The temperature dependence of some kinetic and conductance properties of acetylcholine receptor channels

We examined the temperature dependence of single-channel properties of the nicotinic acetylcholine receptor channel from clonal BC3H-1 cells over a range of 10-40 degrees C. We found temperature sensitivities (Q10 values) of 2-4 for the mean channel open time. The Q10 did not depend strongly on voltage and the voltage dependence of the mean open time was temperature-independent. The Q10 of closing rate of the long-lived open state was 3-4 but the Q10 of closing rate of the brief open state was independent of temperature. The duration of brief closures could be measured only between 10 and 25 degrees C. Since this approached the limit of the experimental time resolution, an accurate determination of the Q10 could not be made. The current decay due to desensitization after rapid application of high concentrations of agonist varied with a Q10 of about 2. The conductance of single channels (the inverse of the ion translocation rate) had a Q10 of 1.3-1.5. We found no obvious nonlinearities in the Arrhenius curves for any of the measured properties.     

The dependence receptor hypothesis

A new family of functionally-related receptors has recently been proposed, dubbed dependence receptors. These proteins, only some of which share sequence similarities, display the common property that they transduce two different intracellular signals: in the presence of ligand, these receptors transduce a positive signal leading to survival, differentiation or migration; conversely, in the absence of ligand, the receptors initiate or amplify a signal for programmed cell death. Thus cells that express these proteins at sufficient concentrations manifest a state of dependence on their respective ligands. The signaling that mediates cell death induction upon ligand withdrawal is in large part uncharacterized, but typically includes a required interaction with, and cleavage by, specific caspases. Here, we review the current knowledge concerning dependence receptors, including the shared mechanisms for cell death induction and their potential relevance in nervous system development and regulation of tumorigenesis.     

Membrane reception of thyroid hormones]

Comparative and competitive analyses of thyroxine (T4) and triiodothyronine (T3) binding to highly purified rat liver, brain and lung cell plasma membranes were carried out. The dependence of hormone binding on the time, temperature and concentration was studied. The effects of trypsin and partial delipidation on the binding parameters of thyroid hormones were investigated. Two thyroid hormone-binding sites were detected in cell plasma membranes of all tissues under study. The maximal binding of T4 to rat liver membranes and the maximal binding of T3 to rat brain membranes was observed in all experiments, the affinity for T3 being higher than that for T4. An important role of both protein and lipid components of plasma membranes in the membrane reception of thyroid hormones is proposed.     

The temperature-related photosynthetic capacity of plants under desert conditions

Summary As described earlier, the native arido-active perennial Hammada scoparia and the cultivated Prunus armeniaca exhibit characteristic seasonal shifts of their temperature optimum of net photosynthesis (OP) under desert conditions in the Negev. In the present paper the OP values were compared with the actual tissue temperatures of the experimental plants.During the growing period from March to September the duration of optimal temperatures for net photosynthesis (OP±3°C) experienced by the plants was 32.2% of the total time at light saturation for P. armeniaca and 27.8% for H. scoparia. For optimal photosynthesis the branchlets of H. scoparia are too cold for 66.1% of the time span and too warm for 6.1% of the time. The respective values for the leaves of the apricot are 28.6% and 39.2%. Simulations at changed tissue temperature show, that for P. armeniaca neither a higher nor a lower temperature regime would lengthen the time span for optimal thermal conditions. For H. scoparia, however, an increase of the general temperature level by 6°C would considerably improve the temperature-related photosynthetic efficiency. The natural temperature responses of the plants were compared with simulations using OP values which are supposed not to shift but to stay constant from March through September at their spring minimum, their summer maximum, or at an intermediate value. For P. armeniaca such constant OP values would result in a shorter duration of optimal temperature conditions. With this plant the natural seasonal shift of the temperature characteristics appears to provide an advantage in respect to its photosynthetic capacity. Contrary to this, for H. scoparia a constant OP value at the low spring level or even at the intermediate level during all the season would result in a substantially prolonged period of favourable temperature conditions for photosynthesis. In this case the seasonal change of optimum temperature for photosynthesis with higher OP values in summer signifies a disadvantage with respect to the temperature-related photosynthetic capacity at the habitat in the central Negev. Apparently this C₄ plant is adapted to higher temperatures than were present. It appears that acclimations of native plants are not always beneficial.     

The temperature dependence of the deformation of bone

The deformation characteristics of compact bone have been measured as a function of temperature in the range from −58 to 90°C. At temperatures below 25°C elastic, anelastic and plastic contributions to the strain could be distinguished. The Young's modulus increased as the temperature was reduced, while the nonelastic component was approximately independent of temperature. An interpretation of these results is made in terms of an idealised hydroxyapatite-collagen composite. At temperatures higher than 50°C the deformation was compounded by non-equilibrium recovery and an irreversible change in the structure of bone.     

The temperature dependence of erythrocyte water diffusion permeability

1. The activation energy of the diffusion water exchange in red blood cells increases with temperature. 2. Fetal blood has a higher activation energy for diffusion water exchange than adult blood. 3. Treatment of red cells with p-chloromercuribenzoate alters the activation energy and apparently allows a lipid and a protein pore pathway to be resolved. The permeability and activation energy of the treated cells is in the range found for lipid membranes; and the difference between treated and untreated cells, the "protein" pathway, has a diffusion activation energy comparable with that of free water. 4. A resolution of the discrepancies between the NMR methods of measuring diffusion water exchange is suggested.     

