The effect of some sodium substitutes on the receptor potential of the crayfish photoreceptor cell.

Abstract

Isolated crayfish retinas were perfused with four solutions in which Li+, ΝΗ4+, Tris H+ and glucose were substituted for the sodium ions in the physiological salt solution. The changes of the extracellularly recorded receptor potential (ReP) evoked by short or long stimuli were measured. The changes in the shape of ReP by test solutions were different for each Na-substitute. For lithium ions as a Na-substitute (Tab. I and Fig. 3) the plateau value he was considerably decreased (to ∼20%) contrary to the peak-amplitude hmax which even slightly increased. Ammonium ions show quite a different effect than all the other substitutes. The ReP is decreased strongly and irreversibly (Tab. II and Fig. 4). When Tris (hydroxymethyl-ammoniummethane-hydrochloride) is substituted for Na, hmax decreased to about 60 per cent and the plateau is even more reduced (to 20 per cent; Tab. III). Only the recovery-value for he (50% smaller) is markedly different contrary to our former experiments where choline was used as Na-substitute (decrease 20%). Glucose as a substitute for sodium chloride caused strongly decreased peak-amplitude hmax 26% (Tab. IV and Fig. 9). Increased osmotic pressure due to excess glucose causes irreversible damage of the ReP (Tab. V). All the changes except those produced by NH4+ were reasonably reversible. The results can be explained by the following assumptions: a) the maximum of the ReP is caused mainly by an increase in the permeability of the cell membrane for sodium. Ca- and Mg-ions also contribute to it to a certain degree. b) The plateau value of the ReP to long light stimuli is determined: 1. by the sodium concentration gradient, 2. by active transport processes, 3. by the Ca++ - and Mg++-gradients, 4. the chloride gradient may perhaps contribute to this value