Single Photon-Evoked Events Of The Ventral Nerve Photoreceptor Cell Of Limulus Facilitation, Adaptation And Dependence Of Lowered External Calcium

Abstract

Bumps, the elementary excitatory events of the Limulus ventral nerve photo receptor following a weak flash of light were recorded under voltage clamp conditions. The statistical distribution of various bump parameters and their changes caused by weak conditioning pre-illumination are described, and the influence of lowered external Ca2+-concentration together with normal or raised Mg2+-concentration (15 °C). 1) Weak conditioning pre-illumination causes desensitization: the bump current amplitude, bump duration , bump area (current-integral), and the bump latency are diminished, the more, the stronger the conditioning flash, i.e. the light adaptation. Very weak conditioning pre-illumination causes facilitation, expressed by an increase in number and size of the observed bumps. The average bump latency, however, is already shortened under these conditions. 2) Lowering the external Ca2+-concentration from 10 mmol/l to 250 (µmol/1 has its primary effect on the dark -adapted photoreceptor (without substantially reducing the ability for light adaptation ). It causes the following average changes: the amplitudes, durations, current-integrals, and the latencies of current bumps are greatly enlarged and the number of bumps is raised. 3) Raised magnesium concentration from 50 to 100 mmol/l can partially compensate for the lack of calcium ; however, it enhances the effect of calcium deficiency on the latency, i.e. it further enlarges the average latencies. The results can be explained on the basis of our model of bump generation by two assumptions. 1) Lowering the external calcium concentration causes a decrease in the cytosolic Ca2+-level without substantially reducing the intracellular calcium stores from which the light-adapting calcium release is fed. The lowered cytosolic Ca2+-concentration induces an “extra” dark adaptation resulting in greater bumps and more bumps exceeding the threshold of recognition. The bump latency, however, which behaves differently from all other bump parameters, is determined by a separate calcium -dependent reaction where magnesium competes with calcium antagonistically. 2) Facilitation is due to cooperativity of transmitter binding in order to open the ion channels