Biosynthesis of the antitumor agent chartreusin involves the oxidative rearrangement of an anthracyclic polyketide

Chartreusin is a potent antitumor agent with a mixed polyketide-carbohydrate structure produced by Streptomyces chartreusis. Three type II polyketide synthase (PKS) gene clusters were identified from an S. chartreusis HKI-249 genomic cosmid library, one of which encodes chartreusin (cha) biosynthesis, as confirmed by heterologous expression of the entire cha gene cluster in Streptomyces albus. Molecular analysis of the approximately 37 kb locus and structure elucidation of a linear pathway intermediate from an engineered mutant reveal that the unusual bis-lactone aglycone chartarin is derived from an anthracycline-type polyketide. A revised biosynthetic model involving an oxidative rearrangement is presented.

A new origin for chartreusin

Unraveling the sequence of reactions that transforms aromatic polyketide backbones into much more elaborate structures, for example the chartreusin bislactone, is challenging, but in this issue of Chemistry & Biology, Xu and coworkers [1] show that sequencing the gene clusters can provide valuable clues.

Contamination of oxygen-consuming organics in the Yellow River of China

Contamination of oxygen-consuming organics (OCOs) was one of the most serious problems in the Yellow River of China. This study was conducted to analyze monitoring of the data on OCOs contamination for the river in 1980 and during 1992-1999 as well as examining the effect of suspended solids (SS) on chemical oxygen demand (CODMn) and biochemical oxygen demand (BOD5) of river water. Several significant results have arisen from the study. First, CODMn and BOD5 of the river water showed an increasing trend from the upper to the lower reaches of the mainstream. BOD5 values of river water in 1992 were significantly higher than those in 1980 and showed an increasing trend during 1992-1999. Second, OCOs in river water of the mainstream was attributed mainly to point sources; the ratio of point to non-point sources of BOD5 was about 2.81. The load from point sources showed an increasing trend during 1992-1998. In contrast, the load from non-point sources manifested a decreasing trend during this period; this was caused by the decreasing trend of SS content in river water. The total load of BOD5 from point and non-point sources displayed an increasing trend during 1992-1998. Third, as the humic substances in SS can hardly be biologically oxidized in natural conditions but can be oxidized by chemical oxidants such as potassium permanganate, CODMn was not suitable for being regarded as a parameter reflecting the pollution degree of OCOs in river water with a high SS content.

Determination of pH by microfluorometry: intracellular and interstitial pH regulation in developing early-stage fish embryos (Danio rerio)

Microfluorometric techniques were applied in vivo for continuous monitoring of specific acid-base parameters in zebrafish (Danio rerio) embryos during early stages of ontogeny. Dextran-coupled pH-sensitive single-excitation/dual-emission dye SNARF-1 was pressure-injected into individual cells or the interstitial space of 16- to 256-cell embryos,and pH was continuously recorded during subsequent development for time periods of up to 8 h. A novel calibration technique was developed, essentially characterized by in vitro inorganic buffer calibration of the optical system and mathematical post-processing according to the effects of in vivo dye modifiers through a correlation established by direct comparison of optical techniques with pH microelectrodes. This approach results in high accuracy of microfluorometry, comparable with that of pH electrodes, and a recovery only limited by the physical stability of the utilized optical system.

Intracellular pH (pHi) in Danio rerio embryos between 1k-cells stage and the end of epiboly was found to be well regulated to a mean value of 7.55±0.13 (± s.d.), a range distinctly more alkaline than typical values for adult fish but in accordance with embryonic pHi of a few non-fish species shortly after fertilization. Also, interstitial pH (pHint) was significantly higher (8.08±0.25) than values for extracellular pH in adult fish. Distributions of HCO3- across membranes and between interstitium and ambient fluid compared with respective potentials strongly suggest that pH in these early stages of ontogeny is already adjusted by active transfer processes. Non-respiratory changes in ambient pH between 7.7 and 8.5 did not significantly affect pHi, a result potentially attributable to low membrane leakage rate or to the potency of active transfer mechanisms. In order to assess the pH regulatory systems more quantitatively,embryos were exposed to ambient changes of carbon dioxide partial pressure(PCO2). The direct impact of PCO2 changes on cell pH was alleviated by cell non-bicarbonate buffering and subsequent rapid, almost complete, compensation by changes in cell[HCO3-] as an expression of transmembrane transfer of acid-base relevant ions. On the basis of these results, we conclude that the regulatory potency of embryonic cells is well developed, is active to resist extensive homoiostatic stress and is efficient to maintain critical metabolism in adverse conditions, even at early stages of ontogeny.

