The use of exogenous fluorescent probes for temperature measurements in single living cells

Quantum dot nano thermometers reveal heterogeneous local thermogenesis in living cells

The local temperature response inside single living cells upon external chemical and physical stimuli was characterized using quantum dots as nano thermometers. The photoluminescence spectral shifts from endocytosed quantum dots were used to map intracellular heat generation in NIH/3T3 cells following Ca(2+) stress and cold shock. The direct observation of inhomogeneous intracellular temperature progression raises interesting new possibilities, including further innovations in nanomaterials for sensing local responses, as well as the concept of subcellular temperature gradient for signaling and regulation in cells.

CdSe quantum dots for two-photon fluorescence thermal imaging

The technological development of quantum dots has ushered in a new era in fluorescence bioimaging, which was propelled with the advent of novel multiphoton fluorescence microscopes. Here, the potential use of CdSe quantum dots has been evaluated as fluorescent nanothermometers for two-photon fluorescence microscopy. In addition to the enhancement in spatial resolution inherent to any multiphoton excitation processes, two-photon (near-infrared) excitation leads to a temperature sensitivity of the emission intensity much higher than that achieved under one-photon (visible) excitation. The peak emission wavelength is also temperature sensitive, providing an additional approach for thermal imaging, which is particularly interesting for systems where nanoparticles are not homogeneously dispersed. On the basis of these superior thermal sensitivity properties of the two-photon excited fluorescence, we have demonstrated the ability of CdSe quantum dots to image a temperature gradient artificially created in a biocompatible fluid (phosphate-buffered saline) and also their ability to measure an intracellular temperature increase externally induced in a single living cell.

Hydrophilic fluorescent nanogel thermometer for intracellular thermometry

The first methodology to measure intracellular temperature is described. A highly hydrophilic fluorescent nanogel thermometer developed for this purpose stays in the cytoplasm and emits stronger fluorescence at a higher temperature. Thus, intracellular temperature variations associated with biological processes can be monitored by this novel thermometer with a temperature resolution of better than 0.5 degrees C.

Fluorescence imaging of membrane dynamics in living cells

Methods of wide-field fluorescence microscopy for measuring membrane dynamics of living cells are described, including spectral imaging as well as anisotropy imaging of the membrane marker 6-dodecanoyl-2-dimethylamino naphthalene (laurdan). Plasma membranes are selected by illumination with an evanescent electromagnetic field and distinguished from intracellular membranes assessed by whole-cell illumination. While fluorescence spectra of laurdan appeared red-shifted with decreasing membrane stiffness, fluorescence anisotropy and rotational correlation times were reduced with increasing membrane fluidity. Membrane stiffness was found to increase with decreasing temperature and increasing amounts of cholesterol and was always higher for the plasma membrane than for intracellular membranes. These effects may have some clinical relevance in the research of drug resistance or cell aging.

Temperature sensing using fluorescent nanothermometers

Acquiring the temperature of a single living cell is not a trivial task. In this paper, we devise a novel nanothermometer, capable of accurately determining the temperature of solutions as well as biological systems such as HeLa cancer cells. The nanothermometer is based on the temperature-sensitive fluorescence of NaYF(4):Er(3+),Yb(3+) nanoparticles, where the intensity ratio of the green fluorescence bands of the Er(3+) dopant ions ((2)H(11/2) --> (4)I(15/2) and (4)S(3/2) --> (4)I(15/2)) changes with temperature. The nanothermometers were first used to obtain thermal profiles created when heating a colloidal solution of NaYF(4):Er(3+),Yb(3+) nanoparticles in water using a pump-probe experiment. Following incubation of the nanoparticles with HeLa cervical cancer cells and their subsequent uptake, the fluorescent nanothermometers measured the internal temperature of the living cell from 25 degrees C to its thermally induced death at 45 degrees C.

