Das Belichtungspotential der Retina des Einsiedlerkrebses in Abh�ngigkeit von der Temperatur

Abstract

1. The action potential of the isolated retina of the hermit crab Eupagurus bernhardus L. resulting from exposure to light has been measured at temperatures between 5° and 25° C. 2. With increasing temperature the time course of the retina action potential (RAP) shortens. Latency tla and peak-amplitude-time tmax of the RAP decrease. The slopes of rising and falling phase of the RAP steepen, the former less than the latter. 3. The temperature-coefficients (Q10's) of all the time-values of the RAP's measured have been larger than 2. 4. At increasing temperature initially the amplitudes of the RAP's rise, while further increase of temperature above 10–15° C the RAP's decrease. 5. All observed temperature-dependent changes of the RAP were fully reversible. 6. The amplitudes of the two components K I and K II of the RAP show a similar temperature-dependence. 7. The range of validity of the Bunsen-Roscoe-law depends on temperature: at short lasting stimuli the amplitude of the RAP depends on the product of stimulus intensity x stimulus duration (I · τ). Under the experimental conditions here described this has been found at temperatures of 25° C up to stimulus durations of 10–20 msec, at 5° C up to stimulus durations of 60–120 msec. 8. The observed RAP's and their changes with temperature have been interpreted in the following way: The RAP occurs as the result of a chemical reaction of the visual pigment induced by exposure to light. This inturn produces a fall of the electrical resistance of the membrane of the photoreceptor cell. The restoration of the dark-adapted state requires metabolic processes. The decrease of the membrane resistance of the photoreceptor cell during illumination is relatively small at low and high temperatures as compared to medium temperatures. The decrease of the resistance during exposure to light and consequently the amplitude of the RAP decrease above 10–15° C with increasing temperature since at higher temperatures the falling phase of the RAP is more accelerated than the rising phase. Among the processes responsible for the rising phase of the RAP there is one photochemical temperature-independent reaction. On the other hand the processes responsible for the falling phase of the RAP all are temperature-dependent. Below 10–15° C the decrease of the electrical resistance during light-exposure rises with increasing temperature. Since in the experiments described the retina has not been fully dark-adapted the amplitude of the RAP largely depends on the speed of dark-adaptation, which shows a strong positive temperature-dependence. The speed of dark-adaptation determines the dependence of the amplitude of the RAP on temperatures below 10–15° C. Furthermore at low temperatures the system which is responsible for the permeability changes of the cell membrane presumably becomes directly inactivated at low temperatures — thereby exhibiting a behaviour similar to that of a nerve fiber showing the phenomenon of cold blockade. The temperature-dependence of the resistances in the retina as well as the temperature-dependence of the range of validity of the Bunsen-Roscoe-law have been taken into account when evaluating the measured curves of the RAP's.