Rapid calcium-dependent exocytosis underlies neurotransmitter release from nerve terminals. Despite the fundamental importance of this process, neither the relationship between presynaptic intracellular calcium ion concentration ([Ca2+]i) and rate of exocytosis, nor the maximal rate of secretion is known quantitatively. To provide this information, we have used flash photolysis of caged Ca2+ to elevate [Ca2+]i rapidly and uniformly in synaptic terminals, while measuring membrane capacitance as an index of exocytosis and monitoring [Ca2+]i with a Ca(2+)-indicator dye. When [Ca2+]i was abruptly increased to > 10 microM, capacitance rose at a rate that increased steeply with [Ca2+]i. The steepness suggested that at least four calcium ions must bind to activate synaptic vesicle fusion. Half-saturation was at 194 microM, and the maximal rate constant was 2,000-3,000 s-1. A given synaptic vesicle can exocytose with high probability within a few hundred microseconds, if [Ca2+]i rises above 100 microM. These properties provide for the extremely rapid signalling required for neuronal communication.
The kinetics of the secretory response in bovine chromaffin cells following flash photolysis of caged Ca2+ were studied by capacitance (Cm) measurements with millisecond time resolution. After elevation of the internal Ca2+ concentration ([Ca2+]i), Cm rises rapidly with one or more exponentials. The time constant of the fastest component decreases for higher [Ca2+]i (range 3-600 microM) over three orders of magnitude before it saturates at approximately 1 ms. The corresponding maximal rates of secretion can be as fast as 100,000 fF/s or 40,000 vesicles/s. There is a Ca(2+)-dependent delay before Cm rises, which may reflect the kinetics of multiple Ca2+ ions binding to the secretory apparatus. The initial rise in Cm is described by models containing a sequence of two to four single Ca(2+)-binding steps followed by a rate-limiting exocytosis step. The predicted Ca2+ dissociation constant (Kd) of a single Ca(2+)-binding site is between 7 and 21 microM. At [Ca2+]i > 30 microM clear indications of a fast endocytotic process complicate the analysis of the secretory response.
Thermodynamic, structural, and magnetic criteria, the properties of the charge distributions, and low-energy ionization processes are theoretically analyzed to learn about the role of π-electron delocalization in recently synthesized stable singlet carbenes (Arduengo et al. J. Am. Chem. Soc. 1991, 113, 361) and silylenes (Denk et al. J. Am. Chem. Soc. 1994, 116, 2691) of the imidazol-2-ylidene type and also in related model systems. The different approaches show consistently that cyclic electron delocalization does indeed occur in the CC unsaturated imidazol-2-ylidene systems, in particular with respect to the corresponding C−C saturated imidazolin-2-ylidenes. However, the conclusion regarding the degree of conjugation and aromaticity depends on the criteria used, being quite small according to the “atoms-in-molecules” charge analysis but more significant according to the energetic and the magnetic properties. According to all criteria, the aromatic character of imidazol-2-ylidenes is less pronounced compared to benzene or the imidazolium cation. π-Electron resonance is found to be less extensive in the silylenes compared to their carbene analogs.
Recent experiments on a variety of neuroendocrine cells indicate that intense stimuli readily depress the secretory response. The most likely explanation for this depression is that a pool of release-ready granules is depleted. We present a two-step model of secretion that allows one to simulate the dynamics of such a pool for different time courses of free intracellular Ca concentration [Ca2+]i. We derive rate constants of the model from two types of experiment and find that, for the simplest type of model, not only the rate of consumption (exocytosis) but also the rate of vesicle supply to the pool of release-ready granules must be made Ca-dependent. Given these functional dependences a variety of results from the literature can be simulated. In particular, the model predicts the occurrence of secretory depression and augmentation under appropriate conditions.
The oxidation of methane with molecular oxygen using the atomic platinum cation as a catalyst, yielding methanol, formaldehyde, and higher oxidation products, has been studied both computationally and experimentally. The most relevant reaction pathways have been followed in detail. To this end a large number of stationary points, both minima and transition states, have been optimized using a hybrid density functional theory method (B3LYP). At these optimized geometries, energies have been calculated using both an empirical scaling scheme (PCI-80) and the B3LYP method employing extended basis sets with several polarization functions. Good agreement with available experimental data has been obtained. For the parts of the catalytic cycle where detailed experimental results have not been available, the new calculated results have complemented the experimental picture to reach an almost complete understanding of the reaction mechanisms. Spin−orbit effects have been incorporated using an empirical approach, which has lead to improved agreement with experiments. The new FTICR experiments reported in the present study have helped to clarify some of the most complicated reaction sequences.
Presynaptic N-type calcium channels interact with syntaxin and synaptosome-associated protein of 25 kDa (SNAP-25) through a binding site in the intracellular loop connecting domains II and III of the ␣ 1 subunit. This binding region was loaded into embryonic spinal neurons of Xenopus by early blastomere injection. After culturing, synaptic transmission of peptide-loaded and control cells was compared by measuring postsynaptic responses under different external Ca 2ϩ concentrations. The relative transmitter release of injected neurons was reduced by ϳ25% at physiological Ca 2ϩ concentration, whereas injection of the corresponding region of the L-type Ca 2ϩ channel had virtually no effect. When applied to a theoretical model, these results imply that 70% of the formerly linked vesicles have been uncoupled after action of the peptide. Our data suggest that severing the physical interaction between presynaptic calcium channels and synaptic proteins will not prevent synaptic transmission at this synapse but will make it less efficient by shifting its Ca 2ϩ dependence to higher values.
Hybrid methods, including a mixture of Hartree–Fock exchange and density functional exchange-correlation treatment have been applied to the cationic methyl complexes MCH+3 of the first and third-row transition metals (M=Sc–Cu,La,Hf–Au). Bond dissociation energies and optimum geometries obtained with the ‘‘Becke-Half-and-Half-Lee–Yang–Parr’’ and ‘‘Becke-3-Lee–Yang–Parr’’ functionals and from calibration calculations employing quadratic configuration interaction with single and double excitations and with a perturbative estimate of triple excitations are reported. A comparison of the results for the 3d-block species to earlier high-level ab initio calculations and experimental data is carried out in order to assess the reliability of hybrid methods as a practical tool in organometallic chemistry. Furthermore, the bond dissociation energies of the cationic 5d-block transition-metal methyl complexes, many of which have not been investigated so far, are predicted.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.