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Rising Phase (rising + phase)

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Life Sciences	100%

Selected Abstracts

Radical Cation Generation from Singlet and Triplet Excited States of All-trans-Lycopene in Chloroform,

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2004
Rui-Min Han
ABSTRACT On direct photoexcitation, subpicosecond time-resolved absorption spectroscopy revealed that the 1Bu -type singlet excited state of all- trans -lycopene in chloroform was about seven times more efficient than all-trans-,-carotene in generating the radical cation. The time constant of radical cation generation from the 1Bu -type state was found to be ,0.14 ps, a value that was comparable for the two carotenoids. On anthracene-sensitized triplet excitation, radical cation generation was found to be much less efficient for lycopene than for ,-carotene. A slow rising phase (20-30 ,s) in the bleaching of ground-state absorption was common for both lycopene and ,-carotene in chloroform and was ascribed to an efficient secondary reaction with a solvent radical leading to the formation of carotenoid radical cations. The reverse ordering in the tendency of the excited states of different multiplicities for the two carotenoids to generate radical cations is discussed in relation to the two carotenoids as scavengers of free radicals. [source]

Replacing the rod with the cone transducin , subunit decreases sensitivity and accelerates response decay

THE JOURNAL OF PHYSIOLOGY, Issue 17 2010
C.-K. Chen
Cone vision is less sensitive than rod vision. Much of this difference can be attributed to the photoreceptors themselves, but the reason why the cones are less sensitive is still unknown. Recent recordings indicate that one important factor may be a difference in the rate of activation of cone transduction; that is, the rising phase of the cone response per bleached rhodopsin molecule (Rh*) has a smaller slope than the rising phase of the rod response per Rh*, perhaps because some step between Rh* and activation of the phosphodiesterase 6 (PDE6) effector molecule occurs with less gain. Since rods and cones have different G-protein , subunits, and since this subunit (T,) plays a key role both in the interaction of G-protein with Rh* and the activation of PDE6, we investigated the mechanism of the amplification difference by expressing cone T, in rod T,-knockout rods to produce so-called GNAT2C mice. We show that rods in GNAT2C mice have decreased sensitivity and a rate of activation half that of wild-type (WT) mouse rods. Furthermore, GNAT2C responses recover more rapidly than WT responses with kinetic parameters resembling those of native mouse cones. Our results show for the first time that part of the difference in sensitivity and response kinetics between rods and cones may be the result of a difference in the G-protein , subunit. They also indicate more generally that the molecular nature of G-protein , may play an important role in the kinetics of G-protein cascades for metabotropic receptors throughout the body. [source]

Subunit-specific desensitization of heteromeric kainate receptors

THE JOURNAL OF PHYSIOLOGY, Issue 4 2010
David D. Mott
Kainate receptor subunits can form functional channels as homomers of GluK1, GluK2 or GluK3, or as heteromeric combinations with each other or incorporating GluK4 or GluK5 subunits. However, GluK4 and GluK5 cannot form functional channels by themselves. Incorporation of GluK4 or GluK5 into a heteromeric complex increases glutamate apparent affinity and also enables receptor activation by the agonist AMPA. Utilizing two-electrode voltage clamp of Xenopus oocytes injected with cRNA encoding kainate receptor subunits, we have observed that heteromeric channels composed of GluK2/GluK4 and GluK2/GluK5 have steady state concentration,response curves that were bell-shaped in response to either glutamate or AMPA. By contrast, homomeric GluK2 channels exhibited a monophasic steady state concentration,response curve that simply plateaued at high glutamate concentrations. By fitting several specific Markov models to GluK2/GluK4 heteromeric and GluK2 homomeric concentration,response data, we have determined that: (a) two strikingly different agonist binding affinities exist; (b) the high-affinity binding site leads to channel opening; and (c) the low-affinity agonist binding site leads to strong desensitization after agonist binding. Model parameters also approximate the onset and recovery kinetics of desensitization observed for macroscopic currents measured from HEK-293 cells expressing GluK2 and GluK4 subunits. The GluK2(E738D) mutation lowers the steady state apparent affinity for glutamate by 9000-fold in comparison to GluK2 homomeric wildtype receptors. When this mutant subunit was expressed with GluK4, the rising phase of the glutamate steady state concentration,response curve overlapped with the wildtype curve, whereas the declining phase was right-shifted toward lower affinity. Taken together, these data are consistent with a scheme whereby high-affinity agonist binding to a non-desensitizing GluK4 subunit opens the heteromeric channel, whereas low-affinity agonist binding to GluK2 desensitizes the whole channel complex. [source]

Response properties of isolated mouse olfactory receptor cells

THE JOURNAL OF PHYSIOLOGY, Issue 1 2001
Johannes Reisert
1Response properties of isolated mouse olfactory receptor cells were investigated using the suction pipette technique. Cells were exposed to the odour cineole or to solutions of modified ionic content by rapidly changing the solution superfusing the cilia. All experiments were performed at 37°C. 2Mouse olfactory receptor cells displayed a steep dependence of action potential frequency on stimulus concentration, a 3-fold increase in stimulus concentration often saturating the firing frequency at 200-300 Hz. The receptor current increased more gradually with increasing cineole concentration and did not saturate within the 100-fold range of cineole concentrations applied. 3When stimulated for 30 s with a low odour concentration, cells responded with sporadic spike firing. Higher concentrations led to the generation of a large receptor current at the onset of stimulation which returned to baseline levels within a few seconds, accompanied during its rising phase by a short burst of action potentials. Thereafter an oscillating response pattern was observed during the remainder of the stimulus, consisting of repetitive increases in receptor current of around 1 s duration accompanied by short bursts of action potentials. 4Olfactory adaptation was studied by comparing the responses to two closely spaced odour stimuli. The response to the second odour stimulus recovered to 80% of its original magnitude when the cell was superfused with Ringer solution during the 5 s interval between odour exposures. In contrast, exposure to a choline-substituted low Na+ solution between odour stimuli had two effects. First, the receptor current response to the first odour stimulus did not terminate as quickly as in the presence of Na+, suggesting the presence of a Na+ -Ca2+ exchanger. Second, the response to the second stimulus only recovered to 55% of its original magnitude, demonstrating the involvement of Na+ -Ca2+ exchange in the recovery of sensitivity in mouse olfactory receptor cells following stimulation. [source]