Thermo-sensitive response based on the membrane fluidity adaptation in Paramecium multimicronucleatum

Relationships between the thermo-sensitive response and membrane lipid fluidity were studied using a ciliated protozoan, Paramecium multimicronucleatum. Paramecium elicits a transient membrane depolarization in response to a cooling stimulus (temperature drop). The depolarization amplitude was largest when the cooling stimulus was started from the culture temperature, whilst when cooling started at a temperature more than 5°C higher or lower than the culture temperature, only a small depolarization was induced. Therefore, the cooling-induced response was dependent on the culture temperature and its sensitivity to the cooling stimulus was highest at the culture temperature. Membrane fluidity measurements of living cells using the fluorescent dye 6-lauroyl-2-dimethylaminonaphthalene (laurdan) showed that the fluidity measured at the culture temperature was almost constant irrespective of the temperature at which the cells had been cultured and adapted, which is consistent with homeoviscous adaptation. The constant fluidity at the culture temperature quickly decreased within a few seconds of application of the cooling stimulus, and the decreased fluidity gradually readapted to a constant level at the decreased temperature within 1 h. When the constant fluidity at culture temperature was modified by the addition of procaine or benzyl alcohol, the cooling-induced depolarization was completely abolished. These results suggest the possibility that the adaptation of fluidity to a constant level and its quick decrease below the constant level activate cooling-sensitive channels to elicit the transient depolarization.

Lipid headgroups mediate organization and dynamics in bilayers

We report on the fluorescence lifetime and anisotropy decay dynamics of the tethered chromophore NBD in unilamellar vesicles comprised of phosphoglycerol and phosphocholine lipids with C(12) and C(18) saturated acyl chains, with or without cholesterol and/or sphingomyelin. For the phosphocholine vesicles, we use the chromophore 2-(12-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)dodecanoyl-1-hexadecanoyl-sn-glycero-3-phosphocholine (NBD-PC), and for the phosphoglycerol vesicles, we use the chromophore 2-(12-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)dodecanoyl-1-hexadecanoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (NBD-PG). The addition of cholesterol and/or sphingomyelin to the PC vesicles restricts the chromophore environment, in agreement with the known rigidizing effect of cholesterol on PC membranes. The PG systems do not exhibit an analogous effect with the addition of cholesterol and/or sphingomyelin. The motional freedom of the NBD chromophore is, in general, more restricted in the PC bilayers than it is in the PG bilayers, and we understand this behavior in the context of the role of the lipid headgroups in mediating bilayer organization.

Measurement of Temperature Differences between Micro-regions in Water Using Near-Infrared Spectroscopy

We have developed a method for measuring the temperature of micro-regions in aqueous solutions using near-infrared spectroscopy that enables us to measure the temperature of biological cells, tissues, and biochemical solutions in vitro. The measurement principle is based on the fact that the peak wavelength of the water absorption band with its center near 1450 nm shifts with changes in temperature. The measurement system, which consists of a biological microscope and two spectrophotometers, can measure respective absorbance spectra for two areas that are each 80 microm in diameter. We formed the temperature distribution in a 500-microm thick water film by heating an immersed Nichrome wire and measured the temperature difference between the two areas. The results of the measurement were compared to a calculated temperature distribution.

The role of phospholipid headgroups in mediating bilayer organization. Perturbations induced by the presence of a tethered chromophore

We report on the dynamics of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-lissamine rhodamine B sulfonyl ammonium salt (Rhodamine-PE), incorporated into unilamellar vesicles composed of 1,2-dimyristoyl-sn-phosphatidylcholine (DMPC). A key question in the investigation of any bilayer system using tethered fluorescent probes is the role that the chromophore itself plays in determining the organization of phospholipid bilayers. In this work, we investigate the role of headgroup-bound chromophores by measuring the steady state and time-resolved fluorescence response of the tethered rhodamine chromophore as a function of concentration in the bilayer. We find that both the steady state and dynamical properties of the chromophores change with concentration, in a manner consistent with the introduction of disorganization to the bilayers. Steady state fluorescence spectra show a clear perturbation of the rhodamine emission spectrum at a chromophore concentration of 0.25 mol%, which is not seen for lower concentrations, and fluorescence anisotropy data show that both the motional freedom and confining volume experienced by the chromophore increase with concentration. Taken collectively, our data point to the importance of using low concentrations of optical probes in the interrogation of bilayer structures.

