Measurement of GABAA receptor function in rat cultured cerebellar granule cells by the Cytosensor microphysiometer

Metabolic activation of A549 human airway epithelial cells by organic dust: a study based on microphysiometry

A Cytosensor microphysiometer, which measures extracellular acidification rate (ECAR), was used to study the early metabolic activation by organic dust from a swine confinement building in a human airway epithelial cell line, A549. The dust is known to cause an intense airway inflammatory reaction following inhalation in vivo and cytokine release in vitro. Dimethyl amiloride (DMA) was used to study sodium/proton exchanger (NHE) activity in cells growing at different cell densities. Exposing cells at low density to dust induced an initial release of acid not involving NHE, followed by a sustained DMA-sensitive NHE activation. In cells near high density, NHE was not activated during exposure resulting in a modest increase in ECAR. Exposing cells at high density resulted in a bi-phasic ECAR pattern; an initial increase in proton release followed by an inhibition of ECAR below baseline. Pretreatment with pertussis toxin (PTX), an inhibitor of receptor/G(i alpha)-coupled signal transductions did not affect ECAR in low and medium density cells, but abolished the inhibition of ECAR in high-density cells. The dust did not prevent forskolin-induced cAMP accumulation and PTX did not affect cAMP in near-confluent cells suggesting the PTX-effect to be cAMP-independent. The ECAR response to organic dust was similar to that of lipopolysaccharide (LPS) except for high-density cells where PTX did not influence the LPS-induced decrease in ECAR below baseline. In summary, the organic dust induces PTX-sensitive (cAMP independent) signalling in near-confluent A549 epithelial cells and, depending on cell density opposing effects on NHE activity during exposure.

Continuous measurement of glucose utilization in heart myoblasts

Understanding quantitative aspects of cell energy metabolism and how it is influenced by environment is central to biology, medicine, and biotechnology. Most methods used for measuring metabolic fluxes associated with energy metabolism require considerable personnel effort or high maintenance instrumentation. The microphysiometer is a commercially available instrument that measures acid extrusion rates, which are commonly used for drug screening. With the addition of oxygen sensors, the instrument can also be used to measure cell oxygen consumption rates and thereby calculate glycolytic fluxes. In the work described here, oxygen consumption and acid extrusion rates were used to measure glucose utilization by the H9c2 rat heart myoblast cell line and these results are compared with fluxes measured with a radiometric assay. Both assays were used to investigate changes in H9c2 energy metabolism due to cell stimulation with carbachol and insulin. The results demonstrate the utility of the microphysiometer method for measuring both transient and sustained changes in partitioning of glucose utilization between glycolysis and oxidation in live cells.

Developmental profile of excitatory GABA(A) responses in cultured rat cerebellar granule cells

GABA induced a transient increase in cytosolic free Ca2+ in cerebellar granule cells, which decreased from 3 to 8 days in vitro (DIV). Cytosolic Ca2+ changes induced by glutamate/glycine were comparable at 3 and 7 DIV. The GABA response was ascribed to GABA(A)-receptor mediated depolarization activating L-type Ca2+ channels since the response was inhibited by bicuculline or nifedipine. GABA-mediated Ca2+ rise at 4 DIV was potentiated by pentobarbital or by the neurosteroid 5beta-pregnan-3alpha-ol-20-one, or by decreasing the extracellular Cl- concentration. Neurons cultured for > 7 DIV showed no rise in intracellular Ca2+ in response to GABA regardless of the Cl- gradient. GABA(A) receptor-mediated cytosolic Ca2+ rise suggests an important role for the excitatory activity of GABA in developing cerebellar granule neurons.

Real-time detection of insulin-like growth factor-1 stimulation of the MAC-T bovine mammary epithelial cell line

Binding of growth factors by cell-surface receptors is an essential means by which cells regulate normal tissue growth and differentiation. Exposure to growth factors is often transient, and our goal was to determine whether short-term exposure to insulin-like growth factor-1 (IGF-1) would lead to activation, assayed as cell proliferation, of mammary epithelial cells. The MAC-T cell line is an immortalized bovine mammary epithelial cell line, chosen as our model mammary cell line because of its known sensitivity to IGF-1. Using the Cytosensor Microphysiometer System, a biosensor capable of measuring extracellular acidification, we were able to measure activation of the cells owing to IGF-1 addition in real time and found that peak acidification occurred in only 14 min. We show that this rapid response to IGF-1 is dose dependent and specific for IGF-1. A significant increase in [3H]thymidine incorporation by cells after a similar short-term exposure to IGF-1 suggests that the measured increase in extracellular acidification following IGF-1 addition is physiologically relevant. This technology offers a new, novel, and rapid means for the study of IGF-1 activity, as well as the screening of IGF-1 inhibitors, in mammary epithelial cells.