The conductance of lecithin bilayers: The dependence upon temperature

The temperature dependence of the area-specific conductance of egg-lecithin/cholesterol bilayers formed with n-hexadecane in 1 mM KCl has been studied. From Arrhenius plots the activation energy for conduction was measured as 35 ± 2 kJ/mol. Comparison of this value with those predicted by various mechanisms whereby charge could be translocated through a bilayer indicate that it is extremely unlikely that ions pass directly through the hydrophobic interior. It is possible however, that ions are translocated across the bilayer through aqueous pores (with radius > 1 nm) which are an intrinsic, if fluctuating, part of the bilayer structure.     

Benzodiazepine receptors: temperature dependence of [3H]flunitrazepam binding.

The characteristics of [³H]flunitrazepam binding to brain specific benzodiazepine receptors were determined at varying temperatures. The rates at which [³H]flunitrazepam associated with and dissociated from benzodiazepine receptors increased with increasing temperatures. The dissociation constant (K_D) also increased with increases in temperature. The (K_D) determined by Scatchard analyses of saturation isotherms showed a similar change with changes in temperature. The maximal binding capacity (Bmax) did not change with changes in temperature. The inhibitory constants of several benzodiazepines to inhibit [³H]flunitrazepam binding to brain were also higher at 37°C than at 0°C, suggesting that the binding affinity of all benzodiazepines to brain benzodiazepine receptors is lower at 37°C than at 0°C. Van't Hoff analysis of [³H]flunitrazepam binding to brain at different temperatures reveals two linear components to this relationship.     

The temperature dependence of salt-protein association is sequence specific

Molecular dynamics (MD) simulations are used to probe the origin of the unexpected temperature dependence of salt accumulation in the C-terminal region of the protein human lymphotactin. As in previous MD simulations, sodium ions accumulate in an enhanced manner near the C-terminal helix at the lower temperature, while the temperature dependence of chloride accumulation is much weaker and slightly positive. In a designed mutant in which all positively charged residues in the C-terminal helix are replaced with neutral polar groups (Ser), the unexpected temperature dependence of the sodium ions is no longer observed. Therefore, these simulations convincingly verified the previous hypothesis that the temperature dependence of ion-protein association is sensitive to the local sequence. This is explained qualitatively in terms of the entropy of association between charged species in solution. These findings have general implications for the interpretation of thermodynamic quantities associated with binding events where ion release is important, such as protein-DNA interactions.     

The temperature dependence of gramicidin conformational States in octanol

In lipid bilayers and organic solvents, the hydrophobic polypeptide gramicidin adopts a number of different conformations, some of which are capable of conducting monovalent cations across phospholipid membranes. The equilibria between conformations have been shown to be influenced by factors such as lipid chain length, solvent, concentration and salt. In this study, the temperature dependence of the equilibrium mixture of double helical ion-free gramicidin in octanol was examined using circular dichroism spectroscopy.     

The elastin-laminin receptor]

Elastin is a major component of the extracellular matrix. Elastin peptides derived from its degradation are present in human sera. Elastin peptides induce on fibroblasts, phagocytic cells, lymphocytes, smooth muscle cells and endothelial cells, a variety of biological effects mediated by the elastin-laminin receptor which has been demonstrated to be present on the membrane of these cells. The transduction pathway of the ELR receptor involves the activation of phospholipase C (PLC) by a pertussis toxin sensitive G-protein. PLC induces the production of inositol trisphosphate (IP3) leading to the increase of the intracellular free calcium on one hand, and of diacylglycerol (DAG) which stimulates the translocation to the membrane of PKC leading to the phosphorylation of members of the MAPK family, such as p42/p44 MAPK. Considering the multiple biological effects of ELR the elucidation of the complexity of the signaling pathways will help to better modulate it, mainly in pathological situations such as atherosclerosis.     

Body temperature-related structural transitions of monotremal and human hemoglobin

In this study, temperature-related structural changes were investigated in human, duck-billed platypus (Ornithorhynchus anatinus, body temperature T(b) = 31-33 degrees C), and echidna (Tachyglossus aculeatus, body temperature T(b) = 32-33 degrees C) hemoglobin using circular dichroism spectroscopy and dynamic light scattering. The average hydrodynamic radius (R(h)) and fractional (normalized) change in the ellipticity (F(obs)) at 222 +/- 2 nm of hemoglobin were measured. The temperature was varied stepwise from 25 degrees C to 45 degrees C. The existence of a structural transition of human hemoglobin at the critical temperature T(c) between 36-37 degrees C was previously shown by micropipette aspiration experiments, viscosimetry, and circular dichroism spectroscopy. Based on light-scattering measurements, this study proves the onset of molecular aggregation at T(c). In two different monotremal hemoglobins (echidna and platypus), the critical transition temperatures were found between 32-33 degrees C, which are close to the species' body temperature T(b). The data suggest that the correlation of the structural transition's critical temperature T(c) and the species' body temperature T(b) is not mere coincidence but, instead, is a more widespread structural phenomenon possibly including many other proteins.