Evaluating the role of chromophore side group identity in mediating solution-phase rotational motion

We report on the rotational diffusion dynamics of the chromophore 7-nitrobenz-2-oxa-1,3-diazole (NBD) in a series of protic and polar aprotic solvents, as a function of the identity of the side group appended to the chromophore amine functionality. The central issue we address is whether or not the side groups play a role in mediating the anisotropic reorientation dynamics of the chromophore. To understand the motional properties of the chromophores in detail, we use both one-photon and two-photon excited fluorescence anisotropy decay measurements, and from these complementary excitation methods, we extract two of the Cartesian components of the rotational diffusion constant, D. The experimental data indicate that, regardless of the functionality of the pendant side group, the reorienting moieties exhibit ratios of Dz/Dx in the range 1.8-2.0. There is a small but discernible difference between the substituted chromophores. For all of the substituted NBD chromophores, dielectric friction plays a discernible role in determining their reorientation dynamics.

A method for measurement of water temperature in micro-region using near infrared light

We have proposed a method for non-invasively measuring the temperature changes of water in a micro-region using near infrared (NIR) light. The goal of this study is to measure the temperature changes of biological cells or tissues. This method is on the basis of the phenomenon that the NIR absorption spectrum of water depends on temperature. First, in order to confirm whether a right irradiation for the temperature measurement itself affects the water temperature, we calculated the temperature rise caused by the optical absorption. Next, absorbance spectra in the wavelength range of 1300 - 2000 nm of water with a thickness of 100 and 500 m were measured by a micro-spectrometer. The measured spectra with temperature-dependence were calibrated using a regression analysis. The standard error of calibration was 0.6 C for 100 m-thickness.

Micro-thermocouple probe for measurement of cellular thermal responses

We have produced micro-thermocouple probes for the measurement of cellular thermal responses. Cells generate heat with their metabolisms and more heat with reactions to a certain physical or chemical stimulation. Therefore, the analysis of the cellular thermal responses would provide new physiological information. However, a real-time thermal measurement technique on a target of a single cell has not been established. In this study, glass micropipettes, which are widely used in bioengineering and medicine, are used for the base of the thermocouple probes. Using microfabrication techniques, the junction of two different metal films is formed at the micropipette tip with a diameter of 1 μm. This probe can inject a chemical substance into a cell and to detect its subsequent temperature changes simultaneously.

Microscopic detection of thermogenesis in a single HeLa cell

We report here the technique for detection and measurement of the temperature changes in single cells using a recently devised microthermometer (a glass micropipette filled with the thermosensitive fluorescent dye Europium (III) thenoyltrifluoroacetonate trihydrate). We found that the heat production in a single HeLa cell occurred with some time delay after the ionomycin-induced Ca(2+) influx from the extracellular space. The time delay inversely depended on extracellular [Ca(2+)], and the increase in temperature was suppressed when Ca(2+)-ATPases were blocked by thapsigargin. These observations strongly suggest that the enzymatic activity of Ca(2+)-ATPases in endoplasmic reticulum leads to the heat production. This study has therefore paved the way for studying the thermogenesis at the single-cell level.

Accurate fluorescent polymeric thermometers containing an ionic component

Fluorescent polymeric thermometers consisting of only N-alkylacrylamide and fluorescent components show rather low temperature resolution in their functional ranges (ca. 15-50 degrees C) because of the occurrence of intermolecular aggregation, which causes hysteresis in their fluorescence response to changes in temperature. By adding an ionic component to prevent such intermolecular aggregation, we obtained four fluorescent polymeric thermometers that offer high temperature resolution (<0.2 degrees C). Each new fluorescent polymeric thermometer covered the temperature range, 9-33 degrees C, 30-51 degrees C, 49-66 degrees C or 4-38 degrees C.

