6 and 8.5 microM, respectively), indicating an association of this site with the site of action of NEM. Also consistent with this hypothesis are the observations that (i) sodium and potassium, both of which are required for high affinity bumetanide binding, increase the rate of inactivation of binding by NEM and (ii) chloride, at concentrations previously shown to competitively inhibit bumetanide binding, protects the cotransporter against NEM. The effects of NEM on bumetanide binding are mimicked by another highly specific sulfhydryl reagent, methyl methanethiolsulfonate. The apparent rate constant for inactivation of high affinity bumetanide binding by NEM is a hyperbolic function of NEM concentration consistent with a model in which the inactivation reaction is first order in [NEM] and proceeds through an intermediate adsorptive complex. The data indicate that the presence of a reduced sulfhydryl group at or closely related to the bumetanide binding site is essential for the operation of the parotid Na/K/Cl cotransporter.