Effects of sodium, potassium, and calcium ions on slow and spike potentials in single photoreceptor cells

The influence of changes in the ionic composition of the bathing medium on responses of the retinula cell of the honeybee drone to light was examined by means of intracellular microelectrodes. The resting potential of the cell was influenced mainly by the concentration of K. The peak of the receptor potential (the transient), which in a normal solution and with strong light approaches zero membrane potential, overshot this level in a K-rich solution. An increase in the concentration of K also raised the level of the steady-state phase of the receptor potential (the plateau). The amplitude of the receptor potential was decreased and the spike potential rapidly abolished when Na was replaced by either sucrose, choline, or Tris. In a Ca-free solution the amplitude of the response and especially that of the plateau, was increased. An increase in Ca had the opposite effects. All these changes were reversible. An attempt was made to interpret the receptor and spike potentials in terms of passive movements of Na and K across the membrane of the retinula cell. The major difficulty encountered was to find an explanation for the persistence of an appreciable fraction of the transient and the plateau in preparations kept up to 12 hr in a solution in which all the Na had been replaced by choline, Tris, or sucrose.

Slow and spike potentials recorded from retinula cells of the honeybee drone in response to light

Responses to light recorded by means of intracellular microelectrodes in isolated heads kept in oxygenated Ringer solution consist of a slow depolarization. Light adaptation increases the rates of depolarization and repolarization and decreases the amplitude of the response. Qualitatively these changes are similar to those observed in Limulus by Fuortes and Hodgkin. They are rapidly reversible during dark adaptation. In retinula cells of the drone eye a large single spike is recorded superimposed on the rising phase of the slow potential. The spike is a regenerative phenomenon; it can be triggered with electric current and is markedly reduced, sometimes abolished by tetrodotoxin. In rare cases cells were found which responded to light with a train of spikes. This behavior was only found under "unusual" experimental conditions; i.e., towards the end of a long experiment, during impalement, or at the beginning of responses to steps of strongly light-adapted preparations.