CYTOSOLIC CALCIUM TRANSIENT AND LIGHT RESPONSE IN LIMULUS VENTRAL NERVE PHOTORECEPTORS - SUPRALINEARITY IN RESPONSE-INTENSITY CURVES

Light-evoked intracellular electrical responses were measured simultan eously with the changes in intracellular foe Ca2+ or Sr2+ concentratio n by means of arsenate III. The response-versus-stimulus intensity cha racteristics was measured in two states of relative light adaptation i n strontium-containing saline (10 mM Sr2+; 0.25 mM Ca2+). Supralinear parts of the curves with slopes > 1 were found for the receptor curren t amplitude and the current-time integral as well as for the amplitude of the arsenate signal within at least 0.3 log units of identical sti mulus intensity range. Maximum slopes of response-versus-intensity cur ves are on average approximately 1.7 for the receptor current and the arsenate signal in both adaptational states. The rise in time of the a rsenate signal evoked by a light flash shows two phases. In strontium saline we observe a fast increase at the onset of the arsenate signal which is detectable within the first 20-100 ms of the signal rise and a slow further increase to the maximum, with a duration of 0.7-6 s. In the detailed analysis of both phases we observed that the amplitude o f each phase increases supralinear, i.e. disproportionate to the stimu lus intensity. The fast rising phase of the arsenate signal is not vis ibly voltage-dependent. This fast rising phase might be a monitor of a n IP3-sensitive calcium or strontium release. Since the amplitude of t he fast rising phase increases supralinear with the stimulus intensity , we propose the interpretation that more than one IP3-molecule must b ind to the IP3-receptor to open this channel of the intracellular calc ium store. Only the slow rising phase is voltage-dependent. The kineti c of this slow rising phase was accelerated with more negative membran e potentials. This predominant slow phase correlates with the receptor current-time integral. Therefore the slow rising phase of the arsenat e signal might be a monitor for influx of calcium or strontium through light activated ion channels in the plasma membrane. The supralineari ty of the amplitude of the slow rising phase might indicate the supral inear increase in calcium or strontium, which is carried by the recept or current. Additionally, we calculated the fraction of strontium and calcium ions of the total receptor current. When calcium was replaced by strontium in the superfusate about 3-30% of the receptor current ar e carried by strontium and calcium ions. In physiological saline calci um contributes about 1-1.5% to the receptor current.