Correlation between thermotolerance and membrane properties in Paramecium aurelia

The relationship between thermotolerance and membrane properties was studied by using a ciliated protozoan, Paramecium aurelia. P. aurelia is a complex of sibling species termed ;syngens' whose cell morphology appear similar on microscopic examination. From the comparison of tolerance to increasing temperature among 14 syngens of P. aurelia, we selected syngens 2 and 3 as low thermotolerant examples, and syngens 8 and 10 as high thermotolerant examples. The membrane resistance of high thermotolerant syngens measured by injection of a constant inward current was greater than that of low thermotolerant syngens. Membrane fluidity measurements of living cells using the fluorescent dye, 6-lauroyl-2-dimethylaminonaphtalene (laurdan) showed that the fluidity at the cultured temperature was decreased in high thermotolerant syngens compared to that of low thermotolerant syngens. However, when the temperature was increased to the killing temperature of each syngens, the fluidity was increased to almost the same level irrespective of syngen. Furthermore, analysis of fatty acids extracted from whole cells showed that the ratios of unsaturated to saturated fatty acids was smaller in high thermotolerant syngens than in low thermotolerant syngens. These results suggest that the thermotolerance of P. aurelia syngens is determined by the membrane fluidity which is related to the fatty acids composition.

Quantitating the Dynamics of NBD Hexanoic Acid in Homogeneous Solution and in Solutions Containing Unilamellar Vesicles

We report here on the motional and fluorescence lifetime dynamics of the chromophore NBDHA (6-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)hexanoic acid) in neat solvents and in aqueous solutions containing unilamellar vesicles of varying composition. We measure the transient response of this chromophore by time-correlated single-photon counting, using one- and two-photon excitation to resolve the Cartesian components of the rotational diffusion constant, D. Our experimental data for NBDHA in selected solvents of varying viscosity demonstrate that one- and two-photon excitation probe different components of the rotational diffusion constant and that this chromophore reorients as a prolate rotor with an aspect ratio of approximately 2. For NBDHA in aqueous solutions containing unilamellar vesicles of varying composition, we recover the same reorientation behavior regardless of vesicle composition. Fluorescence lifetime and steady-state fluorescence data show the chromophore to reside in a polar environment that is different from neat water. We understand these data in the context of the chromophore residing in close proximity to the unilamellar vesicle polar headgroups in all cases.

Nanoelectropulse-induced phosphatidylserine translocation

Nanosecond, megavolt-per-meter, pulsed electric fields induce phosphatidylserine (PS) externalization, intracellular calcium redistribution, and apoptosis in Jurkat T-lymphoblasts, without causing immediately apparent physical damage to the cells. Intracellular calcium mobilization occurs within milliseconds of pulse exposure, and membrane phospholipid translocation is observed within minutes. Pulsed cells maintain cytoplasmic membrane integrity, blocking propidium iodide and Trypan blue. Indicators of apoptosis-caspase activation and loss of mitochondrial membrane potential-appear in nanoelectropulsed cells at later times. Although a theoretical framework has been established, specific mechanisms through which external nanosecond pulsed electric fields trigger intracellular responses in actively growing cells have not yet been experimentally characterized. This report focuses on the membrane phospholipid rearrangement that appears after ultrashort pulse exposure. We present evidence that the minimum field strength required for PS externalization in actively metabolizing Jurkat cells with 7-ns pulses produces transmembrane potentials associated with increased membrane conductance when pulse widths are microseconds rather than nanoseconds. We also show that nanoelectropulse trains delivered at repetition rates from 2 to 2000 Hz have similar effects, that nanoelectropulse-induced PS externalization does not require calcium in the external medium, and that the pulse regimens used in these experiments do not cause significant intra- or extracellular Joule heating.

