Messung der Hell-Dunkel-Adaptation und ihre Beeinflussung durch die extrazelluläre Calciumkonzentration : elektrophysiologische Messungen am Ventralnerv Photorezeptor von Limulus polyphemus
Method: A set-up for intra- and extracellular electrophysiological measurements on the Limulus ventral nerve photoreceptor was built up. A suction electrode method was developed to record the extracellular currents of a single photoreceptor cell at different membrane areas. Simultaneously the voltage response (receptor potential) could be measured intracellularly. 1) The time course of dark adaptation following light adaptation by a bright 1 or 5 s illumination (4,4 ⋅ 1016 photons ⋅ cm−2⋅ s−1 at 543 nm) was measured by two different methods: a) constant−stimulus−curves were determined by measuring the amplitude of the intracellularly recorded receptor potential in response to test stimuli of constant intensity and duration in dependence on the dark adaptation time. b) criterion−response−curves were determined by measuring the sensitivity increase in dependence on the dark adaptation time by determining the light intensity necessary to evoke a criterion response amplitude of the receptor potential. The experimental data for the dark adaptation time tDA and the light intensity I were fitted by two exponential functions I = α⋅Io⋅eτ−tDA and by two power functionsI = a⋅Io⋅tDA−b , respectively. The better fit - that means the higher r2 -values (correlation coefficient) could be obtained by the power functions (fig. 18). 2) The time course of dark adaptation was characterized by two different phases, a fast and a slow one. The first rather fast increase of sensitivity after light adaptation (τI = 5,5 s; sx− = ± 0,6 s, n = 16) was followed by a second slower phase (τ2 = 287 s; sx− = ± 44 s, n = 16). In the double logarithmic plot two different slopes could be determined. The exponent b1 characterizing the first slope was 3,5 (sx− = ± 0,4; n = 16) in the average and b2 characterizing the second slope was 1,0 (sx− = ± 0,1; n = 13) in the average. 3) The influence of the extracellular Ca2+-concentration on the dark adaptation process was investigated. Only the first phase of dark adaptation was strongly dependent on thechanged extracellular Ca2+-concentration - in accordance with the Ca2+-hypothesis (Lisman and Brown, 1972) - while the second phase of dark adaptation was nearly Ca2+-independent (s. Tab. 11). According to this effect the coefficients b and a of the power function were changed: The exponent b1 was decreased to 1,7 (sx− = ± 0,5; n = 6) when the external Ca2+-concentration was lowered from10 mmol/l to 250 μmol/l (see for example fig. 22); b1 was increased to 4,7 (sx− = ± 0,5; n = 3) raising the extracellular Ca2+-concentration to 40 mmol/l (see for example fig. 24). The factor a characterizing the sensitivity for tDA = 1 s varies greatly from experiment to experiment; a1 is decreased - corresponding to a greater sensitivity - to about 5 % in the average by lowering the extracellularCa2+-concentration and strongly increased by a factor of 70 by raising the extracellular Ca2+-concentration