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

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/l Ca2+), or by salines with either lowered (250 μmol/l) or raised (40 mmol/l) 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.