A novel method of thermal activation and temperature measurement in the microscopic region around single living cells

We present a simple approach to bring fast and reversible temperature steps of a wide range of amplitudes from the temperature of the experimental chamber up to the boiling point of water in a desired position, with rise and fall times of around 10 ms in a microvolume of microm in size, such as in a single cell. For this purpose, we applied a technique for illuminating a metal aggregate (1-2 microm in diameter) placed at the tip of a glass micropipette with a focused infrared (1064 nm) laser beam under an optical microscope. Stable temperature gradients were created around the metal aggregate using an appropriate neutral density filter set for the laser output. To monitor the local temperature, we devised a new microthermometer composed of the tip of a micropipette filled with thermosensitive fluorescent dye Europium-TTA possessing steep temperature-dependent phosphorescence upon 365 nm excitation. The microm size of the tip of this pipette was able to measure the local temperature with 0.1 degrees C precision and microm spatial resolution. This new approach is compatible with standard electrophysiological and imaging techniques.

Calcium-independent stimulation of membrane fusion and SNAREpin formation by synaptotagmin I

Neurotransmitter release requires the direct coupling of the calcium sensor with the machinery for membrane fusion. SNARE proteins comprise the minimal fusion machinery, and synaptotagmin I, a synaptic vesicle protein, is the primary candidate for the main neuronal calcium sensor. To test the effect of synaptotagmin I on membrane fusion, we incorporated it into a SNARE-mediated liposome fusion assay. Synaptotagmin I dramatically stimulated membrane fusion by facilitating SNAREpin zippering. This stimulatory effect was topologically restricted to v-SNARE vesicles (containing VAMP 2) and only occurred in trans to t-SNARE vesicles (containing syntaxin 1A and SNAP-25). Interestingly, calcium did not affect the overall fusion reaction. These results indicate that synaptotagmin I can directly accelerate SNARE-mediated membrane fusion and raise the possibility that additional components might be required to ensure tight calcium coupling.

Coexistence of domains with distinct order and polarity in fluid bacterial membranes

In this study we sought the detection and characterization of bacterial membrane domains. Fluorescence generalized polarization (GP) spectra of laurdan-labeled Escherichia coli and temperature dependencies of both laurdan's GP and fluorescence anisotropy of 1,3-diphenyl-1,3,5-hexatriene (DPH) (rDPH) affirmed that at physiological temperatures, the E. coli membrane is in a liquid-crystalline phase. However, the strong excitation wavelength dependence of rlaurdan at 37 degrees C reflects membrane heterogeneity. Time-resolved fluorescence emission spectra, which display distinct biphasic redshift kinetics, verified the coexistence of two subpopulations of laurdan. In the initial phase, <50 ps, the redshift in the spectral mass center is much faster for laurdan excited at the blue edge (350 nm), whereas at longer time intervals, similar kinetics is observed upon excitation at either blue or red edge (400 nm). Excitation in the blue region selects laurdan molecules presumably located in a lipid domain in which fast intramolecular relaxation and low anisotropy characterize laurdan's emission. In the proteo-lipid domain, laurdan motion and conformation are restricted as exhibited by a slower relaxation rate, higher anisotropy and a lower GP value. Triple-Gaussian decomposition of laurdan emission spectra showed a sharp phase transition in the temperature dependence of individual components when excited in the blue but not in the red region. At least two kinds of domains of distinct polarity and order are suggested to coexist in the liquid-crystalline bacterial membrane: a lipid-enriched and a proteolipid domain. In bacteria with chloramphenicol (Cam)-inhibited protein synthesis, laurdan showed reduced polarity and restoration of an isoemissive point in the temperature-dependent spectra. These results suggest a decrease in membrane heterogeneity caused by Cam-induced domain dissipation.

