THE LIGHT-STIMULATED CYTOSOLIC CALCIUM TRANSIENT IN LIMULUS VENTRAL NERVE PHOTORECEPTORS - 2 COMPONENTS IN THE RISING PHASE

1. Light-evoked electrical responses were measured in Limulus ventral nerve photoreceptors simultaneously with changes in the cytosolic free calcium concentration, by means of arsenate III. 2. It has been shown here for the first time that the rise of the arsenate signal consists of two phases. Only the slow phase in the rise of the signal depends on the membrane voltage. The reversal potential of the amplitude of th is slow rising phase was about +196 mV. After removal of external calc ium the reversal potential was about +20 mV. 3. When Na+ in the superf usate was replaced by Li+, the amplitude of the fast rising phase was reduced on the average to 50%, while the slow rising phase was not aff ected. 4. We conclude that the fast rising phase is caused by release of calcium from internal stores while the slow increase in [Ca2+](i) i s due to influx across the plasma membrane, possibly through light-act ivated ion channels.