ZfN: Search Results

1	Author	Helmut Reuß, Hennig Stieve	Requires cookie*
	Title	Two Different Bump Types of the Ventral Photoreceptor of Limulus
	Abstract	The elementary responses, the "bumps", o f the ventral photoreceptor o f Limulus polyphe-mus were measured under voltage clamp conditions. We observed a type o f bump which differs in size and time parameters from the "standard" bumps described previously. The average value o f the amplitude o f these "small, slow " bumps is about 20-fold smaller than that o f the standard bumps (about 1 nA) and the average latency is about 25% longer. The duration o f these bumps is 2 -5 -fo ld longer than that o f a standard bump with the same amplitude. The small, slow bumps are probably light-induced and arise with about the same frequency after a light flash as standard bumps. The different kinetics o f the two bump types can be explained by the assumption that the transduction chain branches into at least two pathways.
	Reference	Z. Naturforsch. 48c, 92 (1993); received December 17 1992
	Published	1993
	Keywords	Limulus Ventral Nerve Photoreceptor, Light-Induced Bumps, Spontaneous Bumps, Small Slow Bumps
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	TEI-XML for	default:Reihe_C/48/ZNC-1993-48c-0092.pdf
	Identifier	ZNC-1993-48c-0092
	Volume	48

2	Author	H. Stieve, M. Pflaum, J. Klomfaß, H. G. Aube	Requires cookie*
	Title	Calcium/Sodium Binding Competition in the Gating of Light-Activated Membrane Conductance Studied by Voltage Clamp Technique in Limulus Ventral Nerve Photoreceptor
	Abstract	The mem brane current vs. voltage dependence was measured in Limulus ventral nerve photo receptors at various external C a2+ and N a+ concentrations, using the voltage clam p technique. Lowering the external concentration of the divalent cations Ca~+ and Mg+ to < 1 \|im ol/l by adding EDTA causes 1) the light-induced transient conductance increase to disappear and 2) Irev J d ! the reversal potential of the mem brane current in the dark to shift to a positive value between + 10 and + 2 0 mV. This value is about the same as the (V^J-l), reversal potential of the total light current under normal ionic conditions. If the external N a+ is lowered to 50 mmol/1 (i.e. 10% of the normal concentration) simultaneously with the lowering o f the divalent cation concentration described above, the light response is not abolished and V TevJ p is shifted less. The extent of this antagonism depends on the sodium sub stitute; it is stronger if choline is used instead of lithium. Lowering of sodium alone to 50 mmol/1, in a saline containing normal C a2+ and Mg2+ concen trations, does not change the mem brane dark current vs. voltage curve and so is not altered; VT ^ A J L, the reversal potential of the light-induced current, however, is reduced by 10 mV (from + 2 0 to + 1 0 mV). This reduction in %evA JL can be accounted for by the reduction of the sodium gradient across the cell membrane. Raising the external C a2+ concentration to 40 or 100 mmol/1 has no conspicuous effect on the mem brane current vs. voltage dependence and the gating o f the light-induced conductance in crease. The results are consistent with our working hypothesis that the gating o f the light-activated ion channels in Limulus photoreceptor is controlled by negative binding sites for which calcium-and sodium ions compete with antagonistic actions. Abbreviations: ReP, receptor potential (m embrane voltage response to the light stimulus); HMAX [mV], peak am pli tude of the ReP; PMP [mV], pre-stimulus mem brane po tential (difference between the extracellularly recorded zero line and the intracellularly recorded base line; initial and final zero were averaged to compensate for drift); VM [mV], mem brane voltage; J M [nA], mem brane current; J d [nA]» dark current, m em brane current during voltage clamp, measured 1 s after clam p onset; J L [nA], am plitude o f total light current; am plitude of total m em brane current following the light flash (J D + A JL)\ AJL, am plitude of light-induced current; ^ ev7D [mV], reversal potential of dark current; ^ ev7 L[mV], reversal potential of light cur rent; I(evj y L[mV], reversal potential of the light-induced current evoked by the light stimulus; g G [\|iS], membrane (slope) conductance in the dark; ^lD 1^], total membrane (slope) conductance during illum ination; J g L[\|iS], light induced mem brane (slope) conductance; PS, physiological saline (see Table I); [N a+]ex[Ca2+]ex, sodium, or calcium ion concentration of the external saline; EDTA, ethylene dimethyl tetra acetic acid; EGTA, ethylene glycol-bis (2-aminoethylether) N ,N '-tetra acetic acid.
	Reference	Z. Naturforsch. 40c, 278—291 (1985); received N ovember 19 1984
	Published	1985
	Keywords	Limulus Ventral Nerve Photoreceptor, Current-Voltage Relation, Varied External Calcium and Sodium Concentration, Calcium /Sodium Binding Competition, Lithium
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	TEI-XML for	default:Reihe_C/40/ZNC-1985-40c-0278.pdf
	Identifier	ZNC-1985-40c-0278
	Volume	40