Laser-assisted optoporation of single cells

The plasma membrane of Chinese hamster ovary cells was made permeable using the focused beam of an argon ion laser (488 nm) and phenol red as a light absorbing dye. Small circular dark spots on the cell surface appeared immediately after laser irradiation and disappeared within about 5 min. They were related to transient changes in membrane properties, which could be visualized using the fluorescent marker laurdan, and were probably due to a local increase in temperature. According to a colony forming assay, cell viability was maintained by using light doses up to 2.5 MJ/cm(2) applied for 1 s. In addition to measurements of the efflux of the cytoplasmic marker calcein, cell transfection using a green fluorescent protein (GFP) coding plasmid was studied: brightly fluorescent GFP with an emission maximum around 510 nm was observed within part of the cells after 24 h. The transfection rates after laser irradiation were around 30% for younger subcultures and less than 10% for aging cells. This may be due to age dependent changes in the phase transition of membrane lipids from gel phase to liquid crystalline phase. High transfection rates, visual control and universality towards various cell lines are possibly the main advantages of laser-assisted optoporation in comparison with presently existing methods of cell transfection.

Lateral organization and domain formation in a two-component lipid membrane system

The thermodynamic phase behavior and lateral lipid membrane organization of unilamellar vesicles made from mixtures of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2 distearoyl-sn-glycero-3-phosphocholine (DSPC) were investigated by fluorescence resonance energy transfer (FRET) as a function of temperature and composition. This was done by incorporating a headgroup-labeled lipid donor (NBD-DPPE) and acceptor (N-Rh-DPPE) in low concentrations into the binary mixtures. Two instances of increased energy transfer efficiency were observed close to the phase lines in the DMPC/DSPC phase diagram. The increase in energy transfer efficiency was attributed to a differential preference of the probes for dynamic and fluctuating gel/fluid coexisting phases. This differential preference causes the probes to segregate (S. Pedersen, K. Jørgensen, T. R. Baekmark, and O. G. Mouritsen, 1996, Biophys. J. 71:554-560). The observed increases in energy transfer match with the boundaries of the DMPC/DSPC phase diagram, as measured by Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). We propose that the two instances of probe segregation are due to the presence of DMPC-rich and DSPC-rich domains, which form a dynamic structure of gel/fluid coexisting phases at two different temperatures. Monitoring the melting profile of each lipid component independently by FTIR shows that the domain structure is formed by DMPC-rich and DSPC-rich domains rather than by pure DMPC and DSPC domains.

Control of a redox reaction on lipid bilayer surfaces by membrane dipole potential

Nitro-2,1,3-benzoxadiazol-4-yl (NBD) group is a widely used, environment-sensitive fluorescent probe. The negatively charged dithionite rapidly reduces the accessible NBD-labeled lipids in liposomes to their corresponding nonfluorescent derivatives. In this study both the phospholipid headgroup and acyl chain NBD-labeled L-alpha-1,2-dipalmitoyl-sn-glycero-3-phospho-[N-(4-nitrobenz-2-oxa-1,3-diazole)-ethanolamine] (DPPN) and 1-acyl-2-[12-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]dodecanoyl]-sn-glycero-3-phosphocholine (NBD-PC), respectively, were employed. The correlation of both the rate coefficient k(1) of the redox reaction and the fluorescence properties of the two probes with the membrane dipole potential Psi in fluid dipalmitoylglycerophosphocholine (DPPC) liposomes is demonstrated. When Psi of the bilayer was varied (decreased by phloretin or increased by 6-ketocholestanol), the value for k1 decreased for both DPPN and NBD-PC with increasing Psi. For both fluorophores a positive correlation to Psi was evident for the relative fluorescence emission intensity (RFI, normalized to the emission of the fluorophore in a DPPC matrix). The relative changes in emission intensity as a function of Psi were approximately equal for both NBD derivatives. Changes similar to those caused by phloretin were seen when dihexadecylglycerophosphocholine (DHPC) was added to DPPC liposomes, in keeping with the lower dipole potential for the former lipid compound compared with DPPC. These effects of Psi on NBD fluorescence should be taken into account when interpreting data acquired using NBD-labeled lipids as fluorescent probes.