Lipopolysaccharide decreases bradykinin receptor-induced acidification responses in cultured bovine aortic endothelial cells

The effects of bacterial lipopolysaccharide (Escherichia coli 0127-B8) on bradykinin receptor function in bovine aortic endothelial cells were investigated using a microphysiometer. Bradykinin and Lys(0)-desArg(10)-bradykinin produced concentration-dependent acidification responses with pEC(50) values of 8.87+/-0.20 and 9.78+/-0.08, respectively. These responses were competitively and selectively antagonised by the bradykinin B(2) receptor antagonist, icatibant and the bradykinin B(1) receptor antagonist, desArg(9)-Leu(8)-bradykinin, respectively. The non-peptide bradykinin B(2) receptor antagonist, FR173657 (0.3 and 3 nM), selectively antagonised bradykinin-induced acidification responses, causing rightward shifts of the concentration-response curves to bradykinin, but at the same time, significantly decreasing the maximum response. A preincubation with lipopolysaccharide (0.01 and 0.1 microg/ml) for 24 h caused a significant concentration-dependent decrease in maximal response to bradykinin (27.2+/-1.9 and 9.7+/-0.4% of control) and the bradykinin B(1) receptor agonist, Lys(0)-desArg(10)-bradykinin (59.0+/-7.14 and 25.3+/-7.8% of control), without affecting the EC(50). These results suggest that bradykinin B(1) receptors are constitutively expressed in cultured bovine aortic endothelial cells and that the microphysiometer provides a rapid, sensitive technique to characterise bradykinin receptors and investigate their regulation by cytokines. Interactions between bradykinin receptors and lipopolysaccharide may play a part in the cascade of deleterious effects that occur during septic shock.

Cytosensor Microphysiometer: technology and recent applications

The Cytosensor Microphysiometer system detects functional responses from living cells in minutes and offers novel information on cell signalling that is often unobtainable with other assay methods. The principle of the system is based on the measurement of small changes in extracellular acidification, using a light addressable potentiometric sensor (LAPS). Energy metabolism in living cells is tightly coupled to cellular ATP usage, so that any event which perturbs cellular ATP levels--such as receptor activation and initiation of signal transduction--will result in a change in acid excretion. As the extrusion of protons is a very general parameter involved in the activation of nearly all kinds of membrane-bound receptors, receptors can be investigated without prior knowledge of the corresponding signalling pathway. However, by blocking certain signalling pathways inside the cell by means of signal transduction probes, specificity can be brought into the system and the corresponding receptor pathways can easily be elucidated. The aim is to give an overview about Cytosensor Microphysiometer technology and to demonstrate, with the help of some recent applications, the capability of the system to measure acidification rates from a wide variety of cell- and receptor-types coupled to different signal transduction pathways. This feature makes the cytosensor system an ideal tool for acting as a single assay system and circumventing the need for multiple assays.

Functional assessment of recombinant human alpha(2)-adrenoceptor subtypes with cytosensor microphysiometry

