Concentrations of caffeine greater than 20 mM increase the indo-1 fluorescence ratio in a Ca(2+)-independent manner

Utilization of the genetically encoded calcium indicator Salsa6F in cardiac applications

Calcium signaling is a critical process required for cellular mechanisms such as cardiomyocyte contraction. The inability of the cell to properly activate or regulate calcium signaling can lead to contractile dysfunction. In isolated cardiomyocytes, calcium signaling has been primarily studied using calcium fluorescent dyes, however these dyes have limited applicability to whole organs. Here, we crossed the Salsa6f mouse which expresses a genetically encoded ratiometric cytosolic calcium indicator with a cardiomyocyte specific inducible cre to temporally-induce expression and studied cytosolic calcium transients in isolated cardiomyocytes and modified Langendorff heart preparations. Isolated cardiomyocytes expressing Salsa6f or Fluo-4AM loaded were compared. We also crossed the Salsa6f mouse with a floxed Polycystin 2 (PC2) mouse to test the feasibility of using the Salsa6f mouse to measure calcium transients in PC2 heterozygous or homozygous knock out mice. Although there are caveats in the applicability of the Salsa6f mouse, there are clear advantages to using the Salsa6f mouse to measure whole heart calcium signals.

Xanthines Studied via Femtosecond Fluorescence Spectroscopy

Xanthines represent a wide class of compounds closely related to the DNA bases adenine and guanine. Ubiquitous in the human body, they are capable of replacing natural bases in double helices and give rise to four-stranded structures. Although the use of their fluorescence for analytical purposes was proposed, their fluorescence properties have not been properly characterized so far. The present paper reports the first fluorescence study of xanthine solutions relying on femtosecond spectroscopy. Initially, we focus on 3-methylxanthine, showing that this compound exhibits non-exponential fluorescence decays with no significant dependence on the emission wavelength. The fluorescence quantum yield (3 × 10⁻⁴) and average decay time (0.9 ps) are slightly larger than those found for the DNA bases. Subsequently, we compare the dynamical fluorescence properties of seven mono-, di- and tri-methylated derivatives. Both the fluorescence decays and fluorescence anisotropies vary only weakly with the site and the degree of methylation. These findings are in line with theoretical predictions suggesting the involvement of several conical intersections in the relaxation of the lowest singlet excited state.

Interactions of antagonists with subtypes of inositol 1,4,5-trisphosphate (IP3) receptor

BACKGROUND AND PURPOSE

Inositol 1,4,5-trisphosphate receptors (IP₃Rs) are intracellular Ca²⁺ channels. Interactions of the commonly used antagonists of IP₃Rs with IP₃R subtypes are poorly understood.

EXPERIMENTAL APPROACH

IP₃-evoked Ca²⁺ release from permeabilized DT40 cells stably expressing single subtypes of mammalian IP₃R was measured using a luminal Ca²⁺ indicator. The effects of commonly used antagonists on IP₃-evoked Ca²⁺ release and ³H-IP₃ binding were characterized.

KEY RESULTS

Functional analyses showed that heparin was a competitive antagonist of all IP₃R subtypes with different affinities for each (IP₃R3 > IP₃R1 ≥ IP₃R2). This sequence did not match the affinities for heparin binding to the isolated N-terminal from each IP₃R subtype. 2-aminoethoxydiphenyl borate (2-APB) and high concentrations of caffeine selectively inhibited IP₃R1 without affecting IP₃ binding. Neither Xestospongin C nor Xestospongin D effectively inhibited IP₃-evoked Ca²⁺ release via any IP₃R subtype.

CONCLUSIONS AND IMPLICATIONS

Heparin competes with IP₃, but its access to the IP₃-binding core is substantially hindered by additional IP₃R residues. These interactions may contribute to its modest selectivity for IP₃R3. Practicable concentrations of caffeine and 2-APB inhibit only IP₃R1. Xestospongins do not appear to be effective antagonists of IP₃Rs.

