Messung der Hell-Dunkel-Adaptation und ihre Beeinflussung durch die extrazelluläre Calciumkonzentration : elektrophysiologische Messungen am Ventralnerv Photorezeptor von Limulus polyphemus

Abstract

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 $\cdot$ 10$^{16}$ photons $\cdot$ cm$^{- 2}$ $\cdot$ s$^{-1}$ at 543 nm) was measured by two different methods: a) $\underline{constant-stimulus-curves}$ 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) $\underline{criterion-response-curves}$ 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 t$_{DA}$ and the light intensity I were fitted by two exponential functions I = $\alpha \cdot I_{o} \cdot e^{\frac{-t_{DA}}{\tau}}$ and by two power functionsI = $a \cdot I_{o} \cdot t^{-b}_{DA}$ , respectively. The better fit - that means the higher r$^{2}$ -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 ($\tau_{I}$ = 5,5 s; s$^{-}_{x}$ = ± 0,6 s, n = 16) was followed by a second slower phase ($\tau_{2}$ = 287 s; s$^{-}_{x}$ = ± 44 s, n = 16). In the double logarithmic plot two different slopes could be determined. The exponent $\underline{b}_{1}$ characterizing the first slope was 3,5 (s$^{-}_{x}$ = ± 0,4; n = 16) in the average and $\underline{b}_{2}$ characterizing the second slope was 1,0 (s$^{-}_{x}$ = ± 0,1; n = 13) in the average. 3) The influence of the extracellular Ca$^{2+}$-concentration on the dark adaptation process was investigated. Only the first phase of dark adaptation was strongly dependent on thechanged extracellular Ca$^{2+}$-concentration - in accordance with the Ca$^{2+}$-hypothesis (Lisman and Brown, 1972) - while the second phase of dark adaptation was nearly Ca$^{2+}$-independent (s. Tab. 11). According to this effect the coefficients $\underline{b}$ and $\underline{a}$ of the power function were changed: The exponent $\underline{b}_{1}$ was decreased to 1,7 (s$^{-}_{x}$ = ± 0,5; n = 6) when the external Ca$^{2+}$-concentration was lowered from10 mmol/l to 250 $\mu$mol/l (see for example fig. 22); $\underline{b}_{1}$ was increased to 4,7 (s$^{-}_{x}$ = $\pm$ 0,5; n = 3) raising the extracellular Ca$^{2+}$-concentration to 40 mmol/l (see for example fig. 24). The factor $\underline{a}$ characterizing the sensitivity for t$_{DA}$ = 1 s varies greatly from experiment to experiment; $\underline{a}_{1}$ is decreased - corresponding to a greater sensitivity - to about 5 % in the average by lowering the extracellularCa$^{2+}$-concentration and strongly increased by a factor of 70 by raising the extracellular Ca$^{2+}$-concentration