Humic acid toxicity in biologically treated soil contaminated with polycyclic aromatic hydrocarbons and pentachlorophenol

Contaminated soil from a land treatment unit at the Libby Groundwater Superfund Site in Libby, MT, was amended with 14C pyrene and incubated for 396 days to promote biodegradation and the formation of soil-associated bound residues. Humic and fulvic acids were extracted from the treated soil microcosms and analyzed for the presence of pyrene residues. Biologic activity promoted 14C association with the fulvic acid fraction, but humic acid-associated 14C did not increase with biologic activity. The Aboatox flash toxicity assay was used to assess the toxicity of humic and fulvic acid fractions. The fulvic acid gave no toxic response, but the humic acid showed significant toxicity. The observed toxicity was likely associated with pentachlorophenol, a known contaminant of the soil that was removed by solvent extraction of the humic acid and that correlated well with toxicity reduction.

Use of flow cytometry and SNARF to calibrate and measure intracellular pH in NS0 cells

BACKGROUND

Two calibration methods have been proposed for determining the relation between the fluorescence ratio of a pH-sensitive fluorescent indicator and intracellular pH (pHi). The first method uses nigericin to clamp pHi to external pH (pHe) and the second is the null point method. We compared these different calibration methods, solution conditions, and temperatures by using flow cytometry and the fluorescent dye 1,5- (and-6)-carboxy seminaphtorhodafluor-1-acetoxymethyl ester with an NS0 cell line.

METHODS

The nigericin method was performed in glucose solutions supplemented with KCl and 2-(N-morpholino)ethane sulphonic acid plus tris(hydroxymethyl)aminomethane (solution 1A), a mixture of K2HPO4/KH2PO4 in glucose-solution supplemented solutions (solution 2A), or bicarbonate buffered growth medium supplemented with K2HPO4/KH2PO4 (solution 2B); this allowed a range of pHe values to be used. The effect of temperature (22 degrees C or 37 degrees C) on the nigericin calibration curve was also investigated. The null point method was performed by using a series of solutions with a mixture of weak acid and base with a known pHi response.

RESULTS

Using solution 1A as the calibration solution resulted in acidic values of pHi for cells cultured in medium as compared with the values achieved with solution 2A. Using solution 2B did not affect the calibration curve. For the temperatures considered in this study, there was no affect on the calibration curve, but temperature did affect the pHi value of cells in phosphate buffered saline. The pseudo-null point method used with flow cytometry resulted in a calibration curve that was significantly different (P<0.05) from that achieved using the nigericin method.

CONCLUSIONS

Our data indicates that the choice of calibration solution can affect the reported pHi value; therefore, careful choice of solution is important.

Application of a pH-sensitive fluoroprobe (C-SNARF-4) for pH microenvironment analysis in Pseudomonas aeruginosa biofilms

An important feature of microbial biofilms is the development of four-dimensional physical and chemical gradients in space and time. There is need for novel approaches to probe these so-called microenvironments to determine their effect on biofilm-specific processes. In this study, we describe the use of seminaphthorhodafluor-4F 5-(and-6) carboxylic acid (C-SNARF-4) for pH microenvironment analysis in Pseudomonas aeruginosa biofilms. C-SNARF-4 is a fluorescent ratiometric probe that allows pH quantification independent of probe concentration and/or laser intensity. By confocal scanning laser microscopy, C-SNARF-4 revealed pH heterogeneity throughout the biofilm in both the x,y and x,z planes, with values ranging from pH 5.6 (within the biofilm) to pH 7.0 (bulk fluid). pH values were typically remarkably different than those just a few micrometers away. Although this probe has been successfully used in a number of eukaryotic systems, problems have been reported which describe spectral emission changes as a result of macromolecular interactions with the fluorophore. To assess how the biofilm environment may influence fluorescent properties of the dye, fluorescence of C-SNARF-4 was quantified via spectrofluorometry while the probe was suspended in various concentrations of representative biofilm matrix components (i.e., proteins, polysaccharides, and bacterial cells) and growth medium. Surprisingly, our data demonstrate that few changes in emission spectra occur as a result of matrix interactions below pH 7. These studies suggest that C-SNARF-4 can be used as a reliable indicator of pH microenvironments, which may help elucidate their influence on the medical and geobiological roles of natural biofilms.