Measurement of localized heating in the focus of an optical trap

Localized heating in the focus of an optical trap operating in water can result in a temperature rise of several kelvins. We present spatially resolved measurements of the refractive-index distribution induced by the localized heating produced in an optical trap and infer the temperature distribution. We have determined a peak temperature rise in water of 4 K in the focus of a 985-nm-wavelength 55-mW laser beam. The localized heating is directly proportional to power and the absorption coefficient. The temperature distribution is in excellent agreement with a model based on the heat equation.

SNAREpins: minimal machinery for membrane fusion

Recombinant v- and t-SNARE proteins reconstituted into separate lipid bilayer vesicles assemble into SNAREpins-SNARE complexes linking two membranes. This leads to spontaneous fusion of the docked membranes at physiological temperature. Docked unfused intermediates can accumulate at lower temperatures and can fuse when brought to physiological temperature. A supply of unassembled v- and t-SNAREs is needed for these intermediates to form, but not for the fusion that follows. These data imply that SNAREpins are the minimal machinery for cellular membrane fusion.

Thermal imaging of receptor-activated heat production in single cells

Changes in enthalpy (i.e., heat content) occur during the diverse intracellular chemical and biophysical interactions that take place in the life cycle of biological cells. Such changes have previously been measured for cell suspensions or cell-free biochemical extracts by using microcalorimetry, thermocouples, or pyroelectric films, all of which afford minimal spatial or temporal resolution. Here we present a novel thermal imaging method that combines both diffraction-limited spatial (approximately 300 nm) and sampling-rate-limited time resolution, using the temperature-dependent phosphorescence intensity of the rare earth chelate Eu-TTA (europium (III) thenoyltrifluoro-acetonate). With this thermosensitive dye, we imaged intracellular heat waves evoked in Chinese hamster ovary cells after activation of the metabotropic m1-muscarinic receptor. Fast application of acetylcholine onto the cells evoked a biphasic heat wave that was blocked by atropine, and after a brief delay was followed by a calcium wave. Atropine applied by itself produced a monophasic heat wave in the cells, suggesting that its interactions with the receptor activate some intracellular metabolic pathways. The thermal imaging technique introduced here should provide new insights into cellular functions by resolving the location, kinetics, and quantity of intracellular heat production.

Physiological monitoring of optically trapped cells: assessing the effects of confinement by 1064-nm laser tweezers using microfluorometry

We report the results of microfluorometric measurements of physiological changes in optically trapped immotile Chinese hamster ovary cells (CHOs) and motile human sperm cells under continuous-wave (CW) and pulsed-mode trapping conditions at 1064 nm. The fluorescence spectra derived from the exogenous fluorescent probes laurdan, acridine orange, propidium iodide, and Snarf are used to assess the effects of optical confinement with respect to temperature, DNA structure, cell viability, and intracellular pH, respectively. In the latter three cases, fluorescence is excited via a two-photon process, using a CW laser trap as the fluorescence excitation source. An average temperature increase of < 0.1 +/- 0.30 degrees C/100 mW is measured for cells when held stationary with CW optical tweezers at powers of up to 400 mW. The same trapping conditions do not appear to alter DNA structure or cellular pH. In contrast, a pulsed 1064-nm laser trap (100-ns pulses at 40 microJ/pulse and average power of 40 mW) produced significant fluorescence spectral alterations in acridine orange, perhaps because of thermally induced DNA structural changes or laser-induced multiphoton processes. The techniques and results presented herein demonstrate the ability to perform in situ monitoring of cellular physiology during CW and pulsed laser trapping, and should prove useful in studying mechanisms by which optical tweezers and microbeams perturb metabolic function and cellular viability.