Calmodulin mutations associated with long QT syndrome prevent inactivation of cardiac L-type Ca(2+) currents and promote proarrhythmic behavior in ventricular myocytes

Recent work has identified missense mutations in calmodulin (CaM) that are associated with severe early-onset long-QT syndrome (LQTS), leading to the proposition that altered CaM function may contribute to the molecular etiology of this subset of LQTS. To date, however, no experimental evidence has established these mutations as directly causative of LQTS substrates, nor have the molecular targets of CaM mutants been identified. Here, therefore, we test whether expression of CaM mutants in adult guinea-pig ventricular myocytes (aGPVM) induces action-potential prolongation, and whether affiliated alterations in the Ca(2+) regulation of L-type Ca(2+) channels (LTCC) might contribute to such prolongation. In particular, we first overexpressed CaM mutants in aGPVMs, and observed both increased action potential duration (APD) and heightened Ca(2+) transients. Next, we demonstrated that all LQTS CaM mutants have the potential to strongly suppress Ca(2+)/CaM-dependent inactivation (CDI) of LTCCs, whether channels were heterologously expressed in HEK293 cells, or present in native form within myocytes. This attenuation of CDI is predicted to promote action-potential prolongation and boost Ca(2+) influx. Finally, we demonstrated how a small fraction of LQTS CaM mutants (as in heterozygous patients) would nonetheless suffice to substantially diminish CDI, and derange electrical and Ca(2+) profiles. In all, these results highlight LTCCs as a molecular locus for understanding and treating CaM-related LQTS in this group of patients.

Pharmaceutical analysis in solids using front face fluorescence spectroscopy and multivariate calibration with matrix correction by piecewise direct standardization

This paper reports the application of piecewise direct standardization (PDS) for matrix correction in front face fluorescence spectroscopy of solids when different excipients are used in a pharmaceutical preparation based on a mixture of acetylsalicylic acid (ASA), paracetamol (acetaminophen) and caffeine. As verified in earlier studies, the use of different excipients and their ratio can cause a displacement, change in fluorescence intensity or band profile. To overcome this important drawback, a standardization strategy was adopted to convert all the excitation-emission fluorescence spectra into those used for model development. An excitation-emission matrix (EEM) for which excitation and emission wavelengths ranging from 265 to 405 nm and 300 to 480 nm, respectively, was used. Excellent results were obtained using unfolded partial least squares (U-PLS), with RMSEP values of 8.2 mg/g, 10.9 mg/g and 2.7 mg/g for ASA, paracetamol and caffeine, respectively, and with relative errors lesser than 5% for the three analytes.

Store-operated Ca2+ entry during intracellular Ca2+ release in mammalian skeletal muscle

Store-operated Ca2+ entry (SOCE) is activated following the depletion of internal Ca2+ stores in virtually all eukaryotic cells. Shifted excitation and emission ratioing of fluorescence (SEER) was used to image mag-indo-1 trapped in the tubular (t) system of mechanically skinned rat skeletal muscle fibres to measure SOCE during intracellular Ca2+ release. Cytosolic Ca2+ transients were simultaneously imaged using the fluorescence of rhod-2. Spatially and temporally resolved images of t system [Ca2+] ([Ca2+]t-sys) allowed estimation of Ca2+ entry flux from the rate of decay of [Ca2+]t-sys. Ca2+ release was induced pharmacologically to activate SOCE without voltage-dependent contributions to Ca2+ flux. Inward Ca2+ flux was monotonically dependent on the [Ca2+] gradient, and strongly dependent on the transmembrane potential. The activation of SOCE was controlled locally. It could occur without full Ca2+ store depletion and in less than a second after initiation of store depletion. These results indicate that the molecular agonists of SOCE must be evenly distributed throughout the junctional membranes and can activate rapidly. Termination of SOCE required a net increase in [Ca2+]SR. Activation and termination of SOCE are also demonstrated, for the first time, during a single event of Ca2+ release. At the physiological [Ca2+]t-sys, near 2 mM (relative to t system volume), SOCE flux relative to accessible cytoplasmic volume was at least 18.6 microM s(-1), consistent with times of SR refilling of 1-2 min measured in intact muscle fibres.