Mechanism of Reversible Fluorescent Staining of Protein with Epicocconone

[reaction: see text] Epicocconone is the active ingredient in Deep Purple Total Protein Stain and responsible for the apparent noncovalent staining of proteins in polyacrylamide gel and electroblots. Reaction of epicocconone with amines has shown that epicocconone reacts reversibly with primary amines to produce a highly fluorescent enamine that is readily hydrolyzed by base or strong acid such as in conditions used in post-electrophoretic analysis such as peptide mass fingerprinting or Edman degradation.

Intracellular pH distribution in Saccharomyces cerevisiae cell populations, analyzed by flow cytometry

Intracellular pH has an important role in the maintenance of the normal functions of yeast cells. The ability of the cell to maintain this pH homeostasis also in response to environmental changes has gained more and more interest in both basic and applied research. In this study we describe a protocol which allows the rapid determination of the intracellular pH of Saccharomyces cerevisiae cells. The method is based on flow cytometry and employs the pH-dependent fluorescent probe carboxy SNARF-4F. The protocol attempts to minimize the perturbation of the system under study, thus leading to accurate information about the physiological state of the single cell. Moreover, statistical analysis performed on major factors that may influence the final determination supported the validity of the optimized protocol. The protocol was used to investigate the effect of external pH on S. cerevisiae cells incubated in buffer. The results obtained showed that stationary cells are better able than exponentially grown cells to maintain their intracellular pH homeostasis independently of external pH changes. Furthermore, analysis of the intracellular pH distribution within the cell populations highlighted the presence of subpopulations characterized by different intracellular pH values. Notably, a different behavior was observed for exponentially grown and stationary cells in terms of the appearance and development of these subpopulations as a response to a changing external pH.

Concurrent measurements of the free cytosolic concentrations of H+ and Na+ ions with fluorescent indicators

We report a method for the concurrent measurement of intracellular [Na+] ([Na+ ]i) and pH (pHi) in cells co-loaded with SBFI, a Na+-sensitive fluorophore, and either carboxy SNARF-1 or SNARF-5F, H+-sensitive fluorophores. With the optical filters specified, fluorescence emissions from SBFI and either SNARF derivative were sufficiently distinct to allow the accurate measurement of [Na+]i and pHi in rat hippocampal neurons. Neither the Na+ sensitivity of SBFI nor the pH sensitivities of carboxy SNARF-1 or SNARF-5F was affected by the presence of a SNARF derivative or SBFI, respectively. In addition, the calibration parameters obtained in neurons single-loaded with SBFI, carboxy SNARF-1 or SNARF-5F were not significantly influenced by the presence of a second fluorophore. In contrast to the established weak sensitivity of SBFI for protons, both SNARF derivatives appeared essentially insensitive to changes in [Na+]i. The utility of the technique was demonstrated in neurons co-loaded with SBFI and SNARF-5F, which was found to have a lower p Ka in situ than carboxy SNARF-1. There were no significant differences in the changes in [Na+]i and pHi observed in response either to intracellular acid loads imposed by the NH4+ prepulse technique or to transient periods of anoxia in neurons single-loaded with SBFI or SNARF-5F or co-loaded with both probes. The findings support the feasibility of using SBFI in conjunction with either carboxy SNARF-1 or SNARF-5F to concurrently and accurately measure [Na+]i and pHi.

A new and simple method to evaluate early membrane changes in frozen-thawed boar spermatozoa

Detection of early changes in the sperm plasma membrane during cryopreservation is of utmost importance when designing freezing protocols and has previously been studied in the pig species using annexin-V detection of phosphatidylserine translocation. In the present study we designed a new assay to detect these changes in boar spermatozoa, based on the slight increase of sperm membrane permeability occurring during the early stages of cryoinjury, using the combination of three fluorescent probes, SNARF-1, YO-PRO-1 and ethidium homodimer. Four ejaculates from five different boars were frozen-thawed and flow cytometrically (FC) evaluated as paired samples. One of the samples was assayed using the annexin-V/propidium iodide staining and the other sample was evaluated using the new triple staining. Using this combination of probes, four sperm subpopulations were easily detected: viable, with stable membranes (SNARF-1 positive cells), and three with compromised membranes, one of YO-PRO-1+/Eth- cells, one ethidium homodimer+ spermatozoa and, finally spermatozoa stained both with YO-PRO-1 and ethidium homodimer (YO-PRO-1+/Eth+). The latter three categories corresponded to dead spermatozoa, but with different degree of membrane damage, being YO-PRO+/Eth- an earlier stage of membrane destabilization, (manifested by an increase in membrane permeability, while still maintaining membrane integrity) than YO-PRO+/Eth+. A method agreement analysis between both methods was performed revealing good agreement, although the percentage of live cells was 9.44% larger for the triple stain than the annexin-V assay. The new assay stained all sperm sub-populations present in the sample, making it especially suitable for both fluorescence microscopy and flow cytometry, facilitating the exclusion of debris and egg-yolk particles when using FC.

Flow cytometric kinetic assay of the activity of Na+/H+ antiporter in mammalian cells

BACKGROUND

The Na(+)/H(+) exchanger (NHE) of mammalian cells is an integral membrane protein that extrudes H(+) ion in exchange for extracellular Na(+) and plays a crucial role in the regulation of intracellular pH (pHi). Thus, when pHi is lowered, NHE extrudes protons at a rate depending of pHi that can be expressed as pH units/s.

METHODS

To abolish the activity of other cellular pH-restoring systems, cells were incubated in bicarbonate-free Dulbecco's modified Eagle's medium buffered with HEPES. Flow cytometry was used to determine pHi with 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethyl ester or 5-(and-6)-carboxy SNARF-1 acetoxymethyl ester acetate, and the appropriate fluorescence ratios were measured. The calibration of fluorescence ratios versus pHi was established by using ionophore nigericin. The activity of NHE was calculated by a kinetic flow cytometric assay as the slope at time 0 of the best-fit curve of pHi recovery versus time after intracellular acidification with a pulse of exogenous sodium propionate.

RESULTS

The kinetic method allowed determination of the pHi-dependent activity of NHE in cell lines and primary cell cultures. NHE activity values were demonstrated to be up to 0.016 pH units/s within the pHi range of 7.3 to 6.3. The inhibition of NHE activity by the specific inhibitor ethyl isopropyl amiloride was easily detected by this method.

CONCLUSIONS

The assay conditions can be used to relate variations in pHi with the activity of NHE and provide a standardized method to compare between different cells, inhibitors, models of ischemia by acidification, and other relevant experimental or clinical situations.

A Proton-Dependent Zinc Uptake in PC12 Cells

Intracellular pH in pheochromocytoma (PC12) cells was manipulated by 'acid loading' the cells and the effect of such a change on radioactive zinc uptake was studied. It was found that zinc uptake was stimulated in cells loaded with protons without causing any measurable change in the intracellular pH. To confirm our assumption that the proton flux due to zinc entry is too small to be measured, we calculated the pH change that one would expect because of zinc influx. The intrinsic buffer capacity of PC12 cells was determined to be 8.03 mM/pH unit and was used in these calculations. It was found that at the five-minute incubation, zinc uptake occurring under our experimental conditions could cause a pH change of 0.000277 pH units per minute (assuming a 1:2 zinc:proton stoichiometry). This study adds a new dimension towards understanding the role played by intracellular pH in causing zinc entry into cells.

Vertical distribution and characteristics of soil humic substances affecting radionuclide distribution

The humic substances extracted from different soil depths are separated into humic (HA) and fulvic (FA) acids, and characterized for their chemical composition, proton exchange capacity, spectroscopic characteristics and binding properties to the europium ion. The chemical and spectroscopic results show that FA compared to HA has a relatively high O/C ratio, high acidic functional group contents and low aromatic contents. The synchronous fluorescence spectroscopic results show that the stability constant (K) of the soil humic substances with Eu(III) ion tends to increase as the soil depth becomes deeper, and HA has a slightly stronger binding ability than FA. The measured total site concentrations (C(L)) reveal that Eu(III) ion is loaded onto HA by 62-77% of the total acid sites, but FA is only approximately 50% covered by Eu(III) ion. Information could be useful in understanding the migration of radionuclides in soil layer.

Preconcentration and determination of priority pollutant phenols in waters at trace levels using a polymeric solid-phase extraction cartridge

A method to preconcentrate and analyse the EPA priority pollutant phenols from water samples is evaluated. Spe-ed Advanta cartridges, containing 100mg of a polymeric matrix modified with polar groups, are used in the extraction and enrichment step. Several parameters are studied in order to find the best experimental conditions to perform a solid-phase extraction process. Sample flow rate and pH appear as variables that could significantly affect the recoveries obtained. Water samples need a pH adjustment to values lower than 3 units and must be percolated through the cartridges with flow rates over 5 mL min(-1). The highest recoveries and preconcentrations are obtained using acetonitrile as the elution solvent. Sample volumes from 100 mL to 1 L with concentrations ranging from 50 to 0.5 microg L(-1) are analysed with quantitative recoveries and similar efficiencies being achieved. After establishing the best conditions, we applied the method to the analysis of spiked natural waters.

Glutamate-induced elevations in intracellular chloride concentration in hippocampal cell cultures derived from EYFP-expressing mice

The homeostasis of intracellular Cl(-) concentration ([Cl(-)](i)) is critical for neuronal function, including gamma-aminobutyric acid (GABA)ergic synaptic transmission. Here, we investigated activity-dependent changes in [Cl(-)](i) using a transgenetically expressed Cl(-)-sensitive enhanced yellow-fluorescent protein (EYFP) in cultures of mouse hippocampal neurons. Application of glutamate (100 microm for 3 min) in a bath perfusion to cell cultures of various days in vitro (DIV) revealed a decrease in EYFP fluorescence. The EYFP signal increased in amplitude with increasing DIV, reaching a maximal response after 7 DIV. Glutamate application resulted in a slight neuronal acidification. Although EYFP fluorescence is sensitive to pH, EYFP signals were virtually abolished in Cl(-)-free solution, demonstrating that the EYFP signal represented an increase in [Cl(-)](i). Similar to glutamate, a rise in [Cl(-)](i) was also induced by specific ionotropic glutamate receptor agonists and by increasing extracellular [K(+)], indicating that an increase in driving force for Cl(-) suffices to increase [Cl(-)](i). To elucidate the membrane mechanisms mediating the Cl(-) influx, a series of blockers of ion channels and transporters were tested. The glutamate-induced increase in [Cl(-)](i) was resistant to furosemide, bumetanide and 4,4'-diisothiocyanato-stilbene-2,2'-disulphonic acid (DIDS), was reduced by bicuculline to about 80% of control responses, and was antagonized by niflumic acid (NFA) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB). We conclude that membrane depolarization increases [Cl(-)](i) via several pathways involving NFA- and NPPB-sensitive anion channels and GABA(A) receptors, but not through furosemide-, bumetanide- or DIDS-sensitive Cl(-) transporters. The present study highlights the vulnerability of [Cl(-)](i) homeostasis after membrane depolarization in neurons.

The effect of fire on soil organic matter--a review

The extent of the soil organic carbon pool doubles that present in the atmosphere and is about two to three times greater than that accumulated in living organisms in all Earth's terrestrial ecosystems. In such a scenario, one of the several ecological and environmental impacts of fires is that biomass burning is a significant source of greenhouse gases responsible for global warming. Nevertheless, the oxidation of biomass is usually incomplete and a range of pyrolysis compounds and particulate organic matter (OM) in aerosols are produced simultaneously to the thermal modification of pre-existing C forms in soil. These changes lead to the evolution of the OM to "pyromorphic humus", composed by rearranged macromolecular substances of weak colloidal properties and an enhanced resistance against chemical and biological degradation. Hence the occurrence of fires in both undisturbed and agricultural ecosystems may produce long-lasting effects on soils' OM composition and dynamics. Due to the large extent of the C pool in soils, small deviations in the different C forms may also have a significant effect in the global C balance and consequently on climate change. This paper reviews the effect of forest fires on the quantity and quality of soils' OM. It is focused mainly on the most stable pool of soil C; i.e., that having a large residence time, composed of free lipids, colloidal fractions, including humic acids (HA) and fulvic acids (FA), and other resilient forms. The main transformations exerted by fire on soil humus include the accumulation of new particulate C forms highly resistant to oxidation and biological degradation including the so-called "black carbon" (BC). Controversial environmental implications of such processes, specifically in the stabilisation of C in soil and their bearing on the global C cycle are discussed.

Transport of anthracene and benz(a)anthracene through iron-quartz and three aquifer materials in laboratory columns

In groundwater systems, dissolved natural organic matter (NOM) can influence the mobility of organic contaminants by altering the contaminant behavior in water and solid phases. The transport of anthracene and benz(a)anthracene (B(a)A) was studied in the presence and absence of NOM and/or soil organic matter (SOM) in column experiments. The results show that sorption are related to the properties of polycyclic aromatic hydrocarbons (PAHs), NOM and SOM. In the Fe-quartz media, the amount of NOM (20 mg/l) in solution had a little effect on increasing the apparent solubility of anthracene and countering increased anthracene sorption. In the natural (Bemidji) soil, Suwannee river fulvic acid (SRFA, 20 mg/l) and Suwannee river humic acid (SRHA) in water did not compete with SOM for anthracene, indicating that SOM has higher partition efficiency for anthracene. It was also observed that slow diffusion through an organic phase apparently caused most of the observed tailing in column breakthrough curves (BTCs). Even though the fOC of washed Bemidji sediment was very low, the transport of B(a)A was retarded significantly, however, and the transport of B(a)A was shown to be facilitated by dissolved NOM.

[The use of nebivolol for the treatment of hypertension in patients with osteoarthrosis]

Nebivolol (5 mg/day) was given for 3 months to 20 patients with essential hypertension and osteoarthrosis treated with diclofenac and to 20 other patients with essential hypertension. After 3 months lowering of blood pressure, decreases of components of 24-hour blood pressure profile, improvements of signs of left ventricular hypertrophy occurred in both groups. There were no significant differences between changes of parameters studied between 2 groups. Thus diclofenac (nonselective cyclooxygenase inhibitor) did not attenuate antihypertensive effect of nebivolol and its action on left ventricular hypertrophy.

Lactate depresses sarcolemmal permeability of Ca2+ in intact arterial smooth muscle

Elevation of ambient lactate concentration has been shown to alter contractile reactivity of vascular smooth muscle. We tested the hypothesis that lactate affects the disposition of intracellular free Ca2+. Porcine carotid artery strips were incubated in normal medium and in medium containing 10 mM sodium lactate or 10 mM sodium pyruvate. The rate and magnitude of contraction in response to K+-depolarization was depressed in lactate when compared to control. This was associated with a decrease in the onset and magnitude of the normal increase in free [Ca2+]i, as reflected by fluorescence of fura-2. Pyruvate had no effect on these variables. Depression in [Ca2+]i could not be attributed to a selective effect of lactate on pHi, membrane potential, or to enhanced superoxide production. Deletion of Ca2+ from the incubation medium negated depression of contractile responsiveness produced by lactate when compared to control. Lactate had no effect on contraction induced by 100 microM norepinephrine, which releases intracellular stored Ca2+. Thus, lactate inhibits arterial smooth muscle contraction by inhibiting influx of Ca2+ across the sarcolemma.

