Sensitivity changes of photoreceptor cells of Hirudo medicinalis caused by changes in extracellular calcium concentration.

Extracellular recordings from the vacuole of photoreceptor cells of Hirudo medicinalis L. were performed using microelectrodes. The cells were adapted by white light flashes given at constant intervals (20 s). Response height versus relative intensity curves obtained from the same cell in physiological saline (PS) and in bathing solutions of either a) lowered calcium contents (2 micron/1 or less) or b) raised calcium contents (15 mM/1) were compared. The cells' adaptation state in PS was operationally defined by the ratio Q = hA/hs where hA is the response height evoked by the adapting flashes, and hs is the corresponding saturation response height. Sensitivity changes were measured by the half saturation intensity shift. Lowering extracellular calcium resulted in: 1. The response height increased and the shape of the response became more rounded and prolonged. 2. The total resistance between the vacuole and outside decreased from 8.2 +/- 1.4 Momega (n = 6) in PS to 4.6 +/- 0.4 Momega (n = 5). The resistance was independent of the cells' adaptation state. 3. A change of the cells' sensitivity occurred either in direction to light adaptation or in direction to dark adaptation. It depended functionally on the ratio Q: a) if Q was less or equal to about 0.6 the cells' sensitivity increased. b) if Q was greater than 0.6 the cells' sensitivity diminished. Raising extracellular calcium decreased the sensitivity of all cells tested independent of their adaptation states in PS. The results can be interpreted under the assumptions that 1. the sensitivity of leech photoreceptor cells is inversely proportional to the intracellular free calcium concentration and 2. intracellular calcium can interact with extracellular calcium in relatively dark adapted cells whereas in relatively light adapted cells the raise of intracellular free calcium is mainly effected by a release from intracellular stores. It is assumed that a Q value of about 0.6 separates relatively light adapted cells from relatively dark adapted cells.