3	Author	Helmut Reuß, Michael Rack, Hennig Stieve	Requires cookie*
	Title	Spontaneous Bumps of the Limulus Photoreceptor Cell are Probably Triggered by the Spontaneous Activation of Single G-Protein Molecules
	Abstract	The elementary responses, the "bumps", o f the ventral photoreceptor o f Limulus polyphemus were studied under voltage clamp conditions. We compared sponta neous bumps with those induced by light and we applied the G-protein activator A1F4-. The amplitude o f the spontaneous bumps is on the average 7 -8 times smaller than that o f the light-induced bumps. Bumps induced by A1F4_ have identical size and time parameters when compared with spontaneous bumps. Thus at least a large part o f the spontaneous bumps is probably triggered by the activation o f single G-protein molecules. However, if the next step o f the transduction chain is gainless, it is also possible the spontaneous bump originate from this step. The different size o f spontaneous and light-induced bumps can be explained assuming that a light-activated metarhodopsin molecule leads to the activation o f 3 to 12 G-protein molecules.
	Reference	Z. Naturforsch. 47c, 932 (1992); received September 7 1992
	Published	1992
	Keywords	Limulus Ventral Nerve Photoreceptor, Spontaneous Bumps, Chemically Induced Bumps, Light-Induced Bumps, G-Protein
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	TEI-XML for	default:Reihe_C/47/ZNC-1992-47c-0932_n.pdf
	Identifier	ZNC-1992-47c-0932_n
	Volume	47

4	Author	H. Stieve, H. Gaube, J. Klomfaß	Requires cookie*
	Title	Effect of External Calcium Concentration on the Intensity Dependence of Light-Induced Membrane Current and Voltage Signals in Two Defined States of Adaptation in the Photo-Receptor of Limulus
	Abstract	The intensity dependence of the response of the Limulus ventral nerve photoreceptor to light flashes was determined in alternating measurements for the membrane current signal (receptor current) under voltage clamp conditions and the membrane voltage signal (receptor potential). Responses were obtained at two reproducible states of adaptation, while the photoreceptor was superfused by physiological saline (10 mmol/1 Ca2+), or by salines with either lowered (250 [j.mol/1) or raised (40 mmol/1) calcium concentration. For the dark-adapted state of the photoreceptor the double logarithmic plot of the response current-time integral F (or the current amplitude) vs. flash intensity rises in a steep, supralinear section (slope 2-4) to a curve knee towards a less steep, sublinear section (slope 0.2—0.6), but does not reach saturation in the intensity range tested. Light adaptation shifts the response size vs. intensity curve towards higher light intensities. This sensitivity shift is enlarged in raised, and almost abolished in low external [Ca2+], The changes of response latency and time-to-peak with stimulus intensity or adaptation are almost identical for receptor current and receptor potential. The decrease-time of the receptor current response, however, depends much less on the stimulus intensity or the state of adaptation than that of the receptor potential. The relative changes in the time course of the receptor current caused by light adaptation are not much influenced by variation of the [Ca2+]ex. Interpretation: The macroscopic receptor current signal consists of a volley of overlapping bumps; the size of these bumps is scaled by a calcium-dependent attenuation function which increases with delay time. This gradual growing attenuation a(t) acts as automatic gain control and may be responsible for the sublinear slope of the intensity dependence of the size of the receptor current. The supralinear slope of this dependence at lower stimulus intensities is probably caused by cooperative effects. Changes in the time course of the macroscopic receptor current due to light adaptation or varied calcium concentration are based on changes in the latency distribution of the underlying bump volley, and the size of the attenuation function.
	Reference	Z. Naturforsch. 41c, 1092—1110 (1986); received June 27 1986
	Published	1986
	Keywords	Limulus Ventral Nerve Photoreceptor, Light-Induced Receptor Current and Receptor Potential, Light and Dark Adaptation, Stimulus/Response Characteristics, Intensity Dependence
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	TEI-XML for	default:Reihe_C/41/ZNC-1986-41c-1092.pdf
	Identifier	ZNC-1986-41c-1092
	Volume	41