Temperature dependence of Na+-H+ exchange, Na+-HCO3- co-transport, intracellular buffering and intracellular pH in guinea-pig ventricular myocytes

Almost all aspects of cardiac function are sensitive to modest changes of temperature. We have examined the thermal sensitivity of intracellular pH regulation in the heart. To do this we determined the temperature sensitivity of pHi, intracellular buffering capacity, and the activity of sarcolemmal acid-extrusion proteins, Na+-H+ exchange (NHE) and Na+-HCO3- co-transport (NBC) in guinea-pig isolated ventricular myocytes. pHi was recorded fluorimetrically with acetoxymethyl (AM)-loaded carboxy-SNARF-1 at either 27 or 37 degrees C. At 27 degrees C, intrinsic (non-CO2-dependent) buffering power (betai) was approximately 60% of that at 37 degrees C. Acid-extrusion (Je) through NHE was approximately 50% slower than at 37 degrees C, consistent with a Q10 of approximately 2. In 5% CO2/HCO3--buffered conditions, in the presence of 30 microM cariporide to inhibit NHE, acid extrusion via NBC was also slowed at 27 degrees C, suggestive of a comparable Q10. Resting pHi at 27 degrees C was similar in Hepes- or 5% CO2/HCO3--buffered superfusates but, in both cases, was approximately 0.1 pH units lower at 37 degrees C. The higher the starting pHi, the larger was the thermally induced fall of pHi, consistent with a mathematical model where intrinsic buffers with a low principal pKa (e.g. close to 6.0) are less temperature-sensitive than those with a higher pKa. The high temperature sensitivity of pHi regulation in mammalian cardiac cells has implications for experimental work conducted at room temperature. It also has implications for the ability of intracellular acidosis to generate intracellular Na+ and Ca2+ overload, cardiac injury and arrhythmia in the heart.

Proton permeation through the myocardial gap junction

Although protons can directly or indirectly gate solute permeability of the myocardial gap junction, there is little information regarding their own permeation, despite their importance in the regulation of myocardial contractility and rhythm. By pipette-loading of acid into guinea pig isolated, ventricular myocyte pairs while imaging pH(i) confocally using SNARF fluorescence, we have observed that protons permeate the junctional region. Permeation is inhibited by glycyrrhetinic acid, an agent that also increases intercellular electrical resistance, suggesting H+ permeation via gap junctions. The rate of spread of acid between cells appears to be limited by junctional permeation rather than by cytoplasmic diffusion. Mathematical analyses, combined with experiments using SNARF as a proton carrier, suggest that gap junctional H+ transmission may be accomplished physiologically by the permeation of intrinsic "proton-porter" molecules. We propose that proton flux through gap junctions will contribute to the dissipation of regional acid loads within the myocardium. This represents a mechanism for the local control of myocardial pH(i).

Vinyl acetate decreases intracellular pH in rat nasal epithelial cells

Vinyl acetate is a synthetic organic ester that has been shown to produce nasal tumors in rats following exposure to 600 ppm in air. The proposed mechanism of action involves the metabolism of vinyl acetate by carboxylesterases and the production of protons leading to cellular acidification. While vinyl acetate-induced decreases in intracellular pH (pHi) have been demonstrated in rat hepatocytes, comparable data from nasal epithelial cells do not exist. Using an in vitro assay system, we have determined the effects of vinyl acetate exposure on pHi in respiratory and olfactory nasal epithelial cells from rats. The respiratory and olfactory epithelial cells were isolated from dissected maxillo- and ethmoturbinates by enzyme digestion. The cells were plated; loaded with the pH-sensitive dye, carboxyseminaphthorhodafluor-1 (SNARF-1); and observed using confocal microscopy. Individual cellular analysis demonstrated that both respiratory and olfactory epithelial cells responded to vinyl acetate exposures with a dose-dependent decrease in pHi. Changes occurred at 100 microM but reached a plateau above 250 microM. Maximal decreases in pHi were 0.3 pH unit in respiratory epithelial cells. While pHi values were normally distributed for the respiratory epithelial cells, the olfactory epithelial cells demonstrated a bimodal distribution, indicating at least two populations of cells, with only one population of cells responding to vinyl acetate. Acidification in these cells did not plateau but continued to increase at 1000 microM. Bis(p-nitrophenyl)phosphate (BNPP), a carboxylesterase inhibitor, was able to attenuate the vinyl acetate-induced decrease in pHi. Data obtained from the isolated cells were validated using tissue explants. These results are consistent with the proposed mode of action for vinyl acetate and supply further data for developing appropriate risk assessments for vinyl acetate exposure.