Imaging single cardiac ryanodine receptor Ca2+ fluxes in lipid bilayers

In this and an accompanying report we describe two steps, single-channel imaging and channel immobilization, necessary for using optical imaging to analyze the function of ryanodine receptor (RyR) channels reconstituted in lipid bilayers. An optical bilayer system capable of laser scanning confocal imaging of fluo-3 fluorescence due to Ca2+ flux through single RyR2 channels and simultaneous recording of single channel currents was developed. A voltage command protocol was devised in which the amplitude, time course, shape, and hence the quantity of Ca2+ flux through a single RyR2 channel is controlled solely by the voltage imposed across the bilayer. Using this system, the voltage command protocol, and concentrations of Ca2+ (25-50 mM) that result in saturating RyR2 Ca2+ currents, proportional fluo-3 fluorescence was recorded simultaneously with Ca2+ currents having amplitudes of 0.25-14 pA. Ca2+ sparks, similar to those obtained with conventional microscope-based laser scanning confocal systems, were imaged in mouse ventricular cardiomyocytes using the optical bilayer system. The utility of the optical bilayer for systematic investigation of how cellular factors extrinsic to the RyR2 channel, such as Ca2+ buffers and diffusion, alter fluo-3 fluorescent responses to RyR2 Ca2+ currents, and for addressing other current research questions is discussed.

Caffeine-induced immobilization of gating charges in isolated guinea-pig ventricular heart cells

The effects of 10 mM caffeine (CAF) on intramembrane charge movements (ICM) were studied in isolated guinea-pig ventricular heart cells with the whole-cell patch-clamp technique. In the presence of CAF, the properties (voltage dependence, maximum Q(ON) [Q(max)], availability with voltage) of Q(ON) charge activated from -110 mV were barely affected. Following a 100 ms prepulse to -50 mV to decrease the participation of charges originating from Na channels, the voltage dependence of Q(ON) was shifted by 5 mV (negative component) and by 10 mV (positive component) towards negative potentials, and Q(max) was depressed by 16.5%. CAF drastically reduced in a time- and voltage-dependent manner Q(OFF) on repolarization to -50 mV, the effects being greater at positive potentials. CAF-induced Q(OFF) immobilization could be almost entirely removed by repolarization to voltages as negative as -170 mV. In these conditions, the voltage-dependence of Q(OFF) (repolarization to +30 to -170 mV) was shifted by 17 mV (negative component) and 30 mV (positive component) towards negative potentials, suggesting an interconversion into charge 2. Most of CAF effects were suppressed when the sarcoplasmic reticulum (SR) was not functional or when the cells were loaded with BAPTA-AM. We conclude that CAF effects on ICM are likely due to Ca(2+) ions released from the SR, and which accumulate in the subsarcolemmal fuzzy spaces in the vicinity of the Ca channels. Because CAF effects were more pronounced on Q(OFF) than on Q(ON) the channels have likely to open before Ca(2+) ions could affect their gating properties. It is speculated that such an effect on gating charges might contribute to the Ca-induced inactivation of the Ca current.

Functional properties of EGFP-tagged skeletal muscle calcium-release channel (ryanodine receptor) expressed in COS-7 cells: sensitivity to caffeine and 4-chloro-m-cresol

We constructed and expressed in COS-7 cells, three E-green fluorescent protein (EGFP) tagged recombinant skeletal muscle ryanodine receptors (RYR). EGFP was tagged to (i) the NH2-terminus (nEGFP-RYR(FL)) and to (ii) the COOH-terminus (cRYR(FL)-EGFP) of the full length RYR; we also tagged the EGFP to (iii) the NH2-terminus of a truncated version of the RYR (nEGFP-RYR(Bhat)) lacking the bulk of the protein. The fluorescent pattern EGFP with all three constructs colocalize with that of an endoplasmic reticulum (ER) membrane tracker fluorescent dye, indicating that the RYR constructs are targeted to ER membranes. Our results show that: (i) COOH-terminal tagging abolishes the sensitivity of the RYR to caffeine, whereas the presence of EGFP at the NH2-terminus does not affect caffeine sensitivity and (ii) 4-Cl-m-cresol sensitivity is lost both with the truncated nEGFP-RYR(Bhat) and the nEGFP-RYR(FL), while COOH-terminal tagging does not affect sensitivity to 4-chloro-m-cresol. The dose-response curves of caffeine-induced calcium release of nEGFP-RYR(FL) differ from those of the truncated nEGFP-RYR(Bhat). Maximal calcium release was approached at 10 mM caffeine with the nEGFP-RYR(FL), while cells expressing the nEGFP-RYR(Bhat) construct displayed a bell shaped curve and the maximal concentration for caffeine-induced calcium release was 5 mM. Equilibrium [3H]-ryanodine binding confirmed the calcium photometry data. Our results demonstrate that EGFP tagging modifies the pharmacological properties of RYR, and suggest that 4-chloro-m-cresol and caffeine act through different mechanisms and probably interact with different sites on the RYR calcium release channel.