We applied the Cytosensor Microphysiometry system to study the three human alpha(2)-adrenoceptor subtypes, alpha(2A), alpha(2B) and alpha(2C), expressed in Chinese hamster ovary (CHO) cells, and assessed its potential in the quantitative monitoring of agonist activity. The natural full agonist, (-)-noradrenaline, was used to define agonist efficacy. The imidazole derivative dexmedetomidine was a potent full agonist of all three receptor subtypes. The imidazolines clonidine and UK 14,304 (5-bromo-N-(4, 5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine) appeared to be partial agonists at alpha(2B)-adrenoceptors (E(max) approximately 60% of (-)-noradrenaline) but full agonists at alpha(2A)- and alpha(2C)-adrenoceptors. The responses mediated by all three alpha(2)-adrenoceptor subtypes were partly inhibited by the sodium-hydrogen (Na(+)/H(+)) exchange inhibitor, MIA (5-(N-methyl-N-isobutyl)-amiloride). The agonist responses were totally abolished by pretreatment with pertussis toxin in cells with alpha(2A)- and alpha(2C)-adrenoceptors, and partly abolished in cells with alpha(2B)-adrenoceptors. The residual signal in alpha(2B)-cells was sensitive to the intracellular Ca(2+)chelator, BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid acetoxymethyl ester). Cholera toxin (which acts on G(s)-proteins) had no effect on the agonist responses. The results suggest that the extracellular acidification responses mediated by all three human alpha(2)-adrenoceptor subtypes are dependent on Na(+)/H(+)exchange and G(i/o) pathways, and that alpha(2B)-adrenoceptors are capable of coupling to another, G(i/o)-independent and Ca(2+)-dependent signaling pathway.

Cytotoxicity of organophosphate anticholinesterases

Organophosphate (OP) anticholinesterases were found to modulate metabolic activities of human neuroblastoma cells and hepatocytes, which was detectable by the Cytosensor microphysiometer. The nerve gas ethyl-S-2-diisopropylaminoethyl methylphosphorothiolate (VX), at 10 microM, produced significant reduction in cell metabolism within 2 min, as measured by changes in the acidification rate of the medium. The reduction was dose- and time-dependent and irreversible after 4 h of exposure. Two alkaline degradation products of VX produced no cytotoxicity. Exposure for 24 h to 3 microM VX caused 36% and 94% irreversible loss of metabolism in hepatocytes and neuroblastoma cells, respectively. The insecticides parathion and chlorpyrifos stimulated hepatocyte metabolism but inhibited neuroblastoma cells. Their oxons were more active. Exposure of neuroblastoma cells for 4 h to VX, parathion, paraoxon, diisopropylfluorophosphate or chlorpyrifos gave an LC50 of 65, 775, 640, 340, or 672 microM, respectively, whereas 24 h gave an LC50 of 0.7, 3.7, 2.5, 29, and 31 microM, respectively. Preincubation of hepatocytes with phenobarbital enhanced their response to parathion and VX due to metabolic bioactivation. Atropine partially blocked the effects of VX and paraoxon on both cell types, which suggests the involvement of a muscarinic receptor as the target for cytotoxicity. There was no correlation between OP in vivo neurotoxicity and in vitro cytotoxicity. It is suggested that the former results from their cholinesterase inhibition, while the latter results from action on different targets and requires much higher concentrations.

Benzodiazepine modulation of recombinant alpha1beta3gamma2 GABA(A) receptor function efficacy determination using the Cytosensor microphysiometer

Gamma-aminobutyric acid (GABA) dose dependently increased extracellular acidification rate in Ltk cells stably expressing human recombinant alpha1beta3gamma2 GABA(A) receptors but had no effect in non-transfected controls. Cells seeded at 1 x 10(5) cells/cup, with 4-5 days induction, had basal acidification rates of 105+/-2 microVs(-1) at 37 degrees C (mean+/-standard error of mean, n=37). GABA responses had a characteristic time-course with an initial alkalinisation followed by a peak of acidification, which was optimized by increasing agonist exposure from 15 s to 25-30 s. The maximum concentration of GABA tested (100 microM) produced a 40+/-2% increase over basal acidification rate (n=3), with an EC50 of 15.5 microM and a Hill slope of 1.5. Responses were specifically antagonized by bicuculline and could be modulated by benzodiazepine ligands with varying efficacies. Full benzodiazepine agonists flunitrazepam (1 microM) and zolpidem (10 microM) significantly potentiated the response to 10 microM GABA by 124+/-15% (n=7) and 117+/-23% (n=3), respectively. The partial agonist bretazenil (100 nM) produced a 45+/-13% (n=3) potentiation whilst the inverse agonist DMCM (10 microM) (methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate) inhibited the response to 20 microM GABA by 53+/-5%. The microphysiometer offers an alternative functional measure for GABA(A) receptors with the sensitivity to measure subtle modulatory effects of benzodiazepine site ligands and to